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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
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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
drivers/gpu/drm/drm_irq.c
View File

@ -1008,6 +1008,31 @@ static void send_vblank_event(struct drm_device *dev,
* period. This helper function implements exactly the required vblank arming
* behaviour.
*
* NOTE: Drivers using this to send out the event in struct &drm_crtc_state
* as part of an atomic commit must ensure that the next vblank happens at
* exactly the same time as the atomic commit is committed to the hardware. This
* function itself does **not** protect again the next vblank interrupt racing
* with either this function call or the atomic commit operation. A possible
* sequence could be:
*
* 1. Driver commits new hardware state into vblank-synchronized registers.
* 2. A vblank happens, committing the hardware state. Also the corresponding
* vblank interrupt is fired off and fully processed by the interrupt
* handler.
* 3. The atomic commit operation proceeds to call drm_crtc_arm_vblank_event().
* 4. The event is only send out for the next vblank, which is wrong.
*
* An equivalent race can happen when the driver calls
* drm_crtc_arm_vblank_event() before writing out the new hardware state.
*
* The only way to make this work safely is to prevent the vblank from firing
* (and the hardware from committing anything else) until the entire atomic
* commit sequence has run to completion. If the hardware does not have such a
* feature (e.g. using a "go" bit), then it is unsafe to use this functions.
* Instead drivers need to manually send out the event from their interrupt
* handler by calling drm_crtc_send_vblank_event() and make sure that there's no
* possible race with the hardware committing the atomic update.
*
* Caller must hold event lock. Caller must also hold a vblank reference for
* the event @e, which will be dropped when the next vblank arrives.
*/
@ -1030,8 +1055,11 @@ EXPORT_SYMBOL(drm_crtc_arm_vblank_event);
* @crtc: the source CRTC of the vblank event
* @e: the event to send
*
* Updates sequence # and timestamp on event, and sends it to userspace.
* Caller must hold event lock.
* Updates sequence # and timestamp on event for the most recently processed
* vblank, and sends it to userspace. Caller must hold event lock.
*
* See drm_crtc_arm_vblank_event() for a helper which can be used in certain
* situation, especially to send out events for atomic commit operations.
*/
void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
struct drm_pending_vblank_event *e)

56
include/drm/drm_crtc.h
View File

@ -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:
*