The pageflip completion timestamps transmitted to userspace
via pageflip completion events are supposed to describe the
time at which the first pixel of the new post-pageflip scanout
buffer leaves the video output of the gpu. This time is
identical to end of vblank, when active scanout starts.
For a crtc in standard fixed refresh rate, the end of vblank
is identical to the vblank timestamps calculated by
drm_update_vblank_count() at each vblank interrupt, or each
vblank dis-/enable. Therefore pageflip events just carry
that vblank timestamp as their pageflip timestamp.
For a crtc switched to variable refresh rate mode (vrr), the
pageflip completion timestamps are identical to the vblank
timestamps iff the pageflip was executed early in vblank,
before the minimum vblank duration elapsed. In this case
the time of display onset is identical to when the crtc
is running in fixed refresh rate.
However, if a pageflip completes later in the vblank, inside
the "extended front porch" in vrr mode, then the vblank will
terminate at a fixed (back porch) duration after flip, so
the display onset time is delayed correspondingly. In this
case the vblank timestamp computed at vblank irq time would
be too early, and we need a way to calculate an estimated
pageflip timestamp that will be later than the vblank timestamp.
How a driver determines such a "late flip" timestamp is hw
and driver specific, but this patch adds a new helper function
that allows the driver to propose such an alternate "late flip"
timestamp for use in pageflip events:
drm_crtc_set_vrr_pageflip_timestamp(crtc, flip_timestamp);
When sending out pageflip events, we now compare that proposed
flip_timestamp against the vblank timestamp of the current
vblank of flip completion and choose to send out the greater/
later timestamp as flip completion timestamp.
The most simple way for a kms driver to supply a suitable
flip_timestamp in vrr mode would be to simply take a timestamp
at start of the pageflip completion handler, e.g., pageflip
irq handler: flip_timestamp = ktime_get(); and then set that
as proposed "late" alternative timestamp via ...
drm_crtc_set_vrr_pageflip_timestamp(crtc, flip_timestamp);
More clever approaches could try to add some corrective offset
for fixed back porch duration, or ideally use hardware features
like hw timestamps to calculate the exact end time of vblank.
Signed-off-by: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@xxxxxxx>
Cc: Harry Wentland <harry.wentland@xxxxxxx>
Cc: Alex Deucher <alexander.deucher@xxxxxxx>
---
drivers/gpu/drm/drm_vblank.c | 49 +++++++++++++++++++++++++++++++++++-
include/drm/drm_vblank.h | 8 ++++++
2 files changed, 56 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 98e091175921..4b3a4c38fabe 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -814,10 +814,21 @@ static void send_vblank_event(struct drm_device *dev,
u64 seq, ktime_t now)
{
struct timespec64 tv;
+ ktime_t alt_flip_time;
switch (e->event.base.type) {
- case DRM_EVENT_VBLANK:
case DRM_EVENT_FLIP_COMPLETE:
+ /*
+ * For flip completion events, override "now" time
+ * with alt_flip_time provided by the driver via
+ * drm_crtc_set_vrr_pageflip_timestamp() in VRR mode
+ * if that time is later than given "now" vblank time.
+ */
+ alt_flip_time = dev->vblank[e->pipe].alt_flip_time;
+ if (alt_flip_time > now)
+ now = alt_flip_time;
+ /* Fallthrough */
+ case DRM_EVENT_VBLANK:
tv = ktime_to_timespec64(now);
e->event.vbl.sequence = seq;
/*
@@ -916,11 +927,47 @@ void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
now = ktime_get();
}
+
e->pipe = pipe;
send_vblank_event(dev, e, seq, now);
}
EXPORT_SYMBOL(drm_crtc_send_vblank_event);
+/**
+ * drm_crtc_set_vrr_pageflip_timestamp - helper to set alternate pageflip time
+ * @crtc: the source CRTC of the pageflip completion event
+ * @flip_time: The alternate pageflip completion timestamp in VRR mode
+ *
+ * In variable refresh rate mode (VRR), a pageflip completion timestamp carried
+ * by the pageflip event can never be earlier than the vblank timestamp of the
+ * vblank of flip completion, as that vblank timestamp defines the end of the
+ * shortest possible vblank duration. In case of a delayed flip completion
+ * inside the extended VRR front porch however, the end of vblank can be much
+ * later, so the driver must assign an estimated timestamp of that later end of
+ * vblank. For a CRTC in VRR mode, the driver should use this helper function to
+ * set an alternate flip completion timestamp in case of late flip completions
+ * in extended vblank. In the most simple case, this @flip_time timestamp could
+ * simply be a ktime_get() timestamp taken at the start of the pageflip
+ * completion routine, with some constant duration of the back porch interval
+ * added, although more precise estimates may be possible on some hardware if
+ * the hardware provides some means of timestamping the true end of vblank.
+ *
+ * When sending out pageflip events, e.g., via drm_crtc_send_vblank_event(), it
+ * will use either the standard vblank timestamp, calculated for a minimum
+ * duration vblank, or the provided @flip_time if that time is later than the
+ * vblank timestamp, to get the best possible estimate of start of display of
+ * the new post-pageflip scanout buffer.
+ */
+void drm_crtc_set_vrr_pageflip_timestamp(struct drm_crtc *crtc,
+ ktime_t flip_time)
+{
+ struct drm_device *dev = crtc->dev;
+ struct drm_vblank_crtc *vblank = &dev->vblank[drm_crtc_index(crtc)];
+
+ vblank->alt_flip_time = flip_time;
+}
+EXPORT_SYMBOL(drm_crtc_set_vrr_pageflip_timestamp);
+
static int __enable_vblank(struct drm_device *dev, unsigned int pipe)
{
if (drm_core_check_feature(dev, DRIVER_MODESET)) {
diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
index 6ad9630d4f48..aacf44694ab6 100644
--- a/include/drm/drm_vblank.h
+++ b/include/drm/drm_vblank.h
@@ -117,6 +117,12 @@ struct drm_vblank_crtc {
* @time: Vblank timestamp corresponding to @count.
*/
ktime_t time;
+ /**
+ * @alt_flip_time: Vblank timestamp for end of extended vblank due to
+ * a late pageflip completion in variable refresh rate mode. Pageflip
+ * events will carry the later one of @time and @alt_flip_time.
+ */
+ ktime_t alt_flip_time;
/**
* @refcount: Number of users/waiters of the vblank interrupt. Only when
@@ -179,6 +185,8 @@ int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs);
u64 drm_crtc_vblank_count(struct drm_crtc *crtc);
u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
ktime_t *vblanktime);
+void drm_crtc_set_vrr_pageflip_timestamp(struct drm_crtc *crtc,
+ ktime_t flip_time);
void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
struct drm_pending_vblank_event *e);
void drm_crtc_arm_vblank_event(struct drm_crtc *crtc,
--
2.17.1
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