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<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>
