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-The Linux SCSI Generic (sg) HOWTO
-!!!The Linux SCSI Generic (sg) HOWTO
-!Douglas Gilbert
-
- dgilbert@interlog.com
-
-
-
-
-Copyright (c) 2001, 2002 by Douglas Gilbert
-
-
-
-2002-05-03
-
-
-__Revision History__Revision 1.22002-05-03Revised by: dpgENOMEM, EPERM; DRIVER_SENSE-bCHECK_CONDITIONRevision 1.12002-01-26Revised by: dpgcorrections, host_status, odd dxfer_lenRevision 1.02001-12-21Revised by: dpgoriginal, displace SCSI-PROGRAMMING-HOWTO
-
-
-
-
-
- This HOWTO describes the SCSI Generic driver (sg) found in the Linux
-2.4 production series of kernels.
-It focuses on the the interface and characteristics of the driver
-that application writers may need to know. The driver's theory of
-operations is covered and some brief examples are included.
-
-
-
-
-
-
-
-
-
-
-
-
- Permission is granted to copy, distribute and/or modify this document
-under the terms of the GNU Free Documentation License, Version 1.1
-or any later version published by the Free Software Foundation;
-with no Invariant Sections, with no Front-Cover Texts, and with
-no Back-Cover Texts.
-
-
-
-
- For an online copy of the license see
- www.fsf.org/copyleft/fdl.html.
-
-
-
-
-
-
-----; __Table of Contents__; 1. Introduction; 2. What the sg driver does; 3. Identifying the version of the SG driver; 4. Interface; 5. Theory of operation; 6. The sg_io_hdr_t structure in detail: ; 6.1. interface_id; 6.2. dxfer_direction; 6.3. cmd_len; 6.4. mx_sb_len; 6.5. iovec_count; 6.6. dxfer_len; 6.7. dxferp; 6.8. cmdp; 6.9. sbp; 6.10. timeout; 6.11. flags; 6.12. pack_id; 6.13. usr_ptr; 6.14. status; 6.15. masked_status; 6.16. msg_status; 6.17. sb_len_wr; 6.18. host_status; 6.19. driver_status; 6.20. resid; 6.21. duration; 6.22. info; 7. System calls: ; 7.1. open(); 7.2. write(); 7.3. read(); 7.4. poll(); 7.5. close(); 7.6. mmap(); 7.7. fcntl(sg_fd, F_SETFL, oflags | FASYNC); 7.8. Errors reported in errno; 8. Ioctl()s: ; 8.1. SG_IO; 8.2. SG_GET_ACCESS_COUNT; 8.3. SG_SET_COMMAND_Q (and _GET_); 8.4. SG_SET_DEBUG; 8.5. SG_EMULATED_HOST; 8.6. SG_SET_KEEP_ORPHAN (and _GET_); 8.7. SG_SET_FORCE_LOW_DMA; 8.8. SG_GET_LOW_DMA; 8.9. SG_NEXT_CMD_LEN; 8.10. SG_GET_NUM_WAITING; 8.11. SG_SET_FORCE_PACK_ID; 8.12. SG_GET_PACK_ID; 8.13. SG_GET_REQUEST_TABLE; 8.14. SG_SET_RESERVED_SIZE (and _GET_ ); 8.15. SG_SCSI_RESET; 8.16. SG_GET_SCSI_ID; 8.17. SG_GET_SG_TABLESIZE; 8.18. SG_GET_TIMEOUT; 8.19. SG_SET_TIMEOUT; 8.20. SG_SET_TRANSFORM; 8.21. SG_GET_TRANSFORM; 8.22. Sg ioctls removed in version 3; 8.23. SCSI_IOCTL_GET_IDLUN; 8.24. SCSI_IOCTL_GET_PCI; 8.25. SCSI_IOCTL_PROBE_HOST; 8.26. SCSI_IOCTL_SEND_COMMAND; 9. Direct and Mmap-ed IO: ; 9.1. Direct IO; 9.2. Mmap-ed IO; 10. Driver and module initialization; 11. Sg and the "proc" file system: ; 11.1. /proc/scsi/sg/debug; 12. Asynchronous usage of sg; A. Sg3_utils package; B. sg_header, the original sg control structure; C. Programming example; D. Debugging; E. Other references----
-!!!Chapter 1. Introduction
-
-This document outlines the Linux SCSI Generic (sg) driver
-interface as found in the 2.4 series kernels. The driver's purpose is to
-allow SCSI commands to be sent directly to SCSI devices. The responses of
-those commands can then be obtained. This type of driver is sometimes termed
-as a "pass through".
-In the case of SCSI disks, the block subsystem which is normally used to
-mount and access a disk, is bypassed permitting low level operations such as
-formatting to be performed. Various specialized applications for writing
-CD-Rs and document scanning use the sg driver.
-
-
-
-Many devices that use other physical buses (e.g. ATAPI cdroms, USB
-mass storage devices and IEEE 1394 sbp2 devices) utilize the SCSI command
-set. By using Linux pseudo SCSI device drivers which bridge between
-the native protocol stack and the SCSI subsystem, the upper level
-SCSI device drivers, including sg, can be used to control "non-SCSI"
-devices.
-
-
-
-This is the third major version of the sg driver.
-A summary of the sg driver history is as follows:
-
-
-
-
-
-*
-
-sg version 1 (original) from 1992 to early 1999
-(lk 2.2.5) . A copy of the original HOWTO (in plain text) is at
-www.torque.net/sg/p/original/SCSI-Programming-HOWTO.txt
-
-
-*
-*
-
-sg version 2 from lk 2.2.6 in the 2.2 series. Its
-documentation is available in abridged form
-[www.torque.net/sg/p/scsi-generic.txt
]
-and a longer form
-[[www.torque.net/sg/p/scsi-generic_long.txt].
-
-
-*
-*
-
-sg version 3 in the linux kernel 2.4 series.
-
-
-*
-This document can be found at the Linux Documentation Project's site at
-www.linuxdoc.org/HOWTO/SCSI-Generic-HOWTO/ .
-It is available in plain text and pdf renderings at that site.
-A (possibly later) version of this document can be found at
-www.torque.net/sg/p/sg_v3_ho.html.
-That is a single html page; drop the ".html" extension for multi-page
-html. There are also postscript, pdf and rtf renderings from the original
-SGML (docbook) file at the same location.
-
-
-
-A more general description of the Linux SCSI subsystem of which sg is a
-part can be found in the
-
-SCSI-2.4-HOWTO.
-
-
-
-This document was last modified on 3rd May 2002.
-
-----
-!!!Chapter 2. What the sg driver does
-
-The sg driver permits user applications to send SCSI commands to devices
-that understand them. SCSI commands are 6, 10, 12 or 16 bytes long
-[[1].
-The SCSI disk driver (sd), once device initialization is complete, only
-sends SCSI READ and WRITE commands. There a several other interesting
-things one might want to do, for example, perform a low level format or turn
-on write caching.
-
-
-
-Associated with some SCSI commands there is data to be written to the device.
-A SCSI WRITE command is one obvious example. When instructed, the sg driver
-arranges for data to be transferred to the device along with the SCSI
-command. It is possible that the lower level driver (often known as
-the "Host Bus Adapter" [[HBA] or simply "adapter" driver) is unable to
-send the command to the device. An example of this occurs when the device does
-not respond in which case a 'host_status' or 'driver-status' error will
-be conveyed back to the user application.
-
-
-
-All going well the SCSI command (and optionally some data) are conveyed
-to the device. The device will respond with a single byte value called
-the 'scsi_status'. GOOD is the scsi status indicating everything has
-gone well. The most common other status is CHECK CONDITION. In this
-latter case, the SCSI mid level issues a REQUEST SENSE SCSI command
-The response of the REQUEST SENSE is 18 bytes or more in length and is
-called the "sense buffer". It will indicate why the original command
-may not have been executed. It is important to realize that a CHECK
-CONDITION may vary in severity from informative (e.g. command needed to
-be retried before succeeding) to fatal (e.g. "medium error" which often
-indicates it is time to replace the disk).
-
-
-
-So in all cases a user application should check the various status values.
-If necessary the "sense buffer" will be copied back to the user
-application. SCSI commands like READ convey data back to the user application
-(if they succeed). The sg driver arranges for this data transfer from the
-device to the user space, if necessary.
-
-
-
-The description so far has concentrated on a disk device, but in reality
-the sg driver is not needed very often for disks because there already
-is a purpose built device driver for that: sd. The same is true of reading
-audio and data CDs (sr [[scd]) and tapes (st). However scanners that
-understand the SCSI command set and CDR "burning" programs tend to use
-the sg driver. Other applications include tape "robots" and music
-CD "ripping".
-
-
-
-To find out more about SCSI (draft) standards and resources visit
-www.t10.org.
-To use the sg device driver you should be familiar with the
-SCSI commands supported by the device that you wish to control.
-Getting hold of such information for devices like scanners can be
-quite challenging (if the vendor does not provide it).
-
-
-
-The first SCSI command sent to a SCSI device when it is initialized
-is an INQUIRY. All SCSI devices should respond promptly to an INQUIRY
-supplying information such as the vendor, product designation and revision.
-Appendix C shows the sg driver being used
-to send an INQUIRY and print out some of the information in the response.
-
-----
-!!!Chapter 3. Identifying the version of the SG driver
-
-Earlier versions of the sg device driver either have no version number
-(e.g. the original driver) or a version number starting with "2". The
-drivers that support this new interface have a major version number of
-"3". The sg version numbers are of the form "x.y.z" and the single number
-given by the SG_GET_VERSION_NUM ioctl() is calculated by
-(x * 10000 + y * 100 + z). The sg driver discussed
here will yield a
-number greater than or equal to 30000 from SG_GET_VERSION_NUM. The version
-number can also be seen using __cat /proc/scsi/sg/version__
-in the new driver. This document describes sg version 3.1.24 for the
-lk 2.4 series. Where some facility has been added during the lk 2.4
-series (e.g. mmap-ed IO) and hence is not available in all versions
-of the lk 2.4 series, this is noted.
-[[2]
-
-
-
-Here is a list of sg versions that have appeared to date during the
-lk 2.4 series.
-
-
-
-
-
-*
-
-lk 2.4.0 : sg version 3.1.17
-
-
-*
-*
-
-lk 2.4.7 : sg version 3.1.19 [[see include/scsi/sg.h in
-that or a later version for the changelog]
-
-
-*
-*
-
-lk 2.4.10 : sg version 3.1.20 [[This version had several changes put into
-it by third parties over the next 6 release kernel versions.]
-
-
-*
-*
-
-lk 2.4.17 : sg version 3.1.22
-
-
-*
-*
-
-lk 2.4.19 : sg version 3.1.24 [[lk 2.4.19 hasn't been released at the time
-of writing. It will most likely contains sg version 3.1.24 .]
-
-
-*
-
-----
-!!!Chapter 4. Interface
-
-This driver supports the following system calls, most of which are typical
-for a character device driver in Linux. They are:
-
-
-
-
-
-*
-
-open()
-
-
-*
-*
-
-close()
-
-
-*
-*
-
-write()
-
-
-*
-*
-
-read()
-
-
-*
-*
-
-ioctl()
-
-
-*
-*
-
-poll()
-
-
-*
-*
-
-fcntl(sg_fd, F_SETFL, oflags | FASYNC)
-
-
-*
-*
-
-mmap()
-
-
-*
-The interface to these calls as seem from Linux applications is well
-documented in the "man" pages (in section 2).
-
-
-
-A user application accesses the sg driver by using the open() system call
-on sg device file name. Each sg device file name corresponds to one
-(potentially) attached SCSI device. These are usually found in the
-/dev directory. Here are some sg device file names:
-
-$ ls -l /dev/sg[[01]
-crw-rw---- 1 root disk 21, 0 Aug 30 16:30 /dev/sg0
-crw-rw---- 1 root disk 21, 1 Aug 30 16:30 /dev/sg1
-The leading "c" at the front of the permissions indicates a character
-device. The absence of read or write permissions for "others" is prudent
-security. The major number of all sg device names is 21 while the minor
-number is the same as the number following "sg" in the device file name.
-When the device file system (devfs) is active on a system then the
-primarily sg device file names are found at the bottom of an informative
-subtree:
-
-$ cd /dev/scsi/host1/bus0/target0/lun0
-$ ls -l generic
-crw-r----- 1 root root 21, 1 Dec 31 1969 generic
-Under devfs (when its daemon [[devfsd] is running) there would usually
-be a symbolic link from /dev/sg1 to
-/dev/scsi/host1/bus0/target0/lun0/generic. This is
-so existing applications looking for the abridged device file name will
-not be surprised. One advantage of devfs is that only attached SCSI devices
-appear in the /dev/scsi subtree.
-
-
-
-A significant addition in sg v3 is an ioctl() called SG_IO which
-is functionally equivalent to a write() followed by a blocking read().
-In certain contexts the write()/read() combination have advantages over
-SG_IO (e.g. command queuing) and continue to be supported.
-
-
-
-The existing (and original) sg interface based on the sg_header structure
-is still available using a write()/read() sequence as before. The SG_IO
-ioctl will only accept the new interface based on the sg_io_hdr_t structure.
-
-
-
-The sg v3 driver thus has a write() call that can accept either the older
-sg_header structure or the new sg_io_hdr_t structure. The write() calls
-decides which interface is being used
-based on the second integer position of the passed header (i.e.
-sg_header::reply_len or sg_io_hdr_t::dxfer_direction). If it is a positive
-number then the old interface is assumed. If it is a negative number then
-the new interface is assumed. The direction constants placed in
-'dxfer_direction' in the new interface have been chosen to have negative
-values.
-
-
-
-If a request is sent to a write() with the sg_io_hdr_t interface then the
-corresponding read() that fetches the response must also use the sg_io_hdr_t
-interface. The same rule applies to the sg_header interface.
-
-
-
-This document concentrates on the sg_io_hdr_t interface introduced in
-the sg version 3 driver. For the definition of the older sg_header
-interface see the sg version 2 documentation. A brief description is
-given in Appendix B.
