[PATCH 06/24] drm/doc: document fallback behaviour for atomic events

Daniel Vetter
2017-03-08T14:12:39Z

Worst case if the hw can't support completion signalling in a
race-free way we want the event to be too late, not too early.

Text adapted from a proposal from Laurent - the other side of how to
make hw work correctly where it's possible is imo already sufficiently
documented.

v2: Review from Laurent.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
include/drm/drm_crtc.h | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index cba29ffedafd..6ef59da3fd8e 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -204,6 +204,12 @@ struct drm_crtc_state {
* drm_crtc_arm_vblank_event(). See the documentation of that function
* for a detailed discussion of the constraints it needs to be used
* safely.
+ *
+ * If the device can't notify of flip completion in a race-free way
+ * at all, then the event should be armed just after the page flip is
+ * committed. In the worst case the driver will send the event to
+ * userspace one frame too late. This doesn't allow for a real atomic
+ * update, but it should avoid tearing.
*/
struct drm_pending_vblank_event *event;

--
2.11.0

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