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drm: Document caveats around atomic event handling

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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
This commit is contained in:
Daniel Vetter
2016-09-30 12:04:56 +02:00
parent 58f0f9f75c
commit 61802130d8
2 changed files with 73 additions and 15 deletions
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56
include/drm/drm_crtc.h
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@@ -109,8 +109,6 @@ struct drm_plane_helper_funcs;
* @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_crtc_state {
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 @@ struct drm_mode_config_funcs {
* 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:
*