drm/i915/audio: add DOC comment describing HDA over HDMI/DP

v2: include the section in the drm docbook.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Documentation/DocBook/drm.tmpl

@@ -3854,6 +3854,11 @@ int num_ioctls;</synopsis>
 	  probing, so those sections fully apply.
         </para>
       </sect2>
+      <sect2>
+	<title>High Definition Audio</title>
+!Pdrivers/gpu/drm/i915/intel_audio.c High Definition Audio over HDMI and Display Port
+!Idrivers/gpu/drm/i915/intel_audio.c
+      </sect2>
       <sect2>
         <title>DPIO</title>
 !Pdrivers/gpu/drm/i915/i915_reg.h DPIO

drivers/gpu/drm/i915/intel_audio.c

@@ -28,6 +28,27 @@
 #include "intel_drv.h"
 #include "i915_drv.h"
 
+/**
+ * DOC: High Definition Audio over HDMI and Display Port
+ *
+ * The graphics and audio drivers together support High Definition Audio over
+ * HDMI and Display Port. The audio programming sequences are divided into audio
+ * codec and controller enable and disable sequences. The graphics driver
+ * handles the audio codec sequences, while the audio driver handles the audio
+ * controller sequences.
+ *
+ * The disable sequences must be performed before disabling the transcoder or
+ * port. The enable sequences may only be performed after enabling the
+ * transcoder and port, and after completed link training.
+ *
+ * The codec and controller sequences could be done either parallel or serial,
+ * but generally the ELDV/PD change in the codec sequence indicates to the audio
+ * driver that the controller sequence should start. Indeed, most of the
+ * co-operation between the graphics and audio drivers is handled via audio
+ * related registers. (The notable exception is the power management, not
+ * covered here.)
+ */
+
 static const struct {
 	int clock;
 	u32 config;