[PATCH] scsi mid to lower level api documentation

Here is some documentation to describe the interface of lower level
drivers (e.g.  aic7xxx) in the scsi subsystem.

The patch is made up of a text file: drivers/scsi/scsi_mid_low_api.txt
and some Documentation/DocBook additions to enable html, ps and pdf
renderings.

Documentation/DocBook/Makefile

@@ -2,7 +2,7 @@ BOOKS	:= wanbook.sgml z8530book.sgml mcabook.sgml videobook.sgml \
 	   kernel-api.sgml parportbook.sgml kernel-hacking.sgml \
 	   kernel-locking.sgml via-audio.sgml mousedrivers.sgml sis900.sgml \
 	   deviceiobook.sgml procfs-guide.sgml tulip-user.sgml \
-	   writing_usb_driver.sgml
+	   writing_usb_driver.sgml scsidrivers.sgml
 
 PS	:=	$(patsubst %.sgml, %.ps, $(BOOKS))
 PDF	:=	$(patsubst %.sgml, %.pdf, $(BOOKS))
@@ -69,6 +69,9 @@ tulip-user.sgml: tulip-user.tmpl
 writing_usb_driver.sgml: writing_usb_driver.tmpl
 	$(TOPDIR)/scripts/docgen <$< >$@
 
+scsidrivers.sgml : scsidrivers.tmpl
+	$(TOPDIR)/scripts/docgen <$< >$@
+
 sis900.sgml: sis900.tmpl $(TOPDIR)/drivers/net/sis900.c
 	$(TOPDIR)/scripts/docgen $(TOPDIR)/drivers/net/sis900.c \
 		<sis900.tmpl >sis900.sgml

