[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:
https://www.tldp.org/HOWTO/SCSI-2.4-HOWTO . The LDP has single
and multiple page HTML renderings as well as postscript and pdf.

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
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), CD-ROM 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
1da177e4 LT	8	the SCSI subsystem in the Linux kernel (lk) 2.4 series. See:
c4e672ac	9	https://www.tldp.org/HOWTO/SCSI-2.4-HOWTO . The LDP has single
1da177e4	10	and multiple page HTML renderings as well as postscript and pdf.
1da177e4 LT	11
	12	Notes on using modules in the SCSI subsystem
	13	============================================
c4e672ac	14	The SCSI support in the Linux kernel can be modularized in a number of
1da177e4 LT	15	different ways depending upon the needs of the end user. To understand
	16	your options, we should first define a few terms.
	17
c4e672ac RD	18	The scsi-core (also known as the "mid level") contains the core of SCSI
	19	support. Without it you can do nothing with any of the other SCSI drivers.
	20	The SCSI core support can be a module (scsi_mod.o), or it can be built into
	21	the kernel. If the core is a module, it must be the first SCSI module
b7223d9b	22	loaded, and if you unload the modules, it will have to be the last one
c4e672ac	23	unloaded. In practice the modprobe and rmmod commands
1da177e4 LT	24	will enforce the correct ordering of loading and unloading modules in
	25	the SCSI subsystem.
	26
b7223d9b	27	The individual upper and lower level drivers can be loaded in any order
c4e672ac RD	28	once the SCSI core is present in the kernel (either compiled in or loaded
	29	as a module). The disk driver (sd_mod.o), CD-ROM driver (sr_mod.o),
	30	tape driver [1]_ (st.o) and SCSI generics driver (sg.o) represent the upper
b7223d9b MCC	31	level drivers to support the various assorted devices which can be
b7223d9b MCC	32	controlled. You can for example load the tape driver to use the tape drive,
1da177e4 LT	33	and then unload it once you have no further need for the driver (and release
	34	the associated memory).
	35
	36	The lower level drivers are the ones that support the individual cards that
	37	are supported for the hardware platform that you are running under. Those
	38	individual cards are often called Host Bus Adapters (HBAs). For example the
b7223d9b MCC	39	aic7xxx.o driver is used to control all recent SCSI controller cards from
b7223d9b MCC	40	Adaptec. Almost all lower level drivers can be built either as modules or
1da177e4 LT	41	built into the kernel.
1da177e4 LT	42
b7223d9b MCC	43	.. [1] There is a variant of the st driver for controlling OnStream tape
b7223d9b MCC	44	devices. Its module name is osst.o .