[PATCH v5 0/2] handle vblank when disabling ctc with interrupt disabled

[PATCH v5 0/2] handle vblank when disabling ctc with interrupt disabled

[Robert Beckett]

Add warning when about to send stale vblank.
Revert change that stops vblank info being updated if interrupts already
disabled. This fixes a stale vblank timestamp issue seen on drm/imx.


Changes since v2:
Split up the patch in to smaller pieces.
Add warning when about to send bogus vblank event.
Update vblank to best guess info during drm_vblank_disable_and_save.

Changes since v3:
Update cover letter text to describe remaining actions.
Drop drm/imx patches as they have now been merged.
Replace vblank info estimation patch with a revert of the issue that
caused the need for estimation.

Changes since v4:
Updated doc for drm_crtc_send_vblank_event() based on feedback.
Confirmed that htmldoc looks good.

Robert Beckett (2):
  drm/vblank: warn on sending stale event
  Revert "drm/vblank: Do not update vblank count if interrupts are
    already disabled."

 drivers/gpu/drm/drm_vblank.c | 36 ++++++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 10 deletions(-)

-- 
2.18.0

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[PATCH v5 1/2] drm/vblank: warn on sending stale event

[Robert Beckett]

Warn when about to send stale vblank info and add advice to
documentation on how to avoid.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/drm_vblank.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 603ab105125d..9395b8c690b8 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -918,6 +918,20 @@ EXPORT_SYMBOL(drm_crtc_arm_vblank_event);
  *
  * 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.
+ *
+ * Care should be taken to avoid stale timestamps. If all of the following are
+ * true:
+ *   - your driver has vblank support (i.e. dev->num_crtcs > 0)
+ *   - the vblank irq is off (i.e. no one called drm_crtc_vblank_get())
+ *   - from the vblank code's pov the pipe is still running (i.e. not after the
+ *     call to drm_crtc_vblank_off() but before the next call to
+ *     drm_crtc_vblank_on())
+ * then drm_crtc_send_vblank_event is going to give you a garbage timestamp and
+ * sequence number (the last recorded before the irq was disabled).
+ *
+ * Drivers must either hold a vblank reference acquired through
+ * drm_crtc_vblank_get() or the vblank must have been shut off by calling
+ * drm_crtc_vblank_off().
  */
 void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
 				struct drm_pending_vblank_event *e)
@@ -925,8 +939,12 @@ void drm_crtc_send_vblank_event(struct drm_crtc *crtc,
 	struct drm_device *dev = crtc->dev;
 	u64 seq;
 	unsigned int pipe = drm_crtc_index(crtc);
+	struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
 	ktime_t now;
 
+	WARN_ONCE(dev->num_crtcs > 0 && !vblank->enabled && !vblank->inmodeset,
+		  "sending stale vblank info\n");
+
 	if (dev->num_crtcs > 0) {
 		seq = drm_vblank_count_and_time(dev, pipe, &now);
 	} else {
-- 
2.18.0

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[PATCH v5 2/2] Revert "drm/vblank: Do not update vblank count if interrupts are already disabled."

[Robert Beckett]

If interrupts are already disabled, then the timestamp for the vblank
does not get updated, causing a stale timestamp to be reported to
userland while disabling crtcs.

This reverts commit 68036b08b91bc491ccc308f902616a570a49227c.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
---
 drivers/gpu/drm/drm_vblank.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 9395b8c690b8..d71c49983be3 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -371,25 +371,23 @@ void drm_vblank_disable_and_save(struct drm_device *dev, unsigned int pipe)
 	spin_lock_irqsave(&dev->vblank_time_lock, irqflags);
 
 	/*
-	 * Update vblank count and disable vblank interrupts only if the
-	 * interrupts were enabled. This avoids calling the ->disable_vblank()
-	 * operation in atomic context with the hardware potentially runtime
-	 * suspended.
+	 * Only disable vblank interrupts if they're enabled. This avoids
+	 * calling the ->disable_vblank() operation in atomic context with the
+	 * hardware potentially runtime suspended.
 	 */
-	if (!vblank->enabled)
-		goto out;
+	if (vblank->enabled) {
+		__disable_vblank(dev, pipe);
+		vblank->enabled = false;
+	}
 
 	/*
-	 * Update the count and timestamp to maintain the
+	 * Always update the count and timestamp to maintain the
 	 * appearance that the counter has been ticking all along until
 	 * this time. This makes the count account for the entire time
 	 * between drm_crtc_vblank_on() and drm_crtc_vblank_off().
 	 */
 	drm_update_vblank_count(dev, pipe, false);
-	__disable_vblank(dev, pipe);
-	vblank->enabled = false;
 
-out:
 	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
 }
 
-- 
2.18.0

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