drm: Document caveats around atomic event handling
It's not that obvious how a driver can all race the atomic commit with
handling the completion event. And there's unfortunately a pile of
drivers with rather bad event handling which misdirect people into the
wrong direction.
Try to remedy this by documenting everything better.
v2: Type fixes Alex spotted.
v3: More typos Alex spotted.
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475229896-6047-1-git-send-email-daniel.vetter@ffwll.ch
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index a544b75..61932f5 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -109,8 +109,6 @@
* @ctm: Transformation matrix
* @gamma_lut: Lookup table for converting pixel data after the
* conversion matrix
- * @event: optional pointer to a DRM event to signal upon completion of the
- * state update
* @state: backpointer to global drm_atomic_state
*
* Note that the distinction between @enable and @active is rather subtile:
@@ -159,6 +157,46 @@
struct drm_property_blob *ctm;
struct drm_property_blob *gamma_lut;
+ /**
+ * @event:
+ *
+ * Optional pointer to a DRM event to signal upon completion of the
+ * state update. The driver must send out the event when the atomic
+ * commit operation completes. There are two cases:
+ *
+ * - The event is for a CRTC which is being disabled through this
+ * atomic commit. In that case the event can be send out any time
+ * after the hardware has stopped scanning out the current
+ * framebuffers. It should contain the timestamp and counter for the
+ * last vblank before the display pipeline was shut off.
+ *
+ * - For a CRTC which is enabled at the end of the commit (even when it
+ * undergoes an full modeset) the vblank timestamp and counter must
+ * be for the vblank right before the first frame that scans out the
+ * new set of buffers. Again the event can only be sent out after the
+ * hardware has stopped scanning out the old buffers.
+ *
+ * - Events for disabled CRTCs are not allowed, and drivers can ignore
+ * that case.
+ *
+ * This can be handled by the drm_crtc_send_vblank_event() function,
+ * which the driver should call on the provided event upon completion of
+ * the atomic commit. Note that if the driver supports vblank signalling
+ * and timestamping the vblank counters and timestamps must agree with
+ * the ones returned from page flip events. With the current vblank
+ * helper infrastructure this can be achieved by holding a vblank
+ * reference while the page flip is pending, acquired through
+ * drm_crtc_vblank_get() and released with drm_crtc_vblank_put().
+ * Drivers are free to implement their own vblank counter and timestamp
+ * tracking though, e.g. if they have accurate timestamp registers in
+ * hardware.
+ *
+ * For hardware which supports some means to synchronize vblank
+ * interrupt delivery with committing display state there's also
+ * drm_crtc_arm_vblank_event(). See the documentation of that function
+ * for a detailed discussion of the constraints it needs to be used
+ * safely.
+ */
struct drm_pending_vblank_event *event;
struct drm_atomic_state *state;
@@ -835,17 +873,9 @@
* CRTC index supplied in &drm_event to userspace.
*
* The drm core will supply a struct &drm_event in the event
- * member of each CRTC's &drm_crtc_state structure. This can be handled by the
- * drm_crtc_send_vblank_event() function, which the driver should call on
- * the provided event upon completion of the atomic commit. Note that if
- * the driver supports vblank signalling and timestamping the vblank
- * counters and timestamps must agree with the ones returned from page
- * flip events. With the current vblank helper infrastructure this can
- * be achieved by holding a vblank reference while the page flip is
- * pending, acquired through drm_crtc_vblank_get() and released with
- * drm_crtc_vblank_put(). Drivers are free to implement their own vblank
- * counter and timestamp tracking though, e.g. if they have accurate
- * timestamp registers in hardware.
+ * member of each CRTC's &drm_crtc_state structure. See the
+ * documentation for &drm_crtc_state for more details about the precise
+ * semantics of this event.
*
* NOTE:
*