Documentation/driver-api/scsi.rst

Source file repositories/reference/linux-study-clean/Documentation/driver-api/scsi.rst

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Documentation/driver-api/scsi.rst
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=====================
SCSI Interfaces Guide
=====================

:Author: James Bottomley
:Author: Rob Landley

Introduction
============

Protocol vs bus
---------------

Once upon a time, the Small Computer Systems Interface defined both a
parallel I/O bus and a data protocol to connect a wide variety of
peripherals (disk drives, tape drives, modems, printers, scanners,
optical drives, test equipment, and medical devices) to a host computer.

Although the old parallel (fast/wide/ultra) SCSI bus has largely fallen
out of use, the SCSI command set is more widely used than ever to
communicate with devices over a number of different buses.

The `SCSI protocol <https://www.t10.org/scsi-3.htm>`__ is a big-endian
peer-to-peer packet based protocol. SCSI commands are 6, 10, 12, or 16
bytes long, often followed by an associated data payload.

SCSI commands can be transported over just about any kind of bus, and
are the default protocol for storage devices attached to USB, SATA, SAS,
Fibre Channel, FireWire, and ATAPI devices. SCSI packets are also
commonly exchanged over Infiniband,
TCP/IP (`iSCSI <https://en.wikipedia.org/wiki/ISCSI>`__), even `Parallel
ports <http://cyberelk.net/tim/parport/parscsi.html>`__.

Design of the Linux SCSI subsystem
----------------------------------

The SCSI subsystem uses a three layer design, with upper, mid, and low
layers. Every operation involving the SCSI subsystem (such as reading a
sector from a disk) uses one driver at each of the 3 levels: one upper
layer driver, one lower layer driver, and the SCSI midlayer.

The SCSI upper layer provides the interface between userspace and the
kernel, in the form of block and char device nodes for I/O and ioctl().
The SCSI lower layer contains drivers for specific hardware devices.

In between is the SCSI mid-layer, analogous to a network routing layer
such as the IPv4 stack. The SCSI mid-layer routes a packet based data
protocol between the upper layer's /dev nodes and the corresponding
devices in the lower layer. It manages command queues, provides error
handling and power management functions, and responds to ioctl()
requests.

SCSI upper layer
================

The upper layer supports the user-kernel interface by providing device
nodes.

sd (SCSI Disk)
--------------

sd (sd_mod.o)

sr (SCSI CD-ROM)
----------------

sr (sr_mod.o)

st (SCSI Tape)
--------------

Annotation

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