-
-----
-!!!Chapter 5. Theory of operation
-
-The path of a request through the sg driver can be broken into 3
-distinct stages:
-
-
-
-
-
-#
-
-The request is received from the user, resources are
-reserved as required (e.g. kernel buffer for indirect IO). If
-necessary, data in the user space is transferred into kernel buffers.
-Then the request is submitted to the SCSI mid level (and then onto
-the adapter) for
-execution. The SCSI mid level maintains a queue so the request may have
-to wait. If a SCSI device supports command queuing then it may be able to
-accommodate multiple outstanding requests.
-
-
-#
-#
-
- Assuming the SCSI adapter supports interrupts, then an interrupt
-is received when the request is completed. When this interrupt arrives
-the data transfer is complete. This means that if the SCSI command was
-a READ then the data is in kernel buffers (indirect IO) or in user
-buffers (direct or mmap-ed IO). The sg driver is informed of this
-interrupt via a kernel mechanism called a "bottom half" handler. Some
-kernel resources are freed up.
-
-
-#
-#
-
- The user makes a call to fetch the result of the request. If necessary,
-data in kernel buffers is transferred to the user space. If necessary,
-the sense buffer is written out to the user space. The remaining
-kernel resources associated with this request are freed up.
-
-
-#
-
-
-
-The write() call performs stage 1 while the read() call performs stage 3.
-If the read() call is made before stage 2 is complete then it will either
-wait or yield EAGAIN (depending on whether the file descriptor is blocking
-or not). If asynchronous notification is being used then stage 2 will send
-a SIGPOLL signal to the user process. The poll() system call will show this
-file descriptor is now readable (unless it was sent by the SG_IO ioctl()).
-
-
-
-The SG_IO ioctl() performs stage 1, waits for stage 2 and then
-performs stage 3. If the file descriptor in question is set O_NONBLOCK
-then SG_IO will ignore this and still block! Also a SG_IO call will not
-effect the poll() state nor cause a SIGPOLL signal to be sent. If you
-really want non-blocking operation (e.g. for command queuing) then don't
-use SG_IO; use the write() read() sequence instead.
-
-
-
-For more information about normal (or indirect), direct and mmap-ed IO
-see Chapter 9 .
-
-
-
-Currently the sg driver uses one Linux major device number (char 21)
-which in the lk 2.4 series limits it to handling 256 SCSI devices.
-Any attempt to attach more than this number will rejected with a
-message being sent to the console and the log file.
-[[3]
-
-----
-!!!Chapter 6. The sg_io_hdr_t structure in detail
-
-The main control structure for the version 3 SCSI generic driver has
-a struct tag name of "sg_io_hdr" and a typedef name of "sg_io_hdr_t".
-The structure is shown in abridged form below. The "[[i]" notation
-indicates an input value while "[[o]" indicates a value that is output.
-The "[[i-bo]" indicates a value that is conveyed from input to output and
-apart from one special case, is not used by the driver. The "[[i-bo]" members
-are meant to aid an application matching the request sent to a write()
-to the corresponding response received by a read(). For pointers the
-"[[*i]" indicates a pointer that is used for reading from user memory into
-the driver, "[[*o]" is a pointer used for writing, and "[[*io]" indicates
-a pointer used for either reading or writing.
-
-typedef struct sg_io_hdr
-{
-int interface_id; /* [[i] 'S' (required) */
-int dxfer_direction; /* [[i] */
-unsigned char cmd_len; /* [[i] */
-unsigned char mx_sb_len; /* [[i] */
-unsigned short iovec_count; /* [[i] */
-unsigned int dxfer_len; /* [[i] */
-void * dxferp; /* [[i], [[*io] */
-unsigned char * cmdp; /* [[i], [[*i] */
-unsigned char * sbp; /* [[i], [[*o] */
-unsigned int timeout; /* [[i] unit: millisecs */
-unsigned int flags; /* [[i] */
-int pack_id; /* [[i-bo] */
-void * usr_ptr; /* [[i-bo] */
-unsigned char status; /* [[o] */
-unsigned char masked_status;/* [[o] */
-unsigned char msg_status; /* [[o] */
-unsigned char sb_len_wr; /* [[o] */
-unsigned short host_status; /* [[o] */
-unsigned short driver_status;/* [[o] */
-int resid; /* [[o] */
-unsigned int duration; /* [[o] */
-unsigned int info; /* [[o] */
-} sg_io_hdr_t; /* 64 bytes long (on i386) */
-
-----
-!!!6.1. interface_id
-
- This must be set to 'S' (capital ess). If not, the ENOSYS error
-message is placed in errno. The idea is to allow interface variants
-in the future that identify themselves with a different value.
-[[The parallel port generic driver (pg) uses the letter 'P' to
-identify itself.]
-The type of interface_id is int.
-
-----
-!!!6.2. dxfer_direction
-
-The type of dxfer_direction is int.
-This is required to be one of the following:
-
-
-
-
-
-*
-
-SG_DXFER_NONE /* e.g. a SCSI Test Unit Ready command */
-
-
-*
-*
-
-SG_DXFER_TO_DEV /* e.g. a SCSI WRITE command */
-
-
-*
-*
-
-SG_DXFER_FROM_DEV /* e.g. a SCSI READ command */
-
-
-*
-*
-
-SG_DXFER_TO_FROM_DEV
-
-
-*
-*
-
-SG_DXFER_UNKNOWN
-
-
-*
-The value SG_DXFER_NONE should be used when there is no data transfer
-associated with a command (e.g. TEST UNIT READY). The value
-SG_DXFER_TO_DEV should be used when data is being moved from user
-memory towards the device (e.g. WRITE). The value SG_DXFER_FROM_DEV
-should be used when data is being moved from the device towards user
-memory (e.g. READ).
-
-
-
- The value SG_DXFER_TO_FROM_DEV is only relevant to indirect IO
-(otherwise it is treated like SG_DXFER_FROM_DEV). Data is moved from
-the user space to the kernel buffers. The command is then performed and
-most likely a READ-like command transfers data from the device into the
-kernel buffers. Finally the kernel buffers are copied back into the user
-space. This technique allows application writers to initialize the
-buffer and perhaps deduce the number of bytes actually read from the
-device (i.e. detect underrun). This is better done by using 'resid' if
-it is supported.
-
-
-
- The value SG_DXFER_UNKNOWN is for those (rare) situations where the
-data direction is not known. It may be useful for backward
-compatibility of existing applications when the relevant direction
-information is not available in the sg interface layer. There is
-a (minor) performance "hit" associated with choosing this
-option (e.g. on the PCI bus). Some recent pseudo device drivers (e.g.
-USB mass storage) may have problems handling this value (especially
-on vendor-specific SCSI commands).
-
-
-
- N.B. 'dxfer_direction' must have one of the five indicated values
-and cannot be uninitialized or zero.
-
-
-
- If 'dxfer_len' is zero then all values are treated like SG_DXFER_NONE.
-
-----
-!!!6.3. cmd_len
-
- This is the length in bytes of the SCSI command that 'cmdp' points
-to. As a SCSI command is expected an EMSGSIZE error number is
-produced if the value is less than 6 or greater than 16. Further,
-if the SCSI mid level has a further limit then EMSGSIZE is produced
-in this case as well.
-[[4]
-The type of cmd_len is unsigned char.
-
-----
-!!!6.4. mx_sb_len
-
- This is the maximum size that can be written back to the 'sbp' pointer
-when a sense_buffer is output which is usually in an error situation.
-The actual number written out is given by 'sb_len_wr'. In all cases
-'sb_len_wr' `= 'mx_sb_len' .
-The type of mx_sb_len is unsigned char.
-
-----
-!!!6.5. iovec_count
-
- This is the number of scatter gather elements in an array pointed
-to by 'dxferp'. If the value is zero then scatter gather (in the
-user space) is _not_ being used and 'dxferp' points to the data
-transfer buffer. If the value is greater than zero then each
-element of the array is assumed to be of the form:
-
- typedef struct sg_iovec
-{
-void * iov_base; /* starting address */
-size_t iov_len; /* length in bytes */
-} sg_iovec_t;
-Note that this structure has been named and defined in such a way to
-parallel "struct iovec" used by the readv() and writev() system calls
-in Linux. See "man 2 readv".
-
-
-
-Note that the scatter gather capability offered by 'iovec_count' is unrelated
-to the scatter gather capability (often associated with DMA) offered by
-most modern SCSI adapters. Furthermore iovec_count's variety of scatter
-gather (into the user space) is only available when normal (or "indirect")
-IO is being used. Hence when the SG_FLAG_DIRECT_IO or SG_FLAG_MMAP_IO are
-set in 'flags' then 'iovec_count' should be zero.
-
-
-
-The type of iovec_count is unsigned short.
-
-----
-!!!6.6. dxfer_len
-
- This is the number of bytes to be moved in the data transfer
-associated with the command. The direction of the transfer is indicated
-by 'dxfer_direction'. If 'dxfer_len' is zero then no data transfer
-takes place.
-[[5]
-
-
-
- If iovec_count is non-zero then 'dxfer_len' should be equal to the sum
-of iov_len lengths. If not, the minimum of the two is the transfer length.
-The type of dxfer_len is unsigned int.
-
-----
-!!!6.7. dxferp
-
- If 'iovec_count' is zero then this value is a pointer to user memory
-of at least 'dxfer_len' bytes in length. If there is a data transfer
-associated with the command then the data will be transferred to or
-from this user memory.
-If 'iovec_count' is greater than zero then this value points to a
-scatter-gather array in user memory. Each element of this array
-should be an object of type sg_iovec_t.
-Note that data is sometimes written to user memory (e.g. from a failed
-SCSI READ) even when an error has occurred.
-
-
-
-If mmap-ed IO is selected then the value in 'dxferp' is ignored and any
-data transfers will be to and from the address returned by the prior
-mmap() call.
-
-
-
-The type of dxferp is void * .
-
-----
-!!!6.8. cmdp
-
- This value points to the SCSI command to be executed. The command is
-assumed to be 'cmd_len' bytes long. If cmdp is NULL then the system
-call yields an EMSGSIZE error number. The user memory pointed to is
-only read (not written to).
-The type of cmdp is unsigned char * .
-
-----
-!!!6.9. sbp
-
- This value points to user memory of at least 'mx_sb_len' bytes length
-where the SCSI sense buffer will be output. Most successful commands
-do not output a sense buffer and this will be indicated by
-'sb_len_wr' being zero. Note that there are error conditions that
-don't result in a sense buffer be generated. The sense buffer results
-from the "auto-sense" mechanism in the SCSI mid-level driver. This
-mechanism detects a CHECK_CONDITION status and issues a REQUEST SENSE
-command and conveys its response back as the "sense buffer".
-The type of sbp is unsigned char * .
-
-----
-!!!6.10. timeout
-
-This value is used to timeout the given command. The units of this
-value are milliseconds. The time being measured is from when a command
-is sent until when sg is informed the request has been completed. A
-following read() can take as long as the user likes. Timeouts are best
-avoided, especially if SCSI bus resets will adversely effect other
-devices on that SCSI bus. When the timeout expires, the SCSI mid level
-attempts error recovery. Error recovery completes when the first action
-in the following list is successful. Note that a more extreme measure
-is being taken at each step.
-
-
-
-
-
-*
-
-
-the SCSI command that has timed out is aborted
-[[6]
-
-
-*
-*
-
-
-a SCSI device reset is attempted
-
-
-*
-*
-
-
-a SCSI bus reset is attempted. Note this may have an adverse effect on
-other devices sharing that SCSI bus.
-
-
-*
-*
-
-
-a SCSI host (bus adapter) reset is attempted. This is an attempt to
-re-initialize the adapter card associated with the SCSI device that
-has the timed out command.
-
-
-*
-If all these fail then the device may be set "offline" which means that
-it is no longer accessible (except by this driver when open()-ed
-O_NONBLOCK) until the machine is rebooted. Offline devices still
-appear in the __cat /proc/scsi/scsi__ listing. The
-last column of the __cat /proc/scsi/sg/devices__
-listing shows the online/offline status of a device ("1" means online
-while "" is offline). The exact status returned depends on which
-level of error recovery succeeded. Most likely the 'host_status' will be
-set to DID_ABORT or DID_RESET.
-
-
-
-The two error statuses containing the word "TIME(_)OUT" are typically
-_not_ related to a command timing out. DID_TIME_OUT in the 'host_status'
-usually means an (unexpected) device selection timeout. DRIVER_TIMEOUT in
-the 'driver_status' byte means the SCSI adapter is unable to control the
-devices on its SCSI bus (and has given up).
-
-
-
-The type of timeout is unsigned int (and it represents milliseconds).
-
-----
-!!!6.11. flags
-
- These are single or multi-bit values that can be "or-ed" together:
-
-
-
-
-
-*
-
- __SG_FLAG_DIRECT_IO__
-This is a request for direct IO on the data transfer. If it cannot
-be performed then the driver automatically performs indirect IO
-instead. If it is important to find out which type of IO was
-performed then check the values from the SG_INFO_DIRECT_IO_MASK in
-'info' when the request packet is completed (i.e. after read() or
-ioctl(,SG_IO,) ). The default action is to do indirect IO.
-
-
-*
-*
-
- __SG_FLAG_LUN_INHIBIT__
-The default action of the sg driver to overwrite internally the top
-3 bits of the second SCSI command byte with the LUN associated with
-the file descriptor's device. To inhibit this action set this flag.
-For SCSI 3 (or later) devices, this internal LUN overwrite does not
-occur.
-
-
-*
-*
-
- __SG_FLAG_MMAP_IO__
-When set the driver will attempt to procure the reserved buffer. If
-the reserved buffer is occupied (EBUSY) or too small (ENOMEM) then
-the operation (write() or ioctl(SG_IO)) fails. No data transfers occur
-between the dxferp pointer and the reserved buffer (dxferp is ignored).
-In order for a user application to access mmap-ed IO, it must have
-successfully executed an appropriate mmap() system call on this sg
-file descriptor. This precondition is not checked by write() or
-ioctl(SG_IO) when this flag is set. Setting this flag and
-SG_FLAG_DIRECT_IO results in a EINVAL error.
