Documentation/DocBook/scsidrivers.tmpl

   1 <?xml version="1.0" encoding="UTF-8"?>
   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
   4
   5 <book id="scsidrivers">
   6  <bookinfo>
   7   <title>SCSI Subsystem Interfaces</title>
   8
   9   <authorgroup>
  10    <author>
  11     <firstname>Douglas</firstname>
  12     <surname>Gilbert</surname>
  13     <affiliation>
  14      <address>
  15       <email>dgilbert@interlog.com</email>
  16      </address>
  17     </affiliation>
  18    </author>
  19   </authorgroup>
  20   <pubdate>2003-08-11</pubdate>
  21
  22   <copyright>
  23    <year>2002</year>
  24    <year>2003</year>
  25    <holder>Douglas Gilbert</holder>
  26   </copyright>
  27
  28   <legalnotice>
  29    <para>
  30      This documentation is free software; you can redistribute
  31      it and/or modify it under the terms of the GNU General Public
  32      License as published by the Free Software Foundation; either
  33      version 2 of the License, or (at your option) any later
  34      version.
  35    </para>
  36
  37    <para>
  38      This program is distributed in the hope that it will be
  39      useful, but WITHOUT ANY WARRANTY; without even the implied
  40      warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  41      See the GNU General Public License for more details.
  42    </para>
  43
  44    <para>
  45      You should have received a copy of the GNU General Public
  46      License along with this program; if not, write to the Free
  47      Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  48      MA 02111-1307 USA
  49    </para>
  50
  51    <para>
  52      For more details see the file COPYING in the source
  53      distribution of Linux.
  54    </para>
  55   </legalnotice>
  56
  57  </bookinfo>
  58
  59 <toc></toc>
  60
  61   <chapter id="intro">
  62       <title>Introduction</title>
  63   <para>
  64 This document outlines the interface between the Linux scsi mid level
  65 and lower level drivers. Lower level drivers are variously called HBA
  66 (host bus adapter) drivers, host drivers (HD) or pseudo adapter drivers.
  67 The latter alludes to the fact that a lower level driver may be a
  68 bridge to another IO subsystem (and the "ide-scsi" driver is an example
  69 of this). There can be many lower level drivers active in a running
  70 system, but only one per hardware type. For example, the aic7xxx driver
  71 controls adaptec controllers based on the 7xxx chip series. Most lower
  72 level drivers can control one or more scsi hosts (a.k.a. scsi initiators).
  73   </para>
  74 <para>
  75 This document can been found in an ASCII text file in the linux kernel
  76 source: <filename>Documentation/scsi/scsi_mid_low_api.txt</filename> .
  77 It currently hold a little more information than this document. The
  78 <filename>drivers/scsi/hosts.h</filename> and <filename>
  79 drivers/scsi/scsi.h</filename> headers contain descriptions of members
  80 of important structures for the scsi subsystem.
  81 </para>
  82   </chapter>
  83
  84   <chapter id="driver-struct">
  85       <title>Driver structure</title>
  86   <para>
  87 Traditionally a lower level driver for the scsi subsystem has been
  88 at least two files in the drivers/scsi directory. For example, a
  89 driver called "xyz" has a header file "xyz.h" and a source file
  90 "xyz.c". [Actually there is no good reason why this couldn't all
  91 be in one file.] Some drivers that have been ported to several operating
  92 systems (e.g. aic7xxx which has separate  files for generic and
  93 OS-specific code) have more than two files. Such drivers tend to have
  94 their own directory under the drivers/scsi directory.
  95   </para>
  96   <para>
  97 scsi_module.c is normally included at the end of a lower
  98 level driver. For it to work a declaration like this is needed before
  99 it is included:
 100 <programlisting>
 101     static Scsi_Host_Template driver_template = DRIVER_TEMPLATE;
 102     /* DRIVER_TEMPLATE should contain pointers to supported interface
 103        functions. Scsi_Host_Template is defined hosts.h */
 104     #include "scsi_module.c"
 105 </programlisting>
 106   </para>
 107   <para>
 108 The scsi_module.c assumes the name "driver_template" is appropriately
 109 defined. It contains 2 functions:
 110 <orderedlist>
 111 <listitem><para>
 112      init_this_scsi_driver() called during builtin and module driver
 113      initialization: invokes mid level's scsi_register_host()
 114 </para></listitem>
 115 <listitem><para>
 116      exit_this_scsi_driver() called during closedown: invokes
 117      mid level's scsi_unregister_host()
 118 </para></listitem>
 119 </orderedlist>
 120   </para>
 121 <para>
 122 When a new, lower level driver is being added to Linux, the following
 123 files (all found in the drivers/scsi directory) will need some attention:
 124 Makefile, Config.help and Config.in . It is probably best to look at what
 125 an existing lower level driver does in this regard.
 126 </para>
 127   </chapter>
 128
 129   <chapter id="intfunctions">
 130      <title>Interface Functions</title>
 131 !EDocumentation/scsi/scsi_mid_low_api.txt
 132   </chapter>
 133
 134   <chapter id="locks">
 135      <title>Locks</title>
 136 <para>
 137 Each Scsi_Host instance has a spin_lock called Scsi_Host::default_lock
 138 which is initialized in scsi_register() [found in hosts.c]. Within the
 139 same function the Scsi_Host::host_lock pointer is initialized to point
 140 at default_lock with the scsi_assign_lock() function. Thereafter
 141 lock and unlock operations performed by the mid level use the
 142 Scsi_Host::host_lock pointer.
 143 </para>
 144 <para>
 145 Lower level drivers can override the use of Scsi_Host::default_lock by
 146 using scsi_assign_lock(). The earliest opportunity to do this would
 147 be in the detect() function after it has invoked scsi_register(). It
 148 could be replaced by a coarser grain lock (e.g. per driver) or a
 149 lock of equal granularity (i.e. per host). Using finer grain locks
 150 (e.g. per scsi device) may be possible by juggling locks in
 151 queuecommand().
 152 </para>
 153   </chapter>
 154
 155   <chapter id="changes">
 156      <title>Changes since lk 2.4 series</title>
 157 <para>
 158 io_request_lock has been replaced by several finer grained locks. The lock
 159 relevant to lower level drivers is Scsi_Host::host_lock and there is one
 160 per scsi host.
 161 </para>
 162 <para>
 163 The older error handling mechanism has been removed. This means the
 164 lower level interface functions abort() and reset() have been removed.
 165 </para>
 166 <para>
 167 In the 2.4 series the scsi subsystem configuration descriptions were
 168 aggregated with the configuration descriptions from all other Linux
 169 subsystems in the Documentation/Configure.help file. In the 2.5 series,
 170 the scsi subsystem now has its own (much smaller) drivers/scsi/Config.help
 171 file.
 172 </para>
 173   </chapter>
 174
 175   <chapter id="credits">
 176      <title>Credits</title>
 177 <para>
 178 The following people have contributed to this document:
 179 <orderedlist>
 180 <listitem><para>
 181 Mike Anderson <email>andmike@us.ibm.com</email>
 182 </para></listitem>
 183 <listitem><para>
 184 James Bottomley <email>James.Bottomley@steeleye.com</email>
 185 </para></listitem>
 186 <listitem><para>
 187 Patrick Mansfield <email>patmans@us.ibm.com</email>
 188 </para></listitem>
 189 </orderedlist>
 190 </para>
 191   </chapter>
 192
 193 </book>