1 files changed, 39 insertions, 1 deletions

diff --git a/Documentation/DocBook/uio-howto.tmpl b/Documentation/DocBook/uio-howto.tmpl
index fdd7f4f887b7..c4d187313963 100644
--- a/Documentation/DocBook/uio-howto.tmpl
+++ b/Documentation/DocBook/uio-howto.tmpl
@@ -30,6 +30,12 @@
 
 <revhistory>
         <revision>
+        <revnumber>0.5</revnumber>
+        <date>2008-05-22</date>
+        <authorinitials>hjk</authorinitials>
+        <revremark>Added description of write() function.</revremark>
+        </revision>
+        <revision>
         <revnumber>0.4</revnumber>
         <date>2007-11-26</date>
         <authorinitials>hjk</authorinitials>
@@ -64,7 +70,7 @@
 <?dbhtml filename="copyright.html"?>
 <title>Copyright and License</title>
 <para>
-      Copyright (c) 2006 by Hans-Jürgen Koch.</para>
+      Copyright (c) 2006-2008 by Hans-Jürgen Koch.</para>
 <para>
 This documentation is Free Software licensed under the terms of the
 GPL version 2.
@@ -189,6 +195,30 @@ interested in translating it, please email me
         represents the total interrupt count. You can use this number
         to figure out if you missed some interrupts.
         </para>
+        <para>
+        For some hardware that has more than one interrupt source internally,
+        but not separate IRQ mask and status registers, there might be
+        situations where userspace cannot determine what the interrupt source
+        was if the kernel handler disables them by writing to the chip's IRQ
+        register. In such a case, the kernel has to disable the IRQ completely
+        to leave the chip's register untouched. Now the userspace part can
+        determine the cause of the interrupt, but it cannot re-enable
+        interrupts. Another cornercase is chips where re-enabling interrupts
+        is a read-modify-write operation to a combined IRQ status/acknowledge
+        register. This would be racy if a new interrupt occurred
+        simultaneously.
+        </para>
+        <para>
+        To address these problems, UIO also implements a write() function. It
+        is normally not used and can be ignored for hardware that has only a
+        single interrupt source or has separate IRQ mask and status registers.
+        If you need it, however, a write to <filename>/dev/uioX</filename>
+        will call the <function>irqcontrol()</function> function implemented
+        by the driver. You have to write a 32-bit value that is usually either
+        0 or 1 to disable or enable interrupts. If a driver does not implement
+        <function>irqcontrol()</function>, <function>write()</function> will
+        return with <varname>-ENOSYS</varname>.
+        </para>
 
         <para>
         To handle interrupts properly, your custom kernel module can
@@ -362,6 +392,14 @@ device is actually used.
 <function>open()</function>, you will probably also want a custom
 <function>release()</function> function.
 </para></listitem>
+
+<listitem><para>
+<varname>int (*irqcontrol)(struct uio_info *info, s32 irq_on)
+</varname>: Optional. If you need to be able to enable or disable
+interrupts from userspace by writing to <filename>/dev/uioX</filename>,
+you can implement this function. The parameter <varname>irq_on</varname>
+will be 0 to disable interrupts and 1 to enable them.
+</para></listitem>
 </itemizedlist>
 
 <para>