-
-
-*
-*
-
- __SG_FLAG_NO_DXFER__
-When set user space data transfers to or from the kernel buffers do
-not take place. This only has effect during indirect IO. This flag
-is for testing bus speed (e.g. the "sg_rbuf" utility uses it).
-
-
-*
-The type of flags is unsigned int.
-
-----
-!!!6.12. pack_id
-
- This value is not normally acted upon by the sg driver. It is provided
-so the user can identify the request. This is useful when command queuing
-is being used.
-The "abnormal" case is when SG_SET_FORCE_PACK_ID is set and a 'pack_id'
-other than -1 is given to read(). In this case the read() will wait
-to fetch a request that matches this 'pack_id'. If this mode is used
-be careful to set 'dxfer_direction' to a valid value (actually any of
-the SG_DXFER_* values will do) on input to the read(), together with
-the wanted pack_id.
-The type of pack_id is int.
-
-----
-!!!6.13. usr_ptr
-
- This value is not acted upon by the sg driver. It is meant to allow the
-user to associate some object with this request (e.g. to maintain state
-information).
-The type of usr_ptr is void * .
-
-----
-!!!6.14. status
-
- This is the SCSI status byte as defined by the SCSI standard. Note that
-it can have vendor information set in bits , 6 and 7 (although this
-is uncommon).
-Further note that this 'status' data does _not_ match the definitions in
-`scsi/scsi.hb (e.g. CHECK_CONDITION). The following 'masked_status'
-does match those definitions.
-[[7]
-The type of status is unsigned char .
-
-----
-!!!6.15. masked_status
-
- Logically: masked_status == ((status 8 0x3e) bb 1) .
-So 'masked_status' strips the vendor information bits off 'status'
-and then shifts it right one position. This makes it easier to do
-things like "if (CHECK_CONDITION == masked_status) ..." using the
-definitions in `scsi/scsi.hb. The defined values in this file are:
-
-
-
-
-
-*
-
- GOOD [[0x00]
-
-
-*
-*
-
- CHECK_CONDITION [[0x01]
-
-
-*
-*
-
- CONDITION_GOOD [[0x02]
-
-
-*
-*
-
- BUSY 0x04
-
-
-*
-*
-
- INTERMEDIATE_GOOD 0x08
-
-
-*
-*
-
- INTERMEDIATE_C_GOOD 0x0a
-
-
-*
-*
-
- RESERVATION_CONFLICT 0x0c
-
-
-*
-*
-
- COMMAND_TERMINATED 0x11
-
-
-*
-*
-
- QUEUE_FULL 0x14
-
-
-*
-N.B. 1 bit offset from usual SCSI status values
-
-
-
- Note that SCSI 3 defines some additional status codes.
-[[8]
-The type of masked_status is unsigned char .
-
-----
-!!!6.16. msg_status
-
- The messaging level in SCSI is under the command level and knowledge
-of what is happening at the messaging level is very rarely needed.
-Furthermore most modern chip-sets used in SCSI adapters completely
-hide this value. Nearly all adapters will return zero in 'msg_status'
-all the time.
-The type of msg_status is unsigned char .
-
-----
-!!!6.17. sb_len_wr
-
- This is the actual number of bytes written to the user memory pointed
-to by 'sbp'. 'sb_len_wr' is always `= 'mx_sb_len'. Linux 2.2 series
-kernels (and earlier) truncate this value to a maximum of 16 bytes.
-The actual number of bytes written will not exceed the length
-indicated by "Additional Sense Length" field (byte 7) of the Request
-Sense response.
-The type of sb_len_wr is unsigned char .
-
-----
-!!!6.18. host_status
-
- These codes potentially come from the firmware on a host adapter
-or from one of several hosts that an adapter driver controls.
-The 'host_status' field has the following values whose #defines mimic
-those which are only visible within the kernel (with the "SG_ERR_"
-removed from the front of each define). A copy of these defines can be
-found in sg_err.h (see Appendix A):
-
-
-
-
-
-*
-
- SG_ERR_DID_OK [[0x00] NO error
-
-
-*
-*
-
- SG_ERR_DID_NO_CONNECT [[0x01] Couldn't connect before timeout period
-
-
-*
-*
-
- SG_ERR_DID_BUS_BUSY [[0x02] BUS stayed busy through time out period
-
-
-*
-*
-
- SG_ERR_DID_TIME_OUT [[0x03] TIMED OUT for other reason (often this
-an unexpected device selection timeout)
-
-
-*
-*
-
- SG_ERR_DID_BAD_TARGET [[0x04] BAD target, device not responding?
-
-
-*
-*
-
- SG_ERR_DID_ABORT [[0x05] Told to abort for some other reason. From
-lk 2.4.15 the SCSI subsystem supports 16 byte commands however few
-adapter drivers do. Those HBA drivers that don't support 16 byte
-commands will yield this error code if a 16 byte command is passed to
-a SCSI device they control.
-
-
-*
-*
-
- SG_ERR_DID_PARITY [[0x06] Parity error. Older SCSI parallel buses
-have a parity bit for error detection. This probably indicates
-a cable or termination problem.
-
-
-*
-*
-
- SG_ERR_DID_ERROR [[0x07] Internal error detected in the host adapter.
-This may not be fatal (and the command may have succeeded). The
-aic7xxx and sym53c8xx adapter drivers sometimes report this for
-data underruns or overruns.
-[[9]
-
-
-*
-*
-
- SG_ERR_DID_RESET [[0x08] The SCSI bus (or this device) has been reset.
-Any SCSI device on a SCSI bus is capable of instigating a reset.
-
-
-*
-*
-
- SG_ERR_DID_BAD_INTR [[0x09] Got an interrupt we weren't expecting
-
-
-*
-*
-
- SG_ERR_DID_PASSTHROUGH [[0x0a] Force command past mid-layer
-
-
-*
-*
-
- SG_ERR_DID_SOFT_ERROR [[0x0b] The low level driver wants a retry
-
-
-*
-The type of host_status is unsigned short .
-
-----
-!!!6.19. driver_status
-
-One driver can potentially control several host adapters. For example
-Advansys provide one Linux adapter driver that controls all adapters made
-by that company - if 2 of more Advansys adapters are in 1 machine, then 1
-driver controls both.
-When ('driver_status' 8 SG_ERR_DRIVER_SENSE) is true the 'sense_buffer'
-is also output. The 'driver_status' field has the following values whose
-#defines mimic those which are only visible within the kernel (with the
-"SG_ERR_" removed from the front of each define). A copy of these defines
-can be found in sg_err.h (see the utilities section):
-
-
-
-
-
-*
-
- SG_ERR_DRIVER_OK [[0x00] Typically no suggestion
-
-
-*
-*
-
- SG_ERR_DRIVER_BUSY [[0x01]
-
-
-*
-*
-
- SG_ERR_DRIVER_SOFT [[0x02]
-
-
-*
-*
-
- SG_ERR_DRIVER_MEDIA [[0x03]
-
-
-*
-*
-
- SG_ERR_DRIVER_ERROR [[0x04]
-
-
-*
-*
-
- SG_ERR_DRIVER_INVALID [[0x05]
-
-
-*
-*
-
- SG_ERR_DRIVER_TIMEOUT [[0x06] Adapter driver is unable to control
-the SCSI bus to its is setting its devices offline (and giving up)
-
-
-*
-*
-
- SG_ERR_DRIVER_HARD [[0x07]
-
-
-*
-*
-
- SG_ERR_DRIVER_SENSE [[0x08] Implies sense_buffer output
-
-
-*
-*
-
- above status 'or'ed with one of the following suggestions
-
-
-*
-*
-
- SG_ERR_SUGGEST_RETRY [[0x10]
-
-
-*
-*
-
- SG_ERR_SUGGEST_ABORT [[0x20]
-
-
-*
-*
-
- SG_ERR_SUGGEST_REMAP [[0x30]
-
-
-*
-*
-
- SG_ERR_SUGGEST_DIE [[0x40]
-
-
-*
-*
-
- SG_ERR_SUGGEST_SENSE [[0x80]
-
-
-*
-The type of driver_status is unsigned short .
-
-----
-!!!6.20. resid
-
-This is the residual count from the data transfer. It is 'dxfer_len'
-less the number of bytes actually transferred. In practice it only
-reports underruns (i.e. positive number) as data overruns should
-never happen. This value will be zero if there was no underrun or
-the SCSI adapter doesn't support this feature.
-[[10]
-The type of resid is int .
-
-----
-!!!6.21. duration
-
- This value will be the number of milliseconds from when a SCSI
-command was sent until sg is informed that it is complete. For i386
-machines the granularity is 10ms while on alpha machines it is 1ms.
-This value is rounded toward zero.
-The type of duration is unsigned int .
-
-----
-!!!6.22. info
-
- This value is designed to convey useful information back to the user
-about the associated request. This information does not necessarily
-indicate an error. Several single bit and multi-bit fields are "or-ed"
-together to make this value.
-
-
-
- A single bit component contained in SG_INFO_OK_MASK indicates whether
-some error or status field is non-zero. If either 'masked_status',
-'host_status' or 'driver_status' are non-zero then SG_INFO_CHECK is
-set. The associated values are:
-
-
-
-
-
-*
-
- SG_INFO_OK_MASK [[0x1]
-
-
-*
-*
-
- SG_INFO_OK [[0x0] no sense, host nor driver "noise"
-
-
-*
-*
-
- SG_INFO_CHECK [[0x1] something abnormal happened. In most but not all
-cases, the sense buffer will be written. If the sense buffer has
-not been written than 'sb_len_wr' will be zero. This flag indicates
-either 'masked_status', 'host_status' or 'driver_status' is
-non-zero.
-
-
-*
-
-
-
- A multi bit component contained in SG_INFO_DIRECT_IO_MASK indicates
-what type of data transfer has just taken place. If indirect IO (or
-no data transfer) has taken place then SG_INFO_INDIRECT_IO is matched.
-Note that even if direct IO was requested in 'flags' the driver may
-choose to do indirect IO instead. If direct IO was requested and
-performed then SG_INFO_DIRECT_IO will be matched. Currently
-SG_INFO_MIXED_IO is never set. The associated values are:
-
-
-
-
-
-*
-
- SG_INFO_DIRECT_IO_MASK [[0x6]
-
-
-*
-*
-
- SG_INFO_INDIRECT_IO [[0x0] data xfer via kernel buffers (or no xfer)
-
-
-*
-*
-
- SG_INFO_DIRECT_IO [[0x2]
-
-
-*
-*
-
- SG_INFO_MIXED_IO [[0x4] part direct, part indirect IO
-
-
-*
-The type of info is unsigned int .
-
-----
-!!!Chapter 7. System calls
-
-System calls that can be used on sg devices are discussed in this chapter.
-The ioctl() system call is discussed in the following chapter [[ see
-Chapter 8 ].
-
-
-
-Successfully opening a sg device file name (e.g. /dev/sg0) establishes a link between a file descriptor and an attached
-SCSI device. The sg driver maintains state information and resources at
-both the SCSI device (e.g. exclusive lock) and the file descriptor (e.g.
-reserved buffer) levels.
-
-
-
-A SCSI device can be detached while an application has a sg file
-descriptor open. An example of this is a "hotplug" device such as a USB
-mass storage device that has just been unplugged. Most subsequent system
-calls that attempt to access the detached SCSI device will yield ENODEV.
-The close() call will complete silently while the poll() call will
-"or" in POLLHUP to its result. A subsequent attempt to open() that
-device name will yield ENODEV.
-
-----
-!!!7.1. open()
-
-__open(const char * filename, int flags). __The filename should be a sg device file name as discussed in the
-Chapter 4.
-Flags can be a number of the following or-ed together:
-
-
-
-
-
-*
-
-O_RDONLY restricts operations to read()s and ioctl()s
-(i.e. can't use write() ).
-
-
-*
-*
-
-O_RDWR permits all system calls to be executed.
-
-
-*
-*
-
-O_EXCL waits for other opens on the associated SCSI device to be closed
-before proceeding. If O_NONBLOCK is set then yields EBUSY when someone
-else has the SCSI device open. The combination of O_RDONLY and O_EXCL is
-disallowed.
-
-
-*
-*
-
-O_NONBLOCK Sets non-blocking mode. Calls that would otherwise block
-yield EAGAIN (e.g. read() ) or EBUSY (e.g. open() ). This flag is ignored by
-ioctl(SG_IO) .
-
-
-*
-Either O_RDONLY or O_RDWR must be set in flag. Either of the other 2
-flags (but not both) can be or-ed in.
-
-
-
-Note that multiple file descriptors may be open to the same SCSI device.
-[[This is a way of side stepping the SG_MAX_QUEUE limit.] At the sg level
-separate state information is maintained. This means that even if
-multiple file descriptors are open to a single SCSI device their
-write() read() sequences are essentially independent.
-
-
-
-Open() calls may be blocked due to exclusive locks (i.e. O_EXCL). An
-exclusive lock applies to a single SCSI device and only to sg's
-use of that device (i.e. it has no effect on access via sd, sr or st
-to that device). If the O_NONBLOCK flag is used then open() calls
-that would have otherwise blocked, yield EBUSY. Applications that
-scan sg devices trying to determine their identity (e.g. whether
-one is a scanner) should use the O_NONBLOCK flag otherwise they
-run the risk of blocking.
-
-
-
-The driver will attempt to reserve SG_DEF_RESERVED_SIZE bytes (32KBytes in
-the current sg.h) on open(). The size of this reserved buffer can
-subsequently be modified with the SG_SET_RESERVED_SIZE ioctl(). In both
-cases these are requests subject to various dynamic constraints. The actual
-amount of memory obtained can be found by the SG_GET_RESERVED_SIZE ioctl().
-The reserved buffer will be used if:
-
-
-
-
-
-*
-
-it is not already in use (e.g. when command queuing is in use)
-
-
-*
-*
-
-a write() or ioctl(SG_IO) requests a data transfer size that is less
-than or equal to the reserved buffer size.
-
-
-*
-
-
-
-Returns a file descriptor if b= 0 , otherwise -1 implies an error.
-
-----
-!!!7.2. write()
-
-__write(int sg_fd, const void * buffer, size_t count). __The action of write() with a control block based on struct sg_header is
-discussed in the earlier document:
-www.torque.net/sg/p/scsi-generic.txt
-(i.e the sg version 2 documentation). This section describes the action of
-write() when it is given a control block based on struct sg_io_hdr.
-
-
-
-The 'buffer' should point to an object of type sg_io_hdr_t and 'count'
-should be sizeof(sg_io_hdr_t) [[it can be larger but the excess is ignored].
-If the write() call succeeds then the 'count' is returned as the result.
-
-
-
-Up to SG_MAX_QUEUE (16) write()s can be queued up before any finished
-requests are completed by read(). An attempt to queue more than that will
-result in an EDOM error.
-[[11]
-The write() command should return more or
-less immediately.
-[[12]
-
-
-
-The version 2 sg driver defaulted the maximum queue length to 1 (and
-made available the SG_SET_COMMAND_Q ioctl() to switch it to SG_MAX_QUEUE).
-So for backward compatibility a file descriptor that only receives
-sg_header structures in its write() will have a default "max" queue length
-of 1. As soon as a sg_io_hdr_t structure is seen by a write() then the
-maximum queue length is switched to SG_MAX_QUEUE on that file descriptor.
-
-
-
-The "const" on the 'buffer' pointer is respected by the sg driver. Data
-is read in from the sg_io_hdr object that is pointed to. Significantly
-this is when the 'sbp' and the 'dxferp' are recorded internally (i.e.
-not from the sg_io_hdr object given to the corresponding read() ).
-
-----
-!!!7.3. read()
-
-__read(int sg_fd, void * buffer, size_t count). __The action of read() with a control block based on struct sg_header is
-discussed in the earlier document:
-www.torque.net/sg/p/scsi-generic.txt
-(i.e. the sg version 2 documentation). This section describes the action of
-read() when it is given a control block based on struct sg_io_hdr.
-
-
-
-The 'buffer' should point to an object of type sg_io_hdr_t and 'count'
-should be sizeof(sg_io_hdr_t) [[it can be larger but the excess is ignored].
-If the read() call succeeds then the 'count' is returned as the result.
-
-
-
-By default, read() will return the oldest completed request that is
-queued up. A read() will not interfere with any request associated
-with the SG_IO ioctl() on this file descriptor except in a special
-case when a SG_IO ioctl() is interrupted by a signal.
-
-
-
-If the SG_SET_FORCE_PACK_ID,1 ioctl() is active then read() will attempt
-to fetch the packet whose pack_id (given earlier to write()) matches the
-sg_io_hdr_t::pack_id given to this read(). If not available it will either
-wait or yield EAGAIN. As a special case, -1 in sg_io_hdr_t::pack_id given
-to read() will match the request whose response has been waiting for
-the longest time. Take care to also set 'dxfer_direction' to any valid
-value (e.g. SG_DXFER_NONE) when in this mode. The 'interface_id' member
-should also be set appropriately.
-
-
-
-Apart from the SG_SET_FORCE_PACK_ID case (and then only for the 3 indicated
-fields), the sg_io_hdr_t object given to read() can be uninitialized. Note
-that the 'sbp' pointer value for optionally outputting a sense buffer was
-recorded from the earlier, corresponding write().
-
-----
-!!!7.4. poll()
-
-__poll(struct pollfd *ufds, unsigned int nfds,
-int timeout). __This call can be used to check the state of a sg file descriptor. It
-will always respond immediately. Typical usages are to periodically
-poll the state of a sg file descriptor and to determine why a SIG_IO
-signal was received.
-
-
-
-For file descriptors associated with sg devices:
-
-
-
-
-
-*
-
- POLLIN one or more responses is awaiting a read()
-
-
-*
-*
-
- POLLOUT command can be sent to write() without causing an EDOM
-error (i.e. sufficient space on sg's queues)
-
-
-*
-*
-
- POLLHUP SCSI device has been detached, awaiting cleanup
-
-
-*
-*
-
- POLLERR internal structures are inconsistent
-
-
-*
-
-
-
-POLLOUT indicates the sg will not block a new write() or SG_IO ioctl().
-However it is still possible (but unlikely) that the mid level or an
-adapter may block (or yield EAGAIN).
-
-----
-!!!7.5. close()
-
-__close(int sg_fd). __Preferably a close() should be done after all issued write()s have had
-their corresponding read() calls completed. Unfortunately this is not
-always possible (e.g. the user may choose to send a kill signal to a
-running process). The sg driver implements "fast" close semantics and thus
-will return more or less immediately (i.e. not wait on any event). This
-is application friendly but requires the sg driver to arrange for an
-orderly cleanup of those packets that are still "in flight".
-
-
-
-When close() leaves outstanding SCSI commands still awaiting responses,
-the sg driver maintains its internal structures for the now defunct
-file descriptor. These internal structures are maintained until all
-outstanding responses (some might be timeouts) are received. When the
-sg driver is loaded as a module and has any open file descriptors or
-"defunct" file descriptors then it cannot be unloaded. An attempt to
-call __rmmod sg__ will report the driver is busy. Defunct
-file descriptors that remain for some time, perhaps awaiting a timeout,
-can be observed with the __cat /proc/scsi/sg/debug__
-command. In this case "closed=1" will be set on the defunct file descriptor
-[[see Section 11.1]. Defunct file descriptors do not impede
-attempts by applications to open() new file descriptors on the same SCSI
-device.
-
-
-
-The kernel arranges for only the last close() on a file descriptor to be
-seen by a driver (and to emphasize this, the corresponding sg driver call
-is named sg_release() rather than sg_close()). This is only significant when
-an application uses fork() or dup().
-
-
-
-Returns 0 if successful, otherwise -1 implies an error.
-
-----
-!!!7.6. mmap()
-
-__mmap(void * start, size_t length, int prot,
-int flags, int sg_fd, off_t offset). __This system call returns a pointer to the beginning of the reserved buffer
-associated with the sg file descriptor 'sg_fd'. The 'start' argument is
-a hint to the kernel and is ignored by this driver; best set it to .
-The 'length' argument should be less than or equal to the size of the
-reserved buffer associated with 'sg_fd'. If it exceeds the
-reserved buffer size (after 'length' has been rounded up to a page size
-multiple) then MAP_FAILED is returned and ENOMEM is placed in errno. The
-'prot' argument should either be PROT_READ or (PROT_READ | PROT_WRITE).
-The 'flags' argument should contain MAP_SHARED. In a sense, the user
-application is "sharing" data with the sg driver. The MAP_PRIVATE flag
-does not play well with compiler optimization flags such as '-O2'.
-The 'offset' argument must be set to 0 (or NULL).
-
-
-
-The mmap() system call can be made multiple times on the same sg_fd.
-The munmap() system call is not required if close() is called on
-sg_fd. Mmap-ed IO is well-behaved when a process is fork()-ed
-(or the equivalent finer grained clone() system call is made).
-In the case of a fork(), 2 processes will be sharing the same memory mapped
-area together with the sg driver for a sg_fd and the last one to
-close the sg_fd (or exit) will cause the shared memory to be freed.
-
-
-
-It is assumed that if the default reserved buffer size of 32 KB is
-not sufficient then a ioctl(SG_SET_RESERVED_SIZE) call is made
-prior to any calls to mmap(). If the required size is not a multiple
-of the kernel's page size (returned by getpagesize() system call) then
-the size passed to ioctl(SG_SET_RESERVED_SIZE) should be rounded up
-to the next page size multiple.
-
-
-
-Mmap-ed IO is requested by setting (or or-ing in) the SG_FLAG_MMAP_IO
-constant into the flag member of the the sg_io_hdr structure prior to
-a call to write() or ioctl(SG_IO). The logic to do mmap-ed IO _assumes_
-that an appropriate mmap() call has been made by the application. In
-other words it does not check.
-[[13]
-
-----
-!!!7.7. fcntl(sg_fd, F_SETFL, oflags | FASYNC)
-
-__fcntl(int sg_fd, int cmd, long arg). __There are several uses for this system call in association with a sg
-file descriptor. The following pseudo code shows code that is useful for
-scanning the sg devices, taking care not to be caught in a wait for
-an O_EXCL lock by another process, and when the appropriate device is
-found, switching to normal blocked io. A working example of this logic
-is in the sg_scan utility program.
-
-open("/dev/sg0", O_RDONLY | O_NONBLOCK)
-/* check device, EBUSY means some other process has O_EXCL lock on it */
-/* when the device you want is found then ... */
-flags = fcntl(sg_fd, F_GETFL)
-fcntl(sg_fd, F_SETFL, flags 8 (~ O_NONBLOCK))
-/* since, with simple apps, it is easier to use normal blocked io */
-
-
-
-The sg driver supports asynchronous notification.
-This is a non-blocking mode of operation in which, when the driver receives
-data back from a device so that a read() can be done, it sends a SIGPOLL
-(aka SIGIO) signal to the owning process. Here is a code snippet from
-the sg_poll test program.
-
-sigemptyset(8sig_set)
-sigaddset(8sig_set, SIGPOLL)
-sigaction(SIGPOLL, 8s_action, )
-fcntl(sg_fd, F_SETOWN, getpid())
-flags = fcntl(sg_fd, F_GETFL);
-fcntl(sg_fd, F_SETFL, flags | O_ASYNC)
-
-----
-!!!7.8. Errors reported in errno
-
-With the original interface almost any string could be accidentally
-given to write() and potentially (but rarely) something nasty could happen.
-If some error was detected then more than likely EIO was placed in errno.
-
-
-
-Unfortunately this can still happen with write() since it can accept
-both the original struct sg_header or the newer sg_io_hdr_t described in
-this note. However since the SG_IO ioctl() will only accept the sg_io_hdr_t
-structure there is less chance of a random string being interpreted
-as a command. Since the sg_io_hdr_t interface does a lot more error
-checking, it attempts to give out more precise errno values to help
-the user pinpoint the problem. [[Admittedly some of these errno values
-are picked in an arbitrary way from the large set of available values.]
-
-
-
-In most cases when a system call on a sg file descriptor fails, the call
-in question will return -1. After an application detects that a system
-call has failed it should read the value in the "errno" variable (prior
-to do any more system calls). Applications should include the `errno.hb
-header.
-
-
-
-Below is a table of errno values indicating which calls to sg will
-generate them and the meaning of the error. A write() call is indicated by
-"w", a read() call by "r" and an open() call by "o".
-
-
-
-
-errno which_calls Meaning
------ ----------- ----------------------------------------------
-EACCES `some ioctlsb Root permission (more precisely CAP_SYS_ADMIN
-or CAP_SYS_RAWIO) required. Also may occur during
-an attempted write to /proc/scsi/sg files.
-EAGAIN r The file descriptor is non-blocking and the request
-has not been completed yet.
-EAGAIN w,SG_IO SCSI sub-system has (temporarily) run out of
-command blocks.
-EBADF w File descriptor was not open()ed O_RDWR.
-EBUSY o Someone else has an O_EXCL lock on this device.
-EBUSY w With mmap-ed IO, the reserved buffer already in use.
-EBUSY `some ioctlsb Attempt to change something (e.g. reserved buffer
-size) when the resource was in use.
-EDOM w,SG_IO Too many requests queued against this file
-descriptor. Limit is SG_MAX_QUEUE active requests.
-If sg_header interface is being used then the
-default queue depth is 1. Use SG_SET_COMMAND_Q
-ioctl() to increase it.
-EFAULT w,r,SG_IO Pointer to user space invalid.
-`most ioctlsb
-EINVAL w,r Size given as 3rd argument not large enough for the
-sg_io_hdr_t structure. Both direct and mmap-ed IO
-selected.
-EIO w Size given as 3rd argument less than size of old
-header structure (sg_header). Additionally a write()
-with the old header will yield this error for most
-detected malformed requests.
-EIO r A read() with the older sg_header structure yields
-this value for some errors that it detects.
-EINTR o While waiting for the O_EXCL lock to clear this call
-was interrupted by a signal.
-EINTR r,SG_IO While waiting for the request to finish this call
-was interrupted by a signal.
-EINTR w [[Very unlikely] While waiting for an internal SCSI
-resource this call was interrupted by a signal.
-EMSGSIZE w,SG_IO SCSI command size ('cmd_len') was too small
-(i.e. ` 6) or too large
-ENODEV o Tried to open() a file with no associated device.
-[[Perhaps sg has not been built into the kernel or
-is not available as a module?]
-ENODEV o,w,r,SG_IO SCSI device has detached, awaiting cleanup.
-User should close fd. Poll() will yield POLLHUP.
-ENOENT o Given filename not found.
-ENOMEM o [[Very unlikely] Kernel was not even able to find
-enough memory for this file descriptor's context.
-ENOMEM w,SG_IO Kernel unable to find memory for internal buffers.
-This is usually associated with indirect IO.
-For mmap-ed IO 'dxfer_len' greater than reserved
-buffer size.
-Lower level (adapter) driver does not support enough
-scatter gather elements for requested data transfer.
-ENOSYS w,SG_IO 'interface_id' of a sg_io_hdr_t object was _not_ 'S'.
-ENXIO o "remove-single-device" may have removed this device.
-ENXIO o, w,r,SG_IO Internal error (including SCSI sub-system busy doing
-error processing - e.g. SCSI bus reset). When a
-SCSI device is offline, this is the response. This
-can be bypassed by opening O_NONBLOCK.
-EPERM o Can't use O_EXCL when open()ing with O_RDONLY
-EPERM w,SG_IO File descriptor open()-ed O_RDONLY but O_RDWR
-`some ioctlsb access mode needed for this operation.
-
-----
-!!!Chapter 8. Ioctl()s
-
-The Linux SCSI upper level drivers, including sg, have a "trickle down"
-ioctl() architecture. This means that ioctl()s whose request value (i.e.
-the second argument) is not understood by the upper level driver, are
-passed down to the SCSI mid-level. Those ioctl()s that are not understood
-by the mid level driver are passed down to the lower level (adapter)
-driver. If none of the 3 levels understands the ioctl() request value
-then -1 is returned and EINVAL is placed in errno. By convention the
-beginning of the request value's symbolic name indicates which level
-will respond to the ioctl(). For example, request values starting with
-"SG_" are processed by the sg driver while those starting with "SCSI_"
-are processed by the mid level.
-
-
-
-Most of the sg ioctl()s read or write information via a pointer given
-as the third argument to the ioctl() call and return 0 on success. A few
-of the older ioctl()s that get a value from the driver return that value
-as the result of the ioctl() call (e.g. ioctl(SG_GET_TIMEOUT) ).
-
-
-
-All sg driver ioctl()s are listed below. They all start with "SG_". They
-are followed by several interesting SCSI mid level ioctl()s which start
-with "SCSI_IOCTL_". The sg ioctl()s are roughly in alphabetical order
-(with _SET_, _GET_ and _FORCE_ ignored). Since ioctl(SG_IO) is a
-complete SCSI command request/response sequence then it is listed first.
-
-----
-!!!8.1. SG_IO
-
-__SG_IO 0x2285. __The idea is deceptively simple: just hand a sg_io_hdr_t object to an
-ioctl() and it will return when the SCSI command is finished. It is
-logically equivalent to doing a write() followed by a blocking read().
-The word "blocking" here implies the read() will wait until the SCSI
-command is complete.
-
-
-
-The same file descriptor can be used both for SG_IO synchronous calls and
-the write() read() sequences at the same time. The sg driver makes sure that
-the response to a SG_IO call will never accidentally be fetched by a read().
-Even though a single file descriptor can be shared in this manner, it is
-probably more sensible (and results in cleaner code) if separate file
-descriptors to the same SCSI device are used in this case.
-
-
-
-It is possible that the wait for the command completion is
-interrupted by a signal. In this case the SG_IO call will yield an
-EINTR error. This is reasonably complex to handle and is discussed
-in the ioctl(SG_SET_KEEP_ORPHAN) description below.
-The following SCSI commands will be permitted by SG_IO when the sg
-file descriptor was opened O_RDONLY:
-
-
-
-
-
-*
-
-TEST UNIT READY
-
-
-*
-*
-
-REQUEST SENSE
-
-
-*
-*
-
-INQUIRY
-
-
-*
-*
-
-READ CAPACITY
-
-
-*
-*
-
-READ BUFFER
-
-
-*
-*
-
-READ(6) (10) and (12)
-
-
-*
-*
-
-MODE SENSE(6) and (10)
-
-
-*
-*
-
-LOG SENSE
-
-
-*
-All commands to SCSI device type SCANNER are accepted. Other cases
-yield an EPERM error. Note
-that the write() read() interface must have the sg file descriptor
-open()-ed with O_RDWR as write permission is required by Linux to execute
-a write() system call.
-
-
-
-The ability of the SG_IO ioctl() to issue certain SCSI commands has led
-to some relaxation on file descriptors open()ed "read-only" compared with
-the version 2 sg driver. The open() call will now attempt to allocate a
-reserved buffer for all newly opened file descriptors. The
-ioctl(SG_SET_RESERVED_SIZE) will now work on "read-only" file descriptors.
-
-----
-!!!8.2. SG_GET_ACCESS_COUNT
-
-__SG_GET_ACCESS_COUNT 0x2289. __This ioctl() yields the access count maintained by the mid level
-for this SCSI device. This number is incremented by each open()
-call done by the upper level SCSI drivers (i.e. sd, sr, st and sg)
-and decremented by those drivers' release(). [[A driver's release()
-corresponds to the last close() on a file descriptor, or is
-supplied by the kernel when a process is aborted.] Each SCSI
-device has a separate access count.
-
-----
-!!!8.3. SG_SET_COMMAND_Q (and _GET_)
-
-__SG_SET_COMMAND_Q 0x2271 [[_GET_ 0x2270] . __The default it the original sg driver was not to allow commands to be
-queued on the same file descriptor (actually it was more restrictive,
-commands could not be queued on a SCSI device). The version 2 sg driver
-kept this action as its default (for backward compatibility) and offered
-these ioctl()s to change and monitor the command queuing state.
-
-----
-!!!8.4. SG_SET_DEBUG
-
-__SG_SET_DEBUG 0x227e. __The third argument is assumed to point to an int. The default value is .
-If this call is made pointing to an int greater than 0 then any SCSI
-request that is issued that results in the SCSI status of CHECK_CONDITION
-(or COMMAND_TERMINATED) will cause a message to be sent to the log (and
-perhaps the console). The message is information derived from the sense
-buffer (i.e. the SCSI error message) and it is prefixed with
-"sg_cmd_done_bh".
-
-
-
-The other actions of debug mode performed in version 2 of the sg driver
-have been removed as they are no longer needed. The internal state
-of the sg driver can now be found by viewing the output of
-__cat /proc/scsi/sg/debug__.
-
-----
-!!!8.5. SG_EMULATED_HOST
-
-__SG_EMULATED_HOST 0x2203. __Assumes 3rd argument points to an int and outputs a flag indicating
-whether the host (adapter) is connected to a "real" SCSI bus or is an
-emulated one (e.g. ide-scsi or usb storage device driver). A value of 1
-means emulated while 0 is not. [[To check: is IEEE1394 a "real" SCSI serial
-bus?]
-
-----
-!!!8.6. SG_SET_KEEP_ORPHAN (and _GET_)
-
-__SG_SET_KEEP_ORPHAN 0x2287 [[_GET_ 0x2288]. __These ioctl()s allow the setting and reading of the "keep_orphan"
-flag. This controls what happens to the request associated with a
-SG_IO ioctl() that is interrupted (i.e. errno is EINTR). The
-default action is to drop the response as soon as it is received.
-This corresponds to the "keep_orphan" flag being . When the
-"keep_orphan" flag is 1 then the response is transformed in such
-a way that it can be fetched by a read(). This is the only
-circumstance in which a request sent by a SG_IO ioctl() can have
-the associated response fetched by a read().
-
-----
-!!!8.7. SG_SET_FORCE_LOW_DMA
-
-__SG_SET_FORCE_LOW_DMA 0x2279. __Assumes 3rd argument points to an int containing 0 or 1. 0 (default)
-means sg decides whether to use memory above 16 Mbyte level (on i386)
-based on the host adapter being used by this SCSI device. Typically
-PCI SCSI adapters will indicate they can DMA to the whole 32 bit address
-space.
-If 1 is given then the host adapter is overridden and only memory below
-the 16MB level is used for DMA. A requirement for this should be
-extremely rare. If the "reserved" buffer allocated on open() is not in
-use then it will be de-allocated and re-allocated under the 16MB level
-(and the latter operation could fail yielding ENOMEM).
-Only the current file descriptor is affected.
-
-----
-!!!8.8. SG_GET_LOW_DMA
-
-__SG_GET_LOW_DMA 0x227a. __Assumes 3rd argument points to an int and places 0 or 1 in it.
-indicates the whole 32 bit address space is being used for DMA transfers
-on this file descriptor. 1 indicates the memory below the 16MB level
-(on i386) is being used (and this may be the case because the host
-adapters setting has been overridden by SG_SET_FORCE_LOW_DMA,1 .
-
-----
-!!!8.9. SG_NEXT_CMD_LEN
-
-__SG_NEXT_CMD_LEN 0x2283. __This ioctl() is not required with sg_io_hdr structure since command length
-is set explicitly for every command.
-Assumes 3rd argument is pointing to an int. The value of the int (if b )
-will be used as the SCSI command length of the next SCSI command sent to
-a write() using the sg_header interface. After that write() the SCSI
-command length logic is reset to use automatic length detection (i.e.
-depending on SCSI command group and the 'twelve_byte' field). If the
-current SCSI command length maximum of 16 is exceeded then the affected
-write() will yield an EDOM error. Giving this ioctl() a value of 0 will
-set automatic length detection for the next write(). N.B. Only the following
-write() on this fd is affected by this ioctl().
-
-----
-!!!8.10. SG_GET_NUM_WAITING
-
-__SG_GET_NUM_WAITING 0x227d. __Assumes 3rd argument points to an int and places the number of packets
-waiting to be read in it. Only those requests that have been issued by
-a write() and are now available to be read() are counted. In other words
-any ioctl(SG_IO) operations underway on this file descriptor will not
-effect this count
-[[14].
-
-----
-!!!8.11. SG_SET_FORCE_PACK_ID
-
-__SG_SET_FORCE_PACK_ID 0x227b. __Assumes 3rd argument is pointing to an int. 0 (default) instructs read()
-to return the oldest (written) packet if multiple packets are
-waiting to be read. 1 instructs read() to view the sg_io_hdr::pack_id
-(or sg_header::pack_id) as input and return the oldest packet matching
-that pack_id or wait until it arrives. If the file descriptor is in
-O_NONBLOCK state, rather than wait this ioctl() will yield EAGAIN.
-As a special case the pack_id of -1
-given to read() in the mode will match the oldest packet.
-Only the current file descriptor is affected by this command.
-
-----
-!!!8.12. SG_GET_PACK_ID
-
-__SG_GET_PACK_ID 0x227c. __Assumes 3rd argument points to an int and places the pack_id of the
-oldest (written) packet in it. If no packet is waiting to be read then
-yields -1.
-
-----
-!!!8.13. SG_GET_REQUEST_TABLE
-
-__SG_GET_REQUEST_TABLE 0x2286. __This ioctl outputs an array of information about the status of
-requests associated with the current file descriptor. Its 3rd
-argument should point to memory large enough to receive SG_MAX_QUEUE
-objects of the sg_req_info_t structure. This structure has the
-following members:
-
- req_state
-0 -b request not in use
-1 -b request has been sent, but is not finished (i.e. it is
-between stages 1 and 2 in the "theory of operation")
-2 -b request is ready to be read() (i.e. it is between stages
-2 and 3 in the "theory of operation")
-orphan
-0 -b normal request
-1 -b request sent by SG_IO ioctl() which has been interrupted
-by a signal
-sg_io_owned
-0 -b request sent by a write()
-1 -b request sent by a SG_IO ioctl()
-problem
-0 -b no problem (or 1 == req_state)
-1 -b req_state is 2 and either masked_status, host_status or
-driver_status is non-zero
-duration
-[[if 1 == req_state] time since request was sent (in millisecs)
-[[if 2 == req_state] duration of request (in millisecs). Clock
-is stopped when stage 2 in "theory of operation" is
-reached
-pack_id
-usr_ptr
-these are user provided values in the sg_io_hdr_t (or
-struct sg_header) that sent the request
-
-----
-!!!8.14. SG_SET_RESERVED_SIZE (and _GET_ )
-
-__SG_SET_RESERVED_SIZE 0x2275 [[_GET_ 0x2272]. __Both ioctl()s assume the 3rd argument is pointing to an int.
-
-
-
-For ioctl(SG_SET_RESERVED_SIZE) the value will be used to
-request a new reserved buffer of that size. The previous reserved buffer
-is freed (if it is not in use; if it was in use then the ioctl() fails and
-EBUSY is placed in errno).
-A new reserved buffer is then allocated and its actual size can be found by
-calling the ioctl(SG_GET_RESERVED_SIZE). The reserved buffer is then used
-for DMA purposes by subsequent write() and ioctl(SG_IO) commands if it is
-not already in use and if the write() is not calling for a buffer size
-larger than that reserved. The reserved buffer may well be a series of
-kernel buffers if the adapter supports scatter-gather. Large buffers can
-be requested (e.g. 4 MB) but not necessarily granted. Once a mmap() call
-has been made on a sg file descriptor, subsequent calls to this ioctl()
-will fail with EBUSY placed in errno.
-
-
-
-In the case of ioctl(SG_GET_RESERVED_SIZE) the size in bytes of
-the reserved buffer from open() or the most recent SG_SET_RESERVED_SIZE
-ioctl() call on this fd. The result can be 0 if memory is very tight. In
-this case it may not be wise to attempt something like burning a CD on
-this file descriptor.
-
-----
-!!!8.15. SG_SCSI_RESET
-
-__SG_SCSI_RESET 0x2284. __Assumes 3rd argument points to an int. That int should be one of the
-following defined in the sg.h header:
-
-
-
-
-
-*
-
-SG_SCSI_RESET_NOTHING (0x0): can be used to poll the device
-after a reset has been issued to see if it has returned to the normal
-state. If it is still being reset or it is offline then EBUSY will be
-placed in errno,
-
-
-*
-*
-
-SG_SCSI_RESET_DEVICE (0x1): issues a reset to the SCSI
-device associated with the current sg file descriptor,
-
-
-*
-*
-
-SG_SCSI_RESET_BUS (0x2): issues a reset to the SCSI
-bus that contains the device associated with the current sg file descriptor.
-This will usually have an adverse effect on any other SCSI device sharing
-this SCSI bus, especially if it was in the middle of an operation,
-
-
-*
-*
-
-SG_SCSI_RESET_HOST (0x3): issues a reset to the host that
-controls the SCSI bus that contains the device associated with the
-current sg file descriptor. This operation can have an adverse effect
-on any SCSI device that is connected to this host.
-
-
-*
-The reset options are in ascending order of severity. Not all levels
-are supported by all linux lower level drivers. Most lower level
-(adapter) drivers support the SCSI bus reset. These boards often issue
-a SCSI bus reset during their initialization.
-
-
-
-Unfortunately this ioctl() doesn't currently do much (but may in the
-future after other issues are resolved). Yields an EBUSY error if the
-SCSI bus or the associated device is being reset when this ioctl() is
-called, otherwise returns .
-N.B. In some recent distributions there is a patch to the SCSI mid level
-code that activates this ioctl. Check your distribution.
-
-----
-!!!8.16. SG_GET_SCSI_ID
-
-__SG_GET_SCSI_ID 0x2276. __Assumes 3rd argument is pointing to an object of type Sg_scsi_id (see
-sg.h) and populates it. That structure contains ints for host_no,
-channel, scsi_id, lun, scsi_type, allowable commands per lun and
-queue_depth. Most of this information is available from other sources
-(e.g. SCSI_IOCTL_GET_IDLUN and SCSI_IOCTL_GET_BUS_NUMBER) but tends to be
-awkward to collect.
-Allowable commands per lun and queue_depth give an insight to the
-command queuing capabilities of the adapters and the device. The latter
-overrides the former (logically) and the former is only of interest
-if it is equal to queue_depth which probably indicates the device
-does not support queuing commands (e.g. most scanners).
-
-typedef struct sg_scsi_id { /* used by SG_GET_SCSI_ID ioctl() */
-int host_no; /* as in "scsi`nb" where 'n' is one of , 1, 2 etc */
-int channel;
-int scsi_id; /* scsi id of target device */
-int lun;
-int scsi_type; /* TYPE_... defined in scsi/scsi.h */
-short h_cmd_per_lun;/* host (adapter) maximum commands per lun */
-short d_queue_depth;/* device (or adapter) maximum queue length */
-int unused[[2]; /* probably find a good use, set 0 for now */
-} sg_scsi_id_t;
-
-----
-!!!8.17. SG_GET_SG_TABLESIZE
-
-__SG_GET_SG_TABLESIZE 0x227F. __Assumes 3rd argument points to an int and places the maximum number of
-scatter gather elements supported by the host adapter associated with
-the current SCSI device. 0 indicates that the adapter does support scatter
-gather.
-
-----
-!!!8.18. SG_GET_TIMEOUT
-
-__SG_GET_TIMEOUT 0x2202. __Ignores its 3rd argument and _returns_ the timeout value (which will be
-b= 0 ). The unit of this timeout is "jiffies" which are currently 10
-millisecond intervals on i386 (less on an alpha). Linux supplies
-a manifest constant HZ which is the number of "jiffies" in 1 second.
-This ioctl() is not relevant to the sg version 3 driver because timeouts
-are specified explicitly for each command in the sg_io_hdr structure.
-
-----
-!!!8.19. SG_SET_TIMEOUT
-
-__SG_SET_TIMEOUT 0x2201. __Assumes 3rd argument points to an int containing the new timeout value
-for this file descriptor. The unit is a "jiffy". Packets that are
-already "in flight" will not be affected. The default value is set
-on open() and is SG_DEFAULT_TIMEOUT (defined in sg.h). This default is
-currently 1 minute and may not be long enough for formats. Negative
-values will yield an EIO error.
-This ioctl() is not relevant to the sg version 3 driver because timeouts
-are specified explicitly for each command in the sg_io_hdr structure.
-Only when the sg_header structure is used is the timeout inherited from
-this value (help on a per file descriptor basis).
-
-----
-!!!8.20. SG_SET_TRANSFORM
-
-__SG_SET_TRANSFORM 0x2204. __Only is meaningful when SG_EMULATED host has yielded 1 (i.e. the low-level
-is the ide-scsi device driver); otherwise an EINVAL error occurs. The
-default state is to _not_ transform SCSI commands to the corresponding
-ATAPI commands but pass them straight through as is. [[Only certain classes
-of SCSI commands need to be transformed to their ATAPI equivalents.]
-The third argument is interpreted as an integer. When it is non-zero then
-a flag is set inside the ide-scsi driver that transforms subsequent
-commands sent to this driver. When zero is passed as the 3rd argument to
-this ioctl then the flag within the ide-scsi driver is cleared and
-subsequent commands are not transformed. Beware, this state will affect
-all devices (and hence all related sg file descriptors) associated with
-this ide-scsi "bus".
-
-----
-!!!8.21. SG_GET_TRANSFORM
-
-__SG_GET_TRANSFORM 0x2205. __Third argument is ignored. Only is meaningful when SG_EMULATED host has
-yielded 1 (i.e. the low-level is the ide-scsi device driver); otherwise
-an EINVAL error occurs. Returns 0 to indicate _not_ transforming SCSI
-to ATAPI commands (default). Returns 1 when it is transforming them.
-
-----
-!!!8.22. Sg ioctls removed in version 3
-
-Some seldom used ioctl()s introduced in the sg 2.x series drivers have
-been withdrawn. They are:
-
-
-
-
-
-*
-
-SG_SET_UNDERRUN_FLAG (and _GET_) [[use 'resid' in this new
-interface]
-
-
-*
-*
-
-SG_SET_MERGE_FD (and _GET) [[added complexity with little
-benefit]
-
-
-*
-
-----
-!!!8.23. SCSI_IOCTL_GET_IDLUN
-
-__SCSI_IOCTL_GET_IDLUN 0x5382. __This ioctl takes a pointer to a "struct scsi_idlun" object as its third
-argument. The "struct scsi_idlun" is not visible to user applications.
-To use this, that structure needs to be replicated in
-the user's program. Something like:
-
-typedef struct my_scsi_idlun {
-int four_in_one; /* 4 separate bytes of info compacted into 1 int */
-int host_unique_id; /* distinguishes adapter cards from same supplier */
-} My_scsi_idlun;
-"four_in_one" is made up as follows:
-
-(scsi_device_id | (lun `` 8) | (channel `` 16) | (host_no `` 24))
-These 4 components are assumed (or masked) to be 1 byte each. These are
-the four numbers that the SCSI subsystem uses to index devices, often
-written as "`host_no, channel, scsi_id, lunb".
-The 'host_unique_id' assigns a different number to each controller from the
-same manufacturer/low-level device driver. Most of the information
-provided by this command is more easily obtained from SG_GET_SCSI_ID.
-
-
-
-The 'host_no' element is a change in lk 2.4 kernels. [[In the lk 2.2 series
-and earlier, it was 'low_inode 8 0xff' from the procfs entry
-corresponding to the host.] This change makes the use of the
-SCSI_IOCTL_GET_BUS_NUMBER ioctl() superfluous.
-
-
-
-The advantage of this ioctl() is that it can be called on any SCSI file
-descriptor.
-
-----
-!!!8.24. SCSI_IOCTL_GET_PCI
-
-__SCSI_IOCTL_GET_PCI 0x5387. __Yields the PCI slot name (pci_dev::slot_name) associated with the lower
-level (adapter) driver that controls the current device. Up to 8 characters
-are output to the location pointed to by 'arg'. If the current device
-is not controlled by a PCI device then errno is set to ENXIO.
-[[This ioctl() was introduced in lk 2.4.4]
-
-----
-!!!8.25. SCSI_IOCTL_PROBE_HOST
-
-__SCSI_IOCTL_PROBE_HOST 0x5385. __This command should be given a pointer to a 'char' array as its 3rd
-argument. That array should be at least sizeof(int) long and have the
-length of the array as an 'int' at the beginning of the array! An ASCII
-string of no greater than that length containing "information" (or the
-name) of SCSI host (i.e. adapter) associated with this file descriptor is
-then placed in the given byte array. N.B. A trailing '\' may need to
-be put on the output string if it has been truncated by the input
-length. Returns 1 if host is present, 0 if it is not and a negative
-value if there is an error.
-
-----
-!!!8.26. SCSI_IOCTL_SEND_COMMAND
-
-__SCSI_IOCTL_SEND_COMMAND 0x1. __This ioctl() also offers a "pass through" SCSI command capability
-which is a subset of what is offered by the sg driver.
-
-
-
-The structure that we are passed should look like:
-
- struct sdata {
-unsigned int inlen; [[i] Length of data written to device
-unsigned int outlen; [[i] Length of data read from device
-unsigned char cmd[[x]; [[i] SCSI command (6 `= x `= 16)
-[[o] Data read from device starts here
-[[o] On error, sense buffer starts here
-unsigned char wdata[[y]; [[i] Data written to device starts here
-};
-Notes:
-
-
-
-
-
-*
-
- The SCSI command length is determined by examining the 1st byte of the
-given command
-[[15]
-. There is no way to override this.
-
-
-*
-*
-
-Data transfers are limited to PAGE_SIZE (4K on i386, 8K on alpha).
-
-
-*
-*
-
-The length (x + y) must be at least OMAX_SB_LEN bytes long to accommodate
-the sense buffer when an error occurs. The sense buffer is truncated to
-OMAX_SB_LEN (16) bytes so that old code will not be surprised.
-
-
-*
-*
-
-If a Unix error occurs (e.g. ENOMEM) then the user will receive a
-negative return and the Unix error code in 'errno'. If the SCSI command
-succeeds then 0 is returned. Positive numbers returned are the compacted
-SCSI error codes (4 bytes in one int) where the lowest byte is the SCSI
-status. See the drivers/scsi/scsi.h file for more information on this.
-
-
-*
-
-----
-!!!Chapter 9. Direct and Mmap-ed IO
-
-The normal action of the sg driver for a read operation (from a device)
-is to request the lower level (adapter) driver to DMA
-[[16]
-data into kernel buffers that the sg driver manages. The sg driver will
-then copy the contents of its buffers into the user space. [[This sequence
-is reversed for a write operation (towards a device)]. While this double
-handling of data is obviously inefficient it does decouple some hardware
-issues from user applications. For these and historical reasons the
-"double-buffered" IO remains the default for the sg driver.
-
-
-
-Both "direct" and "mmap-ed" IO are techniques that permit the data
-to be DMA-ed directly from the lower level (adapter) driver into
-the user application (vice versa for write operations). Both techniques
-result in faster speed, smaller latencies and lower CPU utilization
-but come at the expense of complexity (as always). For example the Linux
-kernel must not attempt to swap out pages in a user application that a
-SCSI adapter is busy DMA-ing data into.
-
-----
-!!!9.1. Direct IO
-
-Direct IO uses the kiobuf mechanism [[see the Linux
-Device Drivers book] to manipulate memory
-allocated within the user space so that a lower level (adapter) driver
-can DMA directly to or from that user space memory. Since the user
-can give a different data buffer to each SCSI command passed through
-the sg interface then the kiobuf mechanism needs to setup its structures
-(and undo that setup) for each SCSI command.
-[[17]
-Direct IO is available as an option in sg 3.1.18 (before that the sg driver
-needed to be recompiled with an altered define). Direct IO support is
-designed in such a way that if it is requested and cannot be performed
-then the command will still be performed using indirect IO. If direct IO
-is requested and has been performed then the SG_INFO_DIRECT_IO bit will
-be set in the 'info' member of the sg_io_hdr_t control structure after
-the request has been completed. Direct IO is not supported on ISA SCSI
-adapters since they only can address a 24 bit address space.
-
-
-
-One limit on direct IO is that sg_io_hdr_t::iovec_count==. So the user
-cannot (currently) use application level scatter gather and direct IO on
-the same request.
-
-
-
-For direct IO to be worthwhile, a reasonable amount of data should be
-requested for data transfer. For transfers less than 8 KByte it is
-probably not worth the trouble. On the other hand "locking down" a
-multiple 512 KB blocks of data for direct IO could adversely impact
-overall system performance. Remember that for the duration of a direct
-IO request, the data transfer buffer is mapped to a fixed memory location
-and locked in such a way that it won't be swapped out. This can "cramp
-the style" of the kernel if it is overdone.
-
-
-
-Prior to sg 3.1.18 the direct IO code was commented out with the
-"SG_ALLOW_DIO" define. In sg 3.1.18 (available for lk 2.4.2 and later)
-the direct IO code is active but is defaulted off by a run time value.
-This value can be accessed via the "proc" file system at
-/proc/scsi/sg/allow_dio . Direct IO is enabled when
-a user with root permissions writes "1" to that file:
-__echo 1 b /proc/scsi/sg/allow_dio__ . If SG_FLAG_DIRECT_IO
-is set in sg_io_hdr::flags but /proc/scsi/sg/allow_dio
-holds "" then indirect IO will be performed (and this is indicated by
-((sg_io_hdr::info 8 SG_INFO_DIRECT_IO_MASK) == SG_INFO_INDIRECT_IO)
-after the request is completed).
-
-----
-!!!9.2. Mmap-ed IO
-
-Memory-mapped IO takes a different approach from direct IO to removing
-the extra data copy performed by normal ("indirect") IO. With mmap-ed
-IO the application calls the mmap() system call to memory map sg's reserved
-buffer. The sg driver maintains one reserved buffer per file descriptor.
-The default size of the reserved buffer is 32 KB and it can be changed
-with the ioctl(SG_SET_RESERVED_SIZE). The mmap() system call only needs
-to be called once prior
-[[18]
-to doing mmap-ed IO. For more details on the mmap() see
-Section 7.6. An application indicates that it wants mmap-ed on
-a SCSI request by setting the SG_FLAG_MMAP_IO value in 'flags'.
-
-
-
-Since there is only reserved buffer per sg file descriptor then only one
-mmap-ed IO command can be active at one time. In order to perform command
-queuing with mmap-ed IO, an application will need to open() multiple
-file descriptors to the same SCSI device.
-With mmap-ed IO the various status values and the sense buffer (if
-required) are conveyed back to an application in the same fashion as
-normal ("indirect") IO.
-
-
-
-Mmap-ed has very low per command latency since the reserved buffer mapping
-only needs to be done once per file descriptor. Also the reserved buffer
-is set up by the sg driver to aid the efficient construction of the
-internal scatter gather list used by the lower level (adapter) driver for
-DMA purposes. This tends to be more efficient than the user memory that
-direct IO requires the sg driver to process into an internal scatter
-gather list. So on both these counts, mmap-ed IO has the edge over direct IO.
-
-----
-!!!Chapter 10. Driver and module initialization
-
-The size of the default reserved buffer can be specified when the
-sg driver is loaded. If it is built into the kernel then use:
-
- sg_def_reserved_size=`nb
-on the boot line (only supported in 2.4 kernels).
-
-
-
-If sg is a module, it can be loaded with __modprobe__
-in either manner:
-
- modprobe sg
-modprobe sg def_reserved_size=`nb
-In the second case "`nb" is an integer (non negative). The default
-value is the value of the SG_DEF_RESERVED_SIZE defined in sg.h .
-This is currently 32768.
-
-
-
-If sg is a module, it can be unloaded with __rmmod__
-like this:
-
- rmmod sg
-However if there is a file descriptor still open with the sg driver
-(or there is an outstanding request awaiting a response) then the
-sg module is considered to be busy and can't be unloaded.
-
-----
-!!!Chapter 11. Sg and the "proc" file system
-
-The sg driver provides information about the SCSI subsystem and the current
-internal state of the sg driver in the /proc/scsi/sg
-directory. Some sg driver defaults can be changed by super user writing values
-to these "pseudo" files
-[[19].
