1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
194 lines
6.5 KiB
XML
194 lines
6.5 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="scsidrivers">
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<bookinfo>
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<title>SCSI Subsystem Interfaces</title>
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<authorgroup>
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<author>
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<firstname>Douglas</firstname>
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<surname>Gilbert</surname>
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<affiliation>
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<address>
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<email>dgilbert@interlog.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<pubdate>2003-08-11</pubdate>
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<copyright>
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<year>2002</year>
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<year>2003</year>
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<holder>Douglas Gilbert</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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This document outlines the interface between the Linux scsi mid level
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and lower level drivers. Lower level drivers are variously called HBA
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(host bus adapter) drivers, host drivers (HD) or pseudo adapter drivers.
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The latter alludes to the fact that a lower level driver may be a
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bridge to another IO subsystem (and the "ide-scsi" driver is an example
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of this). There can be many lower level drivers active in a running
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system, but only one per hardware type. For example, the aic7xxx driver
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controls adaptec controllers based on the 7xxx chip series. Most lower
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level drivers can control one or more scsi hosts (a.k.a. scsi initiators).
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</para>
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<para>
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This document can been found in an ASCII text file in the linux kernel
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source: <filename>Documentation/scsi/scsi_mid_low_api.txt</filename> .
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It currently hold a little more information than this document. The
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<filename>drivers/scsi/hosts.h</filename> and <filename>
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drivers/scsi/scsi.h</filename> headers contain descriptions of members
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of important structures for the scsi subsystem.
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</para>
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</chapter>
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<chapter id="driver-struct">
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<title>Driver structure</title>
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<para>
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Traditionally a lower level driver for the scsi subsystem has been
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at least two files in the drivers/scsi directory. For example, a
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driver called "xyz" has a header file "xyz.h" and a source file
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"xyz.c". [Actually there is no good reason why this couldn't all
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be in one file.] Some drivers that have been ported to several operating
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systems (e.g. aic7xxx which has separate files for generic and
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OS-specific code) have more than two files. Such drivers tend to have
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their own directory under the drivers/scsi directory.
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</para>
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<para>
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scsi_module.c is normally included at the end of a lower
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level driver. For it to work a declaration like this is needed before
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it is included:
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<programlisting>
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static Scsi_Host_Template driver_template = DRIVER_TEMPLATE;
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/* DRIVER_TEMPLATE should contain pointers to supported interface
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functions. Scsi_Host_Template is defined hosts.h */
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#include "scsi_module.c"
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</programlisting>
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</para>
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<para>
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The scsi_module.c assumes the name "driver_template" is appropriately
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defined. It contains 2 functions:
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<orderedlist>
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<listitem><para>
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init_this_scsi_driver() called during builtin and module driver
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initialization: invokes mid level's scsi_register_host()
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</para></listitem>
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<listitem><para>
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exit_this_scsi_driver() called during closedown: invokes
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mid level's scsi_unregister_host()
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</para></listitem>
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</orderedlist>
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</para>
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<para>
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When a new, lower level driver is being added to Linux, the following
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files (all found in the drivers/scsi directory) will need some attention:
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Makefile, Config.help and Config.in . It is probably best to look at what
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an existing lower level driver does in this regard.
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</para>
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</chapter>
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<chapter id="intfunctions">
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<title>Interface Functions</title>
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!EDocumentation/scsi/scsi_mid_low_api.txt
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</chapter>
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<chapter id="locks">
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<title>Locks</title>
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<para>
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Each Scsi_Host instance has a spin_lock called Scsi_Host::default_lock
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which is initialized in scsi_register() [found in hosts.c]. Within the
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same function the Scsi_Host::host_lock pointer is initialized to point
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at default_lock with the scsi_assign_lock() function. Thereafter
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lock and unlock operations performed by the mid level use the
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Scsi_Host::host_lock pointer.
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</para>
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<para>
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Lower level drivers can override the use of Scsi_Host::default_lock by
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using scsi_assign_lock(). The earliest opportunity to do this would
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be in the detect() function after it has invoked scsi_register(). It
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could be replaced by a coarser grain lock (e.g. per driver) or a
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lock of equal granularity (i.e. per host). Using finer grain locks
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(e.g. per scsi device) may be possible by juggling locks in
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queuecommand().
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</para>
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</chapter>
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<chapter id="changes">
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<title>Changes since lk 2.4 series</title>
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<para>
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io_request_lock has been replaced by several finer grained locks. The lock
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relevant to lower level drivers is Scsi_Host::host_lock and there is one
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per scsi host.
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</para>
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<para>
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The older error handling mechanism has been removed. This means the
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lower level interface functions abort() and reset() have been removed.
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</para>
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<para>
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In the 2.4 series the scsi subsystem configuration descriptions were
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aggregated with the configuration descriptions from all other Linux
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subsystems in the Documentation/Configure.help file. In the 2.5 series,
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the scsi subsystem now has its own (much smaller) drivers/scsi/Config.help
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file.
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</para>
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</chapter>
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<chapter id="credits">
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<title>Credits</title>
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<para>
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The following people have contributed to this document:
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<orderedlist>
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<listitem><para>
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Mike Anderson <email>andmike@us.ibm.com</email>
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</para></listitem>
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<listitem><para>
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James Bottomley <email>James.Bottomley@steeleye.com</email>
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</para></listitem>
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<listitem><para>
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Patrick Mansfield <email>patmans@us.ibm.com</email>
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</para></listitem>
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</orderedlist>
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</para>
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</chapter>
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</book>
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