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81f7e3824   Eric Lee   Initial Release, ... 
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              Notes on Linux SCSI Generic (sg) driver
              ---------------------------------------
                                                          20020126
  Introduction
  ============
  The SCSI Generic driver (sg) is one of the four "high level" SCSI device
  drivers along with sd, st and sr (disk, tape and CDROM respectively). Sg
  is more generalized (but lower level) than its siblings and tends to be
  used on SCSI devices that don't fit into the already serviced categories.
  Thus sg is used for scanners, CD writers and reading audio CDs digitally
  amongst other things.
  
  Rather than document the driver's interface here, version information
  is provided plus pointers (i.e. URLs) where to find documentation
  and examples.
  
  
  Major versions of the sg driver
  ===============================
  There are three major versions of sg found in the linux kernel (lk):
        - sg version 1 (original) from 1992 to early 1999 (lk 2.2.5) . 
  	It is based in the sg_header interface structure.
        - sg version 2 from lk 2.2.6 in the 2.2 series. It is based on
  	an extended version of the sg_header interface structure.
        - sg version 3 found in the lk 2.4 series (and the lk 2.5 series).
  	It adds the sg_io_hdr interface structure.
  
  
  Sg driver documentation
  =======================
  The most recent documentation of the sg driver is kept at the Linux
  Documentation Project's (LDP) site: 
  http://www.tldp.org/HOWTO/SCSI-Generic-HOWTO
  This describes the sg version 3 driver found in the lk 2.4 series.
  The LDP renders documents in single and multiple page HTML, postscript
  and pdf. This document can also be found at:
  http://sg.danny.cz/sg/p/sg_v3_ho.html
  
  Documentation for the version 2 sg driver found in the lk 2.2 series can
  be found at http://sg.danny.cz/sg/. A larger version
  is at: http://sg.danny.cz/sg/p/scsi-generic_long.txt.
  
  The original documentation for the sg driver (prior to lk 2.2.6) can be
  found at http://www.torque.net/sg/p/original/SCSI-Programming-HOWTO.txt
  and in the LDP archives.
  
  A changelog with brief notes can be found in the
  /usr/src/linux/include/scsi/sg.h file. Note that the glibc maintainers copy 
  and edit this file (removing its changelog for example) before placing it 
  in /usr/include/scsi/sg.h . Driver debugging information and other notes 
  can be found at the top of the /usr/src/linux/drivers/scsi/sg.c file.
  
  A more general description of the Linux SCSI subsystem of which sg is a 
  part can be found at http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO .
  
  
  Example code and utilities
  ==========================
  There are two packages of sg utilities:
    - sg3_utils   for the sg version 3 driver found in lk 2.4
    - sg_utils    for the sg version 2 (and original) driver found in lk 2.2
                  and earlier
  Both packages will work in the lk 2.4 series however sg3_utils offers more
  capabilities. They can be found at: http://sg.danny.cz/sg/sg3_utils.html and 
  freecode.com
  
  Another approach is to look at the applications that use the sg driver.
  These include cdrecord, cdparanoia, SANE and cdrdao.
  
  
  Mapping of Linux kernel versions to sg driver versions
  ======================================================
  Here is a list of linux kernels in the 2.4 series that had new version
  of the sg driver:
        lk 2.4.0 : sg version 3.1.17
        lk 2.4.7 : sg version 3.1.19 
        lk 2.4.10 : sg version 3.1.20  **
        lk 2.4.17 : sg version 3.1.22 
  
  ** There were 3 changes to sg version 3.1.20 by third parties in the
     next six linux kernel versions.
  
  For reference here is a list of linux kernels in the 2.2 series that had 
  new version of the sg driver:
        lk 2.2.0 : original sg version [with no version number]
        lk 2.2.6 : sg version 2.1.31
        lk 2.2.8 : sg version 2.1.32
        lk 2.2.10 : sg version 2.1.34 [SG_GET_VERSION_NUM ioctl first appeared]
        lk 2.2.14 : sg version 2.1.36
        lk 2.2.16 : sg version 2.1.38
        lk 2.2.17 : sg version 2.1.39
        lk 2.2.20 : sg version 2.1.40
  
  The lk 2.5 development series has recently commenced and it currently
  contains sg version 3.5.23 which is functionally equivalent to sg
  version 3.1.22 found in lk 2.4.17 .
  
  
  Douglas Gilbert
  26th January 2002
  dgilbert@interlog.com