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.
Cc: Andrzej Hajda <a.hajda@xxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
drivers/gpu/drm/drm_irq.c | 32 +++++++++++++++++++++++++--
include/drm/drm_crtc.h | 56 ++++++++++++++++++++++++++++++++++++-----------
2 files changed, 73 insertions(+), 15 deletions(-)
diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 10611a936059..8874b564ec76 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -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 even 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 registes.
+ * 2. A vblank happes, 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 savely is to prevent the vblank from firing
+ * (and the hardware from committig 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)
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index f88f9a2d05c1..b381be639fd8 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -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 out scanning out the current
+ * framebuffers. It should contain the timestampe 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 timestampe 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:
*
--
2.7.4
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