Documentation/gpu: Add asynchronous flip documentation for i915

Add the details of the implementation of asynchronous flips for i915.

v7: -Rebased.

v8: -Rebased.

v9: -Rebased.

v10: Move all documentation changes to this patch. (Ville)
Signed-off-by: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200921110210.21182-8-karthik.b.s@intel.com

Documentation/gpu/i915.rst

@@ -118,6 +118,12 @@ Atomic Plane Helpers
 .. kernel-doc:: drivers/gpu/drm/i915/display/intel_atomic_plane.c
    :internal:
 
+Asynchronous Page Flip
+----------------------
+
+.. kernel-doc:: drivers/gpu/drm/i915/display/intel_display.c
+   :doc: asynchronous flip implementation
+
 Output Probing
 --------------
 

drivers/gpu/drm/i915/display/intel_display.c

@@ -14929,6 +14929,24 @@ static bool intel_cpu_transcoders_need_modeset(struct intel_atomic_state *state,
 	return false;
 }
 
+/**
+ * DOC: asynchronous flip implementation
+ *
+ * Asynchronous page flip is the implementation for the DRM_MODE_PAGE_FLIP_ASYNC
+ * flag. Currently async flip is only supported via the drmModePageFlip IOCTL.
+ * Correspondingly, support is currently added for primary plane only.
+ *
+ * Async flip can only change the plane surface address, so anything else
+ * changing is rejected from the intel_atomic_check_async() function.
+ * Once this check is cleared, flip done interrupt is enabled using
+ * the skl_enable_flip_done() function.
+ *
+ * As soon as the surface address register is written, flip done interrupt is
+ * generated and the requested events are sent to the usersapce in the interrupt
+ * handler itself. The timestamp and sequence sent during the flip done event
+ * correspond to the last vblank and have no relation to the actual time when
+ * the flip done event was sent.
+ */
 static int intel_atomic_check_async(struct intel_atomic_state *state)
 {
 	struct drm_i915_private *i915 = to_i915(state->base.dev);