scsi: docs: convert scsi-changer.txt to ReST

Link: https://lore.kernel.org/r/433d073fa982174a19783c2e59412b724e2cf946.1583136624.git.mchehab+huawei@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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diff --git a/Documentation/scsi/scsi-changer.rst b/Documentation/scsi/scsi-changer.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+README for the SCSI media changer driver
+========================================
+
+This is a driver for SCSI Medium Changer devices, which are listed
+with "Type: Medium Changer" in /proc/scsi/scsi.
+
+This is for *real* Jukeboxes.  It is *not* supported to work with
+common small CD-ROM changers, neither one-lun-per-slot SCSI changers
+nor IDE drives.
+
+Userland tools available from here:
+	http://linux.bytesex.org/misc/changer.html
+
+
+General Information
+-------------------
+
+First some words about how changers work: A changer has 2 (possibly
+more) SCSI ID's. One for the changer device which controls the robot,
+and one for the device which actually reads and writes the data. The
+later may be anything, a MOD, a CD-ROM, a tape or whatever. For the
+changer device this is a "don't care", he *only* shuffles around the
+media, nothing else.
+
+
+The SCSI changer model is complex, compared to - for example - IDE-CD
+changers. But it allows to handle nearly all possible cases. It knows
+4 different types of changer elements:
+
+  ===============   ==================================================
+  media transport   this one shuffles around the media, i.e. the
+                    transport arm.  Also known as "picker".
+  storage           a slot which can hold a media.
+  import/export     the same as above, but is accessible from outside,
+                    i.e. there the operator (you !) can use this to
+                    fill in and remove media from the changer.
+		    Sometimes named "mailslot".
+  data transfer     this is the device which reads/writes, i.e. the
+		    CD-ROM / Tape / whatever drive.
+  ===============   ==================================================
+
+None of these is limited to one: A huge Jukebox could have slots for
+123 CD-ROM's, 5 CD-ROM readers (and therefore 6 SCSI ID's: the changer
+and each CD-ROM) and 2 transport arms. No problem to handle.
+
+
+How it is implemented
+---------------------
+
+I implemented the driver as character device driver with a NetBSD-like
+ioctl interface. Just grabbed NetBSD's header file and one of the
+other linux SCSI device drivers as starting point. The interface
+should be source code compatible with NetBSD. So if there is any
+software (anybody knows ???) which supports a BSDish changer driver,
+it should work with this driver too.
+
+Over time a few more ioctls where added, volume tag support for example
+wasn't covered by the NetBSD ioctl API.
+
+
+Current State
+-------------
+
+Support for more than one transport arm is not implemented yet (and
+nobody asked for it so far...).
+
+I test and use the driver myself with a 35 slot cdrom jukebox from
+Grundig.  I got some reports telling it works ok with tape autoloaders
+(Exabyte, HP and DEC).  Some People use this driver with amanda.  It
+works fine with small (11 slots) and a huge (4 MOs, 88 slots)
+magneto-optical Jukebox.  Probably with lots of other changers too, most
+(but not all :-) people mail me only if it does *not* work...
+
+I don't have any device lists, neither black-list nor white-list.  Thus
+it is quite useless to ask me whenever a specific device is supported or
+not.  In theory every changer device which supports the SCSI-2 media
+changer command set should work out-of-the-box with this driver.  If it
+doesn't, it is a bug.  Either within the driver or within the firmware
+of the changer device.
+
+
+Using it
+--------
+
+This is a character device with major number is 86, so use
+"mknod /dev/sch0 c 86 0" to create the special file for the driver.
+
+If the module finds the changer, it prints some messages about the
+device [ try "dmesg" if you don't see anything ] and should show up in
+/proc/devices. If not....  some changers use ID ? / LUN 0 for the
+device and ID ? / LUN 1 for the robot mechanism. But Linux does *not*
+look for LUNs other than 0 as default, because there are too many
+broken devices. So you can try:
+
+  1) echo "scsi add-single-device 0 0 ID 1" > /proc/scsi/scsi
+     (replace ID with the SCSI-ID of the device)
+  2) boot the kernel with "max_scsi_luns=1" on the command line
+     (append="max_scsi_luns=1" in lilo.conf should do the trick)
+
+
+Trouble?
+--------
+
+If you insmod the driver with "insmod debug=1", it will be verbose and
+prints a lot of stuff to the syslog.  Compiling the kernel with
+CONFIG_SCSI_CONSTANTS=y improves the quality of the error messages a lot
+because the kernel will translate the error codes into human-readable
+strings then.
+
+You can display these messages with the dmesg command (or check the
+logfiles).  If you email me some question because of a problem with the
+driver, please include these messages.
+
+
+Insmod options
+--------------
+
+debug=0/1
+	Enable debug messages (see above, default: 0).
+
+verbose=0/1
+	Be verbose (default: 1).
+
+init=0/1
+	Send INITIALIZE ELEMENT STATUS command to the changer
+	at insmod time (default: 1).
+
+timeout_init=<seconds>
+	timeout for the INITIALIZE ELEMENT STATUS command
+	(default: 3600).
+
+timeout_move=<seconds>
+	timeout for all other commands (default: 120).
+
+dt_id=<id1>,<id2>,... / dt_lun=<lun1>,<lun2>,...
+	These two allow to specify the SCSI ID and LUN for the data
+	transfer elements.  You likely don't need this as the jukebox
+	should provide this information.  But some devices don't ...
+
+vendor_firsts=, vendor_counts=, vendor_labels=
+	These insmod options can be used to tell the driver that there
+	are some vendor-specific element types.  Grundig for example
+	does this.  Some jukeboxes have a printer to label fresh burned
+	CDs, which is addressed as element 0xc000 (type 5).  To tell the
+	driver about this vendor-specific element, use this::
+
+		$ insmod ch			\
+			vendor_firsts=0xc000	\
+			vendor_counts=1		\
+			vendor_labels=printer
+
+	All three insmod options accept up to four comma-separated
+	values, this way you can configure the element types 5-8.
+	You likely need the SCSI specs for the device in question to
+	find the correct values as they are not covered by the SCSI-2
+	standard.
+
+
+Credits
+-------
+
+I wrote this driver using the famous mailing-patches-around-the-world
+method.  With (more or less) help from:
+
+	- Daniel Moehwald <moehwald@hdg.de>
+	- Dane Jasper <dane@sonic.net>
+	- R. Scott Bailey <sbailey@dsddi.eds.com>
+	- Jonathan Corbet <corbet@lwn.net>
+
+Special thanks go to
+
+	- Martin Kuehne <martin.kuehne@bnbt.de>
+
+for a old, second-hand (but full functional) cdrom jukebox which I use
+to develop/test driver and tools now.
+
+Have fun,
+
+   Gerd
+
+Gerd Knorr <kraxel@bytesex.org>