Noticed while reviewing code. I'm not sure whether this might or might
not explain some of the missed vblank hilarity we've been seeing. I
think those all go through the vblank completion event, which has
unconditional barriers - it always takes the spinlock. Therefore no
cc stable.
v2:
- Barrriers are hard, put them in in the right order (Chris).
- Improve the comments a bit.
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
drivers/gpu/drm/drm_vblank.c | 38 +++++++++++++++++++++++++++++++++++-
include/drm/drm_vblank.h | 13 +++++++++++-
2 files changed, 49 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 603ab105125d..eb2a8304536c 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -295,11 +295,23 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
static u64 drm_vblank_count(struct drm_device *dev, unsigned int pipe)
{
struct drm_vblank_crtc *vblank = &dev->vblank[pipe];
+ u64 count;
if (WARN_ON(pipe >= dev->num_crtcs))
return 0;
- return vblank->count;
+ count = vblank->count;
+
+ /*
+ * This read barrier corresponds to the implicit write barrier of the
+ * write seqlock in store_vblank(). Note that this is the only place
+ * where we need an explicit barrier, since all other access goes
+ * through drm_vblank_count_and_time(), which already has the required
+ * read barrier curtesy of the read seqlock.
+ */
+ smp_rmb();
+
+ return count;
}
/**
@@ -764,6 +776,14 @@ drm_get_last_vbltimestamp(struct drm_device *dev, unsigned int pipe,
* vblank interrupt (since it only reports the software vblank counter), see
* drm_crtc_accurate_vblank_count() for such use-cases.
*
+ * Note that for a given vblank counter value drm_crtc_handle_vblank()
+ * and drm_crtc_vblank_count() or drm_crtc_vblank_count_and_time()
+ * provide a barrier: Any writes done before calling
+ * drm_crtc_handle_vblank() will be visible to callers of the later
+ * functions, iff the vblank count is the same or a later one.
+ *
+ * See also &drm_vblank_crtc.count.
+ *
* Returns:
* The software vblank counter.
*/
@@ -818,6 +838,14 @@ static u64 drm_vblank_count_and_time(struct drm_device *dev, unsigned int pipe,
* vblank events since the system was booted, including lost events due to
* modesetting activity. Returns corresponding system timestamp of the time
* of the vblank interval that corresponds to the current vblