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phoenix |
Notes on Linux SCSI Generic (sg) driver
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---------------------------------------
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20020126
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Introduction
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============
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The SCSI Generic driver (sg) is one of the four "high level" SCSI device
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drivers along with sd, st and sr (disk, tape and CDROM respectively). Sg
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is more generalized (but lower level) than its siblings and tends to be
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used on SCSI devices that don't fit into the already serviced categories.
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Thus sg is used for scanners, CD writers and reading audio CDs digitally
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amongst other things.
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Rather than document the driver's interface here, version information
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is provided plus pointers (i.e. URLs) where to find documentation
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and examples.
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Major versions of the sg driver
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===============================
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There are three major versions of sg found in the linux kernel (lk):
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- sg version 1 (original) from 1992 to early 1999 (lk 2.2.5) .
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It is based in the sg_header interface structure.
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- sg version 2 from lk 2.2.6 in the 2.2 series. It is based on
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an extended version of the sg_header interface structure.
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- sg version 3 found in the lk 2.4 series (and the lk 2.5 series).
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It adds the sg_io_hdr interface structure.
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Sg driver documentation
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=======================
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The most recent documentation of the sg driver is kept at the Linux
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Documentation Project's (LDP) site:
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http://www.tldp.org/HOWTO/SCSI-Generic-HOWTO
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This describes the sg version 3 driver found in the lk 2.4 series.
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The LDP renders documents in single and multiple page HTML, postscript
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and pdf. This document can also be found at:
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http://www.torque.net/sg/p/sg_v3_ho.html
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Documentation for the version 2 sg driver found in the lk 2.2 series can
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be found at http://www.torque.net/sg/p/scsi-generic.txt . A larger version
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is at: http://www.torque.net/sg/p/scsi-generic_long.txt .
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The original documentation for the sg driver (prior to lk 2.2.6) can be
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found at http://www.torque.net/sg/p/original/SCSI-Programming-HOWTO.txt
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and in the LDP archives.
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A changelog with brief notes can be found in the
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/usr/src/linux/include/scsi/sg.h file. Note that the glibc maintainers copy
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and edit this file (removing its changelog for example) before placing it
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in /usr/include/scsi/sg.h . Driver debugging information and other notes
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can be found at the top of the /usr/src/linux/drivers/scsi/sg.c file.
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A more general description of the Linux SCSI subsystem of which sg is a
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part can be found at http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO .
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Example code and utilities
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==========================
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There are two packages of sg utilities:
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- sg3_utils for the sg version 3 driver found in lk 2.4
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- sg_utils for the sg version 2 (and original) driver found in lk 2.2
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and earlier
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Both packages will work in the lk 2.4 series however sg3_utils offers more
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capabilities. They can be found at: http://www.torque.net/sg and
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freshmeat.net
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Another approach is to look at the applications that use the sg driver.
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These include cdrecord, cdparanoia, SANE and cdrdao.
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Mapping of Linux kernel versions to sg driver versions
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======================================================
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Here is a list of linux kernels in the 2.4 series that had new version
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of the sg driver:
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lk 2.4.0 : sg version 3.1.17
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lk 2.4.7 : sg version 3.1.19
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lk 2.4.10 : sg version 3.1.20 **
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lk 2.4.17 : sg version 3.1.22
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** There were 3 changes to sg version 3.1.20 by third parties in the
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next six linux kernel versions.
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For reference here is a list of linux kernels in the 2.2 series that had
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new version of the sg driver:
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lk 2.2.0 : original sg version [with no version number]
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lk 2.2.6 : sg version 2.1.31
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lk 2.2.8 : sg version 2.1.32
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lk 2.2.10 : sg version 2.1.34 [SG_GET_VERSION_NUM ioctl first appeared]
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lk 2.2.14 : sg version 2.1.36
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lk 2.2.16 : sg version 2.1.38
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lk 2.2.17 : sg version 2.1.39
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lk 2.2.20 : sg version 2.1.40
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The lk 2.5 development series has recently commenced and it currently
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contains sg version 3.5.23 which is functionally equivalent to sg
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version 3.1.22 found in lk 2.4.17 .
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Douglas Gilbert
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26th January 2002
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dgilbert@interlog.com
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