drm: Documentation style guide

Every time I type or review docs this seems a bit different. Try to
document the common style so we can try to unify at least new docs.

v2: Spelling fixes from Pierre, Laurent and Jani.

v3: More spelling fixes from Lukas.

Cc: Pierre Moreau <pierre.morrow@free.fr>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Lukas Wunner <lukas@wunner.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449564561-3896-1-git-send-email-daniel.vetter@ffwll.ch

Documentation/DocBook/gpu.tmpl

@@ -124,6 +124,43 @@
     <para>
       [Insert diagram of typical DRM stack here]
     </para>
+  <sect1>
+    <title>Style Guidelines</title>
+    <para>
+      For consistency this documentation uses American English. Abbreviations
+      are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
+      on. To aid in reading, documentations make full use of the markup
+      characters kerneldoc provides: @parameter for function parameters, @member
+      for structure members, &amp;structure to reference structures and
+      function() for functions. These all get automatically hyperlinked if
+      kerneldoc for the referenced objects exists. When referencing entries in
+      function vtables please use -&gt;vfunc(). Note that kerneldoc does
+      not support referencing struct members directly, so please add a reference
+      to the vtable struct somewhere in the same paragraph or at least section.
+    </para>
+    <para>
+      Except in special situations (to separate locked from unlocked variants)
+      locking requirements for functions aren't documented in the kerneldoc.
+      Instead locking should be check at runtime using e.g.
+      <code>WARN_ON(!mutex_is_locked(...));</code>. Since it's much easier to
+      ignore documentation than runtime noise this provides more value. And on
+      top of that runtime checks do need to be updated when the locking rules
+      change, increasing the chances that they're correct. Within the
+      documentation the locking rules should be explained in the relevant
+      structures: Either in the comment for the lock explaining what it
+      protects, or data fields need a note about which lock protects them, or
+      both.
+    </para>
+    <para>
+      Functions which have a non-<code>void</code> return value should have a
+      section called "Returns" explaining the expected return values in
+      different cases and their meanings. Currently there's no consensus whether
+      that section name should be all upper-case or not, and whether it should
+      end in a colon or not. Go with the file-local style. Other common section
+      names are "Notes" with information for dangerous or tricky corner cases,
+      and "FIXME" where the interface could be cleaned up.
+    </para>
+  </sect1>
   </chapter>
 
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