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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://www.torque.net/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://www.torque.net/sg/p/scsi-generic.txt . A larger version
+is at:  http://www.torque.net/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://www.torque.net/sg and 
+freshmeat.net
+
+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