Documentation/DocBook/scsidrivers.tmpl

+<!-- -*- sgml -*- -->
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.1//EN"[]>
+
+<book id="scsidrivers">
+ <bookinfo>
+  <title>SCSI Subsystem Interfaces</title>
+  
+  <authorgroup>
+   <author>
+    <firstname>Douglas</firstname>
+    <surname>Gilbert</surname>
+    <affiliation>
+     <address>
+      <email>dgilbert@interlog.com</email>
+     </address>
+    </affiliation>
+   </author>
+  </authorgroup>
+  <pubdate>2002-04-27</pubdate>
+
+  <copyright>
+   <year>2002</year>
+   <holder>Douglas Gilbert</holder>
+  </copyright>
+  <legalnotice>
+   <para>
+    Permission is granted to copy, distribute and/or modify this
+    document under the terms of the GNU Free Documentation License,
+    Version 1.1 or any later version published by the Free Software
+    Foundation; with no Invariant Sections, with no Front-Cover Texts,
+    and with no Back-Cover Texts.  A copy of the license is included
+    in the section entitled "GNU Free Documentation License".
+   </para>
+  </legalnotice>
+
+ </bookinfo>
+
+<toc></toc>
+
+  <chapter id="intro">
+      <title>Introduction</title>
+  <para>
+This document outlines the interface between the Linux scsi mid level
+and lower level drivers. Lower level drivers are variously called HBA
+(host bus adapter) drivers, host drivers (HD) or pseudo adapter drivers.
+The latter alludes to the fact that a lower level driver may be a
+bridge to another IO subsystem (and the "ide-scsi" driver is an example
+of this). There can be many lower level drivers active in a running
+system, but only one per hardware type. For example, the aic7xxx driver
+controls adaptec controllers based on the 7xxx chip series. Most lower
+level drivers can control one or more scsi hosts (a.k.a. scsi initiators).
+  </para>
+<para>
+This document can been found in an ASCII text file in the linux kernel 
+source: <filename>drivers/scsi/scsi_mid_low_api.txt</filename> .
+It currently hold a little more information than this document. The
+<filename>drivers/scsi/hosts.h</filename> and <filename>
+drivers/scsi/scsi.h</filename> headers contain descriptions of members
+of important structures for the scsi subsystem.
+</para>
+  </chapter>
+
+  <chapter id="driver_struct">
+      <title>Driver structure</title>
+  <para>
+Traditionally a lower level driver for the scsi subsystem has been
+at least two files in the drivers/scsi directory. For example, a
+driver called "xyz" has a header file "xyz.h" and a source file
+"xyz.c". [Actually there is no good reason why this couldn't all
+be in one file.] Some drivers that have been ported to several operating
+systems (e.g. aic7xxx which has separate  files for generic and
+OS-specific code) have more than two files. Such drivers tend to have
+their own directory under the drivers/scsi directory.
+  </para>
+  <para>
+scsi_module.c is normally included at the end of a lower
+level driver. For it to work a declaration like this is needed before
+it is included:
+<programlisting>
+    static Scsi_Host_Template driver_template = DRIVER_TEMPLATE;
+    /* DRIVER_TEMPLATE should contain pointers to supported interface
+       functions. Scsi_Host_Template is defined hosts.h */
+    #include "scsi_module.c"
+</programlisting>
+  </para>
+  <para>
+The scsi_module.c assumes the name "driver_template" is appropriately
+defined. It contains 2 functions:
+<orderedlist>
+<listitem><para>
+     init_this_scsi_driver() called during builtin and module driver
+     initialization: invokes mid level's scsi_register_host()
+</para></listitem>
+<listitem><para>
+     exit_this_scsi_driver() called during closedown: invokes
+     mid level's scsi_unregister_host()
+</para></listitem>
+</orderedlist>
+  </para>
+<para>
+When a new, lower level driver is being added to Linux, the following 
+files (all found in the drivers/scsi directory) will need some attention: 
+Makefile, Config.help and Config.in . It is probably best to look at what 
+an existing lower level driver does in this regard.
+</para>
+  </chapter>
+
+  <chapter id="intfunctions">
+     <title>Interface Functions</title>
+!Edrivers/scsi/scsi_mid_low_api.txt
+  </chapter>
+
+  <chapter id="locks">
+     <title>Locks</title>
+<para>
+Each Scsi_Host instance has a spin_lock called Scsi_Host::default_lock
+which is initialized in scsi_register() [found in hosts.c]. Within the
+same function the Scsi_Host::host_lock pointer is initialized to point
+at default_lock with the scsi_assign_lock() function. Thereafter
+lock and unlock operations performed by the mid level use the
+Scsi_Host::host_lock pointer.
+</para>
+<para>
+Lower level drivers can override the use of Scsi_Host::default_lock by
+using scsi_assign_lock(). The earliest opportunity to do this would
+be in the detect() function after it has invoked scsi_register(). It
+could be replaced by a coarser grain lock (e.g. per driver) or a
+lock of equal granularity (i.e. per host). Using finer grain locks
+(e.g. per scsi device) may be possible by juggling locks in
+queuecommand().
+</para>
+  </chapter>
+
+  <chapter id="changes">
+     <title>Changes since lk 2.4 series</title>
+<para>
+io_request_lock has been replaced by several finer grained locks. The lock
+relevant to lower level drivers is Scsi_Host::host_lock and there is one
+per scsi host.
+</para>
+<para>
+The older error handling mechanism has been removed. This means the
+lower level interface functions abort() and reset() have been removed.
+</para>
+<para>
+In the 2.4 series the scsi subsystem configuration descriptions were
+aggregated with the configuration descriptions from all other Linux
+subsystems in the Documentation/Configure.help file. In the 2.5 series,
+the scsi subsystem now has its own (much smaller) drivers/scsi/Config.help
+file.
+</para>
+  </chapter>
+
+  <chapter id="credits">
+     <title>Credits</title>
+<para>
+The following people have contributed to this document:
+<orderedlist>
+<listitem><para>
+Mike Anderson <email>andmike@us.ibm.com</email>
+</para></listitem>
+<listitem><para>
+James Bottomley <email>James.Bottomley@steeleye.com</email>
+</para></listitem>
+<listitem><para>
+Patrick Mansfield <email>patmans@us.ibm.com</email>
+</para></listitem>
+</orderedlist>
+</para>
+  </chapter>
+
+</book>