-
-
-
-The following files which are readable by all:
-
-allow_dio 0 indicates direct IO disable, 1 for enabled
-debug debug information including active request data
-def_reserved_size default buffer size reserved for each file descriptor
-devices one line of numeric data per device
-device_hdr single line of column names corresponding to 'devices'
-device_strs one line of vendor, product and rev info per device
-hosts one line of numeric data per host
-host_hdr single line of column names corresponding to 'hosts'
-host_strs one line of host information (string) per host
-version sg version as a number followed by a string representation
-
-
-
-Each line in 'devices' and 'device_strs' corresponds to an sg device. For
-example the first line corresponds to /dev/sg0. The line
-number (origin ) also corresponds to the sg minor device number. This
-mapping is local to sg and is normally the same as given by th
-__cat /proc/scsi/scsi__ command which is reported by the SCSI
-mid level driver. The two mappings may diverge when 'remove-single-device'
-and 'add-single-device' are used (see the SCSI-2.4-HOWTO for more information).
-
-
-
-Each line in 'hosts' and 'host_strs' corresponds to a SCSI host. For example
-the first line corresponds to the host normally represented as "scsi0".
-This mapping is invariant across the SCSI sub system. [[So these entries
-could arguably be migrated to the mid level.]
-
-
-
-The column headers in 'device_hdr' are given below. If the device is not
-present (and one is present after it) then a line of "-1" entries is
-output. Each entry is separated by a whitespace (currently a tab):
-
-host host number (indexes 'hosts' table, origin )
-chan channel number of device
-id SCSI id of device
-lun Logical Unit number of device
-type SCSI type (e.g. -bdisk, 5-bcdrom, 6-bscanner)
-opens number of opens (by sd, sr, sr and sg) at this time
-depth maximum queue depth supported by device
-busy number of commands being processed by host for this device
-online 1 indicates device is in normal online state, -boffline
-A SCSI device is set offline by the SCSI mid level when it decides that
-a device is no longer responding (e.g. the device does not respond to
-an SCSI INQUIRY command after it has been reset).
-
-
-
-The column headers in 'host_hdr' are given below. Each entry is separated
-by a whitespace (currently a tab):
-
-uid unique id (non-zero if multiple hosts of same type)
-busy number of commands being processed for this host
-cpl maximum number of command per lun (may be 0 if "device depth"
-is given
-sgat maximum elements of scatter gather the adapter (pseudo)
-DMA can accommodate
-isa 0 -b non-ISA adapter, 1 -b ISA adapter. ISA adapters are
-assumed to have a 24 bit address bus limit (16 MB).
-emu 0 -b real SCSI adapter, 1 -b emulated SCSI adapter
-(e.g. ide-scsi device driver)
-
-
-
-The 'def_reserved_size' is both readable and writable. It is only writable
-by root. It is initialized to the value of DEF_RESERVED_SIZE in the "sg.h"
-file. Values between 0 and 1048576 (which is 2 ** 20) are accepted and can
-be set from the command line with the following syntax:
-
-$ echo "262144" b /proc/scsi/sg/def_reserved_size
-Note that the actual reserved buffer associated with a file descriptor could
-be less than 'def_reserved_size' if appropriate memory is not available. If
-the sg driver is compiled into the kernel (but not when it is a module)
-this value can also be read at /proc/sys/kernel/sg-big-buff . This latter
-feature is deprecated.
-
-
-
-The 'allow_dio' is both readable and writable. It is only writable by
-root. When it is 0 (default) any request to do direct IO (i.e. by
-setting SG_FLAG_DIRECT_IO) will be ignored and indirect IO will be done
-instead.
-
-----
-!!!11.1. /proc/scsi/sg/debug
-
-This appendix explains the output from the /proc/scsi/sg/debug which is typically viewed by the command
-__cat /proc/scsi/sg/debug__. Below is the (slightly
-abridged) output while this command: __sgp_dd if=/dev/sg0
-of=/dev/null bs=512__ is executing on the system. That sgp_dd
-command is using command queuing to read a disk (and the data is
-written to /dev/null which forgets it).
-
-$ cat /proc/scsi/sg/debug
-dev_max(currently)=7 max_active_device=1 (origin 1)
-scsi_dma_free_sectors=416 sg_pool_secs_aval=320 def_reserved_size=32768
-bbb device=sg0 scsi0 chan=0 id=0 lun=0 em=0 sg_tablesize=255 excl=
-FD(1): timeout=60000ms bufflen=65536 (res)sgat=2 low_dma=
-cmd_q=1 f_packid=1 k_orphan=0 closed=
-fin: id=3949312 blen=65536 dur=10ms sgat=2 op=0x28
-act: id=3949440 blen=65536 t_o/elap=60000/10ms sgat=2 op=0x28
-rbbb act: id=3949568 blen=65536 t_o/elap=60000/10ms sgat=2 op=0x28
-act: id=3949696 blen=65536 t_o/elap=60000/0ms sgat=2 op=0x28
-Those items output above that are significant to user applications
-are described below.
-
-
-
-Broadly speaking the above output shows everything is going fine.
-Four SCSI READ(10) commands (SCSI opcode 0x28) for different ids
-are underway. Three commands are active while one is finished with its
-status and data read() and the request structure is pending deletion.
-The "id" corresponds to the pack_id given in the sg_io_hdr structure
-(or the sg_header structure). In the case if sgp_dd the pack_id
-value is the block number being given to the SCSI READ (or WRITE).
-You will notice the 4 ids are 128 apart.
-
-
-
-The "bbb" line shows the sg device name followed by the linux scsi
-adapter, channel, scsi id and lun numbers. The "em=" argument indicates
-whether the driver emulates a SCSI HBA. The ide-scsi driver would
-set "em=1". The "sg_tablesize" is the maximum number of scatter gather
-elements supported by the adapter driver. The "excl=" indicates no
-sg open() on this device is currently using the O_EXCL flag.
-
-
-
-The next two lines starting with "FD(1)" supply data about the first
-(and only in this case) open file descriptor on /dev/sg0. The default timeout is 60 seconds however this is only
-significant if the sg_header interface is being used since the
-sg_io_hdr interface explicits sets the timeout on a per command basis.
-"bufflen=65536" is the reserved buffer size for this file descriptor.
-The "(res)sgat=2" indicates that this reserved buffer requires 2
-scatter gather elements. The "low_dma" will be set to 1 for ISA HBAs
-indicating only the bottom 16 MB of RAM can be used for its kernel
-buffers. The "cmd_q=1" indicates command queuing is being allowed.
-The "f_packid=1" indicates the SG_SET_FORCE_PACK_ID mode is on.
-The "k_orphan" value is 1 in the rare cases when a SG_IO is interrupted
-while a SCSI command is "in flight". The "closed" value is 1 in the
-rare cases the file descriptor has been closed while a SCSI command
-is "in flight".
-
-
-
-Each line indented with 5 spaces represents a SCSI command. The state
-of the command is either:
-
-
-
-
-
-*
-
-prior: command hasn't been sent to mid level (rare)
-
-
-*
-*
-
-act: mid level (adapter driver or device) has command
-
-
-*
-*
-
-rcv: sg bottom half handler has received response to this
-command (awaiting read() or SG_IO ioctl to complete
-
-
-*
-*
-
-fin: SCSI response (and optionally data) has been or is
-being read but the command data structures have not been removed
-
-
-*
-These states can be optionally prefixed by "rbbb" which means the reserved
-buffer is being used, "diobb" which means this command is using direct IO,
-or "mmapbb" which means that mmap-ed IO is being used by this command.
-The "id" is the pack_id from this command's interface structure. The "blen"
-is the buffer length used by the data transfer associated with this command.
-For commands that a response has been received "dur" shows its duration
-in milliseconds. For commands still "in flight" an indication of
-"t_o/elap=60000/10ms" means this command has a timeout of 60000 milliseconds
-of which 10 milliseconds has already elapsed. The "sgat=2" argument
-indicates that this command's "blen" requires 2 scatter gather elements.
-The "op" value is the hexadecimal value of the SCSI command being executed.
-
-
-
-If sg has lots of activity then the "debug" output may span many lines
-and in some cases appear to be corrupted. This occurs because procfs requests
-fixed buffer sizes of information and, if there is more data to output,
-returns later to get the remainder. The problem with this strategy is that
-sg's internal state may have changed. Rather than double buffering, the sg
-driver just continues from the same offset. While procfs is very useful,
-ioctl()s (such as SG_GET_REQUEST_TABLE) still have their place.
-
-----
-!!!Chapter 12. Asynchronous usage of sg
-
-It is recommended that synchronous sg-based applications use the new
-SG_IO ioctl() command. Existing applications (which are mainly synchronous)
-can continue to use the older sg_header based interface which is still
-supported.
-
-
-
-Asynchronous usage allows multiple SCSI commands to be queued up to the
-device. If the device supports command queuing then there can be a major
-performance gain. Even if the device doesn't support command queuing (or
-is temporarily busy) then queuing up commands in the mid level or the host
-driver can be a minor performance win (since there will be a lower latency
-to transmit the next command when the device becomes free).
-
-
-
-Asynchronous usage usually starts with setting the O_NONBLOCK flag on open()
-[[or thereafter by using the fcntl(fd, SETFD, old_flags | O_NONBLOCK)
-system call]. A similar effect can be obtained without using O_NONBLOCK
-when POSIX threads are used. There are several strategies that can then
-be followed:
-
-
-
-
-
-#
-
-set O_NONBLOCK and use a poll() loop
-
-
-#
-#
-
-set O_NONBLOCK and use SIGPOLL signal to alert app when readable
-
-
-#
-#
-
-use POSIX threads and a single sg file descriptor
-
-
-#
-#
-
-use POSIX threads and multiple sg file descriptors to same device
-
-
-#
-
-
-
-The O_NONBLOCK flag also permits open(), write() and read() [[but not
-the ioctl(SG_IO)] to access a SCSI device even though it has been
-marked offline. SCSI devices are marked offline when they are detected
-and don't respond to the initial SCSI commands as expected, or,
-some SCSI error condition is detected on that device and the
-mid level error recovery logic is unable to "resurrect" the device.
-A SCSI device that is being reset (and still settling) could be
-accessed during this period by using the O_NONBLOCK flag; this
-could lead to unexpected behaviour so the sg user should take care.
-
-
-
-In Linux SIGIO and SIGPOLL are the same signal. If POSIX real time
-signals are used (e.g. when SA_SIGINFO is used with sigaction() and
-fcntl(fd, F_SETSIG, SIGRTMIN + `nb) ) then the file descriptor with which
-the signal is associated is available to the signal handler. The
-associated file descriptor is in the si_fd member of the siginfo_t
-structure. The poll() system call that is often used after a signal
-is received can thus be bypassed.
-
-----
-!!!Appendix A. Sg3_utils package
-
-The sg3_utils package is a collection of programs that use the sg interface.
-The utilities can be categorized as follows:
-
-
-
-
-
-*
-
-variants of the Unix __dd__ command: sg_dd, sgp_dd, sgq_dd
-and sgm_dd,
-
-
-*
-*
-
-scanning and mapping utilities: sg_scan, sg_map and scsi_devfs_scan,
-
-
-*
-*
-
-SCSI support: sg_inq, scsi_inquiry, sginfo, sg_readcap, sg_start and sg_reset,
-
-
-*
-*
-
-timing and testing: sg_rbuf, sg_test_rwbuf, sg_read, sg_turs and sg_debug,
-
-
-*
-*
-
-example programs: sg_simple1..4 and sg_simple16,
-
-
-*
-
-
-
-The "dd" family of utilities take a sg device file name as input (i.e.
-if=`sg_dev_filen_nameb), as output of both. They can also take raw
-device file names
-[[20]
-instead of sg device file names. One important difference from the
-standard __dd__ command is that the value given to
-the block size (bs=) argument must be the exact block size of that device
-and not a integral multiple as allowed by __dd__.
-These "dd" variants are suitable for SCSI Direct Access Devices such
-as disk and CDROMs (but are not suitable for SCSI tape devices).
-
-
-
-The sg3_utils package is designed to be used with the sg version 3
-driver found in the lk 2.4 series. There is also a sg_utils package
-that supports a subset of these commands for the sg version 2 driver
-(with some support for the original sg driver) which is found in
-the lk 2.2 series (from and after lk 2.2.6). There are links to
-the most recent sg3_utils (and sg_utils) packages at the sg website
-at
-www.torque.net/sg. There are tarballs and
-both source and binary rpm packages. At the time of writing the latest
-sg3_utils tarball is at
-
-www.torque.net/sg/p/sg3_utils-.97.tgz.
-There is a README file in that tarball that should be examined for
-up to date information. The more important utility commands (e.g.
-sg_dd) have "man" pages.
-[[21]
-
-
-
-Almost all of the sg device driver capabilities discussed in this
-document appear in code in one or more of these programs. For example
-the recently added mmap-ed IO can be found in sgm_dd, sg_read and
-sg_rbuf.
-
-
-
-The sg3_utils package also provides some functions that may be useful for
-applications that use sg. The functions declared in sg_err.h and defined in sg_err.c
-categorize SCSI subsystem errors that are returned to an application
-in a read() or a ioctl(SG_IO). In the case of sense buffers, they
-are decoded into text message (as per SCSI 2 definitions). There is
-also a function to do a 64 bit seek (llseek.h).
-
-----
-!!!Appendix B. sg_header, the original sg control structure
-
-Following is the original interface structure of the sg driver that
-dates back to 1991. Those field elements with a "[[o]+" are added by
-the sg version 2 driver which was first placed in lk 2.2.6 in April
-1999.
-
-struct sg_header
-{
-int pack_len; /* [[o] */
-int reply_len; /* [[i] */
-int pack_id; /* [[i-bo] */
-int result; /* [[o] */
-unsigned int twelve_byte:1; /* [[i] */
-unsigned int target_status:5; /* [[o]+ */
-unsigned int host_status:8; /* [[o]+ */
-unsigned int driver_status:8; /* [[o]+ */
-unsigned int other_flags:10; /* unused */
-unsigned char sense_buffer[[SG_MAX_SENSE]; /* [[o] */
-}; /* This structure is 36 bytes long on i386 */
-SCSI commands are sent via write() calls to an sg device name (e.g. /dev/sg0).
-The data written to write() is of the form
-`a_sg_header_obj + scsi_command [[ + data_to_write]b. The "data_to_write"
-component is only needed for SCSI commands that transfer data towards the
-SCSI device. The corresponding read() to the sg device name will yield
-data of the form `a_sg_header_obj [[ + data_to_read]b.
-
-
-
-This interface is fully described in the
-www.torque.net/sg/p/scsi-generic.txt
-file which documents the sg version 2 driver.
-
-
-
-Since many Linux applications use this interface, it is still supported
-in this version (i.e. version 3) of the driver. Only its most perverse
-idiosyncrasies have been modified and no major applications have reported
-any problems running old applications atop this newer driver.
-
-----
-!!!Appendix C. Programming example
-
-This appendix contains an example program. It is an abridged version of
-sg_simple2.c found in the sg3_utils
-package. It send a SCSI INQUIRY command to the nominated sg device and
-prints out some of the response or outputs error information. Hopefully
-showing the error processing does not cloud what is being illustrated.
-
-#include `unistd.hb
-#include `fcntl.hb
-#include `stdio.hb
-#include `string.hb
-#include `errno.hb
-#include `sys/ioctl.hb
-#include `scsi/sg.hb /* take care: fetches glibc's /usr/include/scsi/sg.h */
-/* This is a simple program executing a SCSI INQUIRY command using the
-sg_io_hdr interface of the SCSI generic (sg) driver.
-* Copyright (C) 2001 D. Gilbert
-* This program is free software. Version 1.01 (20020226)
-*/
-#define INQ_REPLY_LEN 96
-#define INQ_CMD_CODE 0x12
-#define INQ_CMD_LEN 6
-int main(int argc, char * argv[[])
-{
-int sg_fd, k;
-unsigned char inqCmdBlk[[INQ_CMD_LEN] =
-{INQ_CMD_CODE, , , , INQ_REPLY_LEN, };
-/* This is a "standard" SCSI INQUIRY command. It is standard because the
-* CMDDT and EVPD bits (in the second byte) are zero. All SCSI targets
-* should respond promptly to a standard INQUIRY */
-unsigned char inqBuff[[INQ_REPLY_LEN];
-unsigned char sense_buffer[[32];
-sg_io_hdr_t io_hdr;
-if (2 != argc) {
-printf("Usage: 'sg_simple0 `sg_deviceb'\n");
-return 1;
-}
-if ((sg_fd = open(argv[[1], O_RDONLY)) ` ) {
-/* Note that most SCSI commands require the O_RDWR flag to be set */
-perror("error opening given file name");
-return 1;
-}
-/* It is prudent to check we have a sg device by trying an ioctl */
-if ((ioctl(sg_fd, SG_GET_VERSION_NUM, 8k) ` ) || (k ` 30000)) {
-printf("%s is not an sg device, or old sg driver\n", argv[[1]);
-return 1;
-}
-/* Prepare INQUIRY command */
-memset(8io_hdr, , sizeof(sg_io_hdr_t));
-io_hdr.interface_id = 'S';
-io_hdr.cmd_len = sizeof(inqCmdBlk);
-/* io_hdr.iovec_count = ; */ /* memset takes care of this */
-io_hdr.mx_sb_len = sizeof(sense_buffer);
-io_hdr.dxfer_direction = SG_DXFER_FROM_DEV;
-io_hdr.dxfer_len = INQ_REPLY_LEN;
-io_hdr.dxferp = inqBuff;
-io_hdr.cmdp = inqCmdBlk;
-io_hdr.sbp = sense_buffer;
-io_hdr.timeout = 20000; /* 20000 millisecs == 20 seconds */
-/* io_hdr.flags = ; */ /* take defaults: indirect IO, etc */
-/* io_hdr.pack_id = ; */
-/* io_hdr.usr_ptr = NULL; */
-if (ioctl(sg_fd, SG_IO, 8io_hdr) ` ) {
-perror("sg_simple0: Inquiry SG_IO ioctl error");
-return 1;
-}
-/* now for the error processing */
-if ((io_hdr.info 8 SG_INFO_OK_MASK) != SG_INFO_OK) {
-if (io_hdr.sb_len_wr b ) {
-printf("INQUIRY sense data: ");
-for (k = ; k ` io_hdr.sb_len_wr; ++k) {
-if ((k b ) 88 (0 == (k % 10)))
-printf("\n ");
-printf("0x%02x ", sense_buffer[[k]);
-}
-printf("\n");
-}
-if (io_hdr.masked_status)
-printf("INQUIRY SCSI status=0x%x\n", io_hdr.status);
-if (io_hdr.host_status)
-printf("INQUIRY host_status=0x%x\n", io_hdr.host_status);
-if (io_hdr.driver_status)
-printf("INQUIRY driver_status=0x%x\n", io_hdr.driver_status);
-}
-else { /* assume INQUIRY response is present */
-char * p = (char *)inqBuff;
-printf("Some of the INQUIRY command's response:\n");
-printf(" %.8s %.16s %.4s\n", p + 8, p + 16, p + 32);
-printf("INQUIRY duration=%u millisecs, resid=%d\n",
-io_hdr.duration, io_hdr.resid);
-}
-close(sg_fd);
-return ;
-}
-
-
-
-The sg_simple4.c program is an example of using mmap-ed IO in the sg3_utils
-package. An example of using direct IO can be found in sg_rbuf.c in the
-same package.
-
-----
-!!!Appendix D. Debugging
-
-There are various ways to debug what is happening with the sg driver.
-The information provided in the /proc/scsi/sg
-directory can be useful, especially the debug
-pseudo file. It outputs the state of the sg driver when it is called.
-Invoking it at the right time can be a challenge. One approach (used in
-SANE) is to invoke the system() system call like this:
-
- system("cat /proc/scsi/sg/debug");
-at appropriate times within an application that is using the sg driver.
-
-
-
-Another debugging technique is to trace all system calls a program makes
-with the __strace__ command (see its "man" page). This
-command can also be used to obtain timing information (with the
-"-r" and "t" options).
-
-
-
-To debug the sg driver itself then the kernel needs to be built with
-CONFIG_SCSI_LOGGING selected. Then copious output will be sent by the sg
-driver whenever it is invoked to the log (normally
-/var/log/messages) and/or the console.
-This debug output is turned on by:
-
- $ echo "scsi log timeout 7" b /proc/scsi/scsi
-As the number (i.e. 7) is reduced, less output is generated. To turn off
-this type of debugging use:
-
- $ echo "scsi log timeout " b /proc/scsi/scsi
-
-
-
-If you want the system to log SCSI (CHECK_CONDITION related) errors that
-sg detects rather than process them within the application using sg
-then set ioctl(SG_SET_DEBUG) to a value greater than zero.
-Processing SCSI errors within the application using sg is my preference.
-
-----
-!!!Appendix E. Other references
-
-The primary site for SCSI information, standards (draft and emerging) and
-related reseources is
-www.t10.org.
-
-
-
-The most recent news on the sg driver can be found at:
-www.torque.net/sg .
-
-
-
-Some notes on the sg v3 driver can be found at:
-www.torque.net/sg/s_packet.html .
-For some timings (and CPU utilizations) comparisons between direct and
-indirect IO see:
-www.torque.net/sg/rbuf_tbl.html
-
-
-
-
-The Linux Documentation Project's SCSI-2.4-HOWTO may help to put this
-driver into perspective:
-linuxdoc.org/HOWTO/SCSI-2.4-HOWTO .
-The most recent version of that document can be found at
-www.torque.net/scsi/SCSI-2.4-HOWTO .
-
-
-
-
-To understand the inner workings of device drivers there is a fine book
-called "Linux Device Drivers", second edition by Alessandro Rubini and
-Jonathan Corbet published by O'Reilly [[ISBN -596-00008-1]. The authors
-and the publisher have unselfishly made this book available under the GNU
-Free Documentation License (version 1.1). It can be found in html at
-www.oreilly.com/catalog/linuxdrive2/chapter/book .
-
-
-!Notes
-[[1]
-
-SCSI command opcode 0x7f does allow for variable length commands but
-that is not supported in Linux currently.
-
-[[2]
-
-There is an sg version 3..19 which is an optional driver for the lk 2.2
-series. It has the following limitations:
-
-
-
-
-
-*
-
-maximum size of SCSI commands is 12 bytes
-
-
-*
-*
-
-sense buffer limited to 16 bytes
-
-
-*
-*
-
-resid (residual data transfer count) is always
-
-
-*
-*
-
-direct and mmap-ed IO not supported (defaults to indirect IO)
-
-
-*
-
-[[3]
-
-Patches exist for sg to extend the number of SCSI devices past the
-256 limit when the device file system (devfs) is being used.
-
-[[4]
-
-Linux kernel prior to 2.4.15 limited SCSI commands to a length of 12
-bytes. In lk 2.4.15 this was raised to 16 bytes. However unless lower
-level drivers (e.g. aic7xxx) indicate that they can handle 16 byte
-commands (and few currently do) then the command is aborted with a
-DID_ABORT host status.
-
-[[5]
-
-Some HBA - SCSI device combinations have difficulties with an odd valued
-dxfer_len . In some cases the operation succeeds but a DID_ERROR host status
-is returned. So unless there is a good reason, applications that want
-maximum portability should avoid an odd valued dxfer_len .
-
-[[6]
-
-Whether aborting individual commands is supported or not is left to
-the adapter. Many adapters are unable to abort SCSI commands "in flight"
-because these details are handled in silicon by embedded processors
-in hardware. SCSI device or bus resets are required.
-
-[[7]
-
-Some lower level drivers (e.g. ide-scsi) clear this status field
-even when a CHECK_CONDITION or COMMAND_TERMINATED status has
-occurred. However they do set DRIVER_SENSE in driver_status field.
-Also a (sb_len_wr b ) indicates there is a sense buffer.
-
-[[8]
-
-Some lower level drivers (e.g. ide-scsi) clear this masked_status field
-even when a CHECK_CONDITION or COMMAND_TERMINATED status has
-occurred. However they do set DRIVER_SENSE in driver_status field.
-Also a (sb_len_wr b ) indicates there is a sense buffer.
-
-[[9]
-
-In some cases the sym53cxx driver reports a DID_ERROR when it internally
-rounds up an odd transfer length by 1. This is an example of a "non-error".
-
-[[10]
-
-Unfortunately some adapters drivers report an incorrect number for 'resid'.
-This is due to some "fuzziness" in the internal interface definitions within
-the Linux scsi subsystem concerning the _exact_ number of bytes to be
-transferred. Therefore only applications tied to a specific adapter that
-is known to give the correct figure should use this feature. Hopefully
-this will be cleared up in the near future.
-
-[[11]
-
-The command queuing capabilities of the SCSI device and the adapter driver
-should also be taken into account. To this end the sg_scsi_id::h_cmd_per_lun
-and sg_scsi_id::d_queue_depth values returned bu ioctl(SG_GET_SCSI_ID) may
-be useful. Also some devices that indicate in their INQUIRY response that
-they can accept command queuing react badly when queuing is actually
-attempted.
-
-[[12]
-
-There is a small probability it will spend some time waiting for a
-command block to become available. In this case the wait is interruptible.
-If O_NONBLOCK is active then this scenario will cause a EAGAIN.
-
-[[13]
-
-The sg driver does record that the mmap() system call has been invoked
-at least once on a file descriptor. This is not sufficient because the
-given 'length' may be too short for the current IO. Also the driver is
-unaware of munmap() calls so it could easily be tricked.
-
-[[14]
-
-If ioctl(SG_SET_KEEP_ORPHAN) is set to 1 and a ioctl(SG_IO) operation
-is interrupted (e.g. by control-C by the user) then when the response
-arrives then the "num_waiting" will be incremented to indicate a
-read() can now pick up the response.
-
-[[15]
-
-Here is the mapping from the SCSI opcode "group" (top 3 bits of opcode)
-to the assumed length (in lk 2.4.15):
-
-unsigned char scsi_command_size[[8] =
-{
-6, 10, 10, 12,
-16, 12, 10, 10
-};
-The assumed length of group 4 commands changed from 12 to 16 in lk 2.4.15
-reflecting support for 16 byte SCSI commands being added to the kernel.
-
-[[16]
-
-Older SCSI adapters and some pseudo adapter drivers don't have DMA
-capability in which case the CPU is used to copy the data.
-
-[[17]
-
-Unfortunately that setup time is large enough in some versions of
-the lk 2.4 series to adversely impact direct IO performance.
-Also memory malloc()-ed in the user space tends to be made up of
-discontinuous pages seen from the SCSI adapter. This requires the
-sg driver to build heavily splintered scatter gather lists which
-is less than desirable. This limits the maximum transfer size to
-[[(max_scsi_adapter_scatter_gather_elements - 1) * PAGE_SIZE].
-[[This is a _different_ scatter gather mechanism to that which the user
-sees in the sg interface based on iovec.]
-
-[[18]
-
-When a write() or ioctl(SG_IO) attempts mmap-ed IO there is no check
-performed that a prior mmap() system call has been performed. If no
-mmap() has been issued then random data is written to the device or data
-read from the device in inaccessible. Also once mmap() has been called
-on a file descriptor then all subsequent calls to ioctl(SG_SET_RESERVED_SIZE)
-will yield EBUSY.
-
-[[19]
-
-One strange quirk is that the /proc/scsi/sg directory
-will not appear if there are no SCSI devices (or pseudo devices such as
-USB mass storage) attached to the system. The reason for this is that
-in the absence of SCSI devices, the SCSI mid level does not initialize
-the sg driver (even if it has been loaded as a module).
-When the sg driver is a module and the __rmmod sg__ is
-successfully executed then the /proc/scsi/sg directory
-and its contents are removed.
-
-[[20]
-
-Raw device names are of the form /dev/raw/raw`nb
-and can be bound to block devices (e.g. an IDE disk partition
-such as /dev/hda3). The binding is done with the
-__raw__ command (see "man raw").
-
-[[21]
-
-Although the author wrote most of these programs, initially to test
-facilities within the sg driver, some have been contributed by others.
-See www.torque.net/sg/u_index.html
-for more information
.
+Describe
[HowToSCSIGenericHOWTO
] here.
