[linux-block.git] / Documentation / scsi / scsi.rst

.. SPDX-License-Identifier: GPL-2.0

============================
SCSI subsystem documentation
============================

The Linux Documentation Project (LDP) maintains a document describing
the SCSI subsystem in the Linux kernel (lk) 2.4 series. See:
http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO . The LDP has single
and multiple page HTML renderings as well as postscript and pdf.
It can also be found at:
http://web.archive.org/web/%2E/http://www.torque.net/scsi/SCSI-2.4-HOWTO

Notes on using modules in the SCSI subsystem
============================================
The scsi support in the linux kernel can be modularized in a number of
different ways depending upon the needs of the end user.  To understand
your options, we should first define a few terms.

The scsi-core (also known as the "mid level") contains the core of scsi
support.  Without it you can do nothing with any of the other scsi drivers.
The scsi core support can be a module (scsi_mod.o), or it can be built into
the kernel. If the core is a module, it must be the first scsi module
loaded, and if you unload the modules, it will have to be the last one
unloaded.  In practice the modprobe and rmmod commands (and "autoclean")
will enforce the correct ordering of loading and unloading modules in
the SCSI subsystem.

The individual upper and lower level drivers can be loaded in any order
once the scsi core is present in the kernel (either compiled in or loaded
as a module).  The disk driver (sd_mod.o), cdrom driver (sr_mod.o),
tape driver [1]_ (st.o) and scsi generics driver (sg.o) represent the upper
level drivers to support the various assorted devices which can be
controlled.  You can for example load the tape driver to use the tape drive,
and then unload it once you have no further need for the driver (and release
the associated memory).

The lower level drivers are the ones that support the individual cards that
are supported for the hardware platform that you are running under. Those
individual cards are often called Host Bus Adapters (HBAs). For example the
aic7xxx.o driver is used to control all recent SCSI controller cards from
Adaptec. Almost all lower level drivers can be built either as modules or
built into the kernel.

.. [1] There is a variant of the st driver for controlling OnStream tape
       devices. Its module name is osst.o .
Commit	Line	Data
b7223d9b MCC	1	.. SPDX-License-Identifier: GPL-2.0
	2
	3	============================
1da177e4 LT	4	SCSI subsystem documentation
1da177e4 LT	5	============================
b7223d9b	6
1da177e4 LT	7	The Linux Documentation Project (LDP) maintains a document describing
	8	the SCSI subsystem in the Linux kernel (lk) 2.4 series. See:
	9	http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO . The LDP has single
	10	and multiple page HTML renderings as well as postscript and pdf.
0ea6e611	11	It can also be found at:
b7223d9b	12	http://web.archive.org/web/%2E/http://www.torque.net/scsi/SCSI-2.4-HOWTO
1da177e4 LT	13
	14	Notes on using modules in the SCSI subsystem
	15	============================================
b7223d9b	16	The scsi support in the linux kernel can be modularized in a number of
1da177e4 LT	17	different ways depending upon the needs of the end user. To understand
	18	your options, we should first define a few terms.
	19
b7223d9b	20	The scsi-core (also known as the "mid level") contains the core of scsi
1da177e4 LT	21	support. Without it you can do nothing with any of the other scsi drivers.
1da177e4 LT	22	The scsi core support can be a module (scsi_mod.o), or it can be built into
b7223d9b MCC	23	the kernel. If the core is a module, it must be the first scsi module
b7223d9b MCC	24	loaded, and if you unload the modules, it will have to be the last one
1da177e4 LT	25	unloaded. In practice the modprobe and rmmod commands (and "autoclean")
	26	will enforce the correct ordering of loading and unloading modules in
	27	the SCSI subsystem.
	28
b7223d9b	29	The individual upper and lower level drivers can be loaded in any order
1da177e4 LT	30	once the scsi core is present in the kernel (either compiled in or loaded
1da177e4 LT	31	as a module). The disk driver (sd_mod.o), cdrom driver (sr_mod.o),
b7223d9b MCC	32	tape driver [1]_ (st.o) and scsi generics driver (sg.o) represent the upper
	33	level drivers to support the various assorted devices which can be
	34	controlled. You can for example load the tape driver to use the tape drive,
1da177e4 LT	35	and then unload it once you have no further need for the driver (and release
	36	the associated memory).
	37
	38	The lower level drivers are the ones that support the individual cards that
	39	are supported for the hardware platform that you are running under. Those
	40	individual cards are often called Host Bus Adapters (HBAs). For example the
b7223d9b MCC	41	aic7xxx.o driver is used to control all recent SCSI controller cards from
b7223d9b MCC	42	Adaptec. Almost all lower level drivers can be built either as modules or
1da177e4 LT	43	built into the kernel.
1da177e4 LT	44
b7223d9b MCC	45	.. [1] There is a variant of the st driver for controlling OnStream tape
b7223d9b MCC	46	devices. Its module name is osst.o .
1da177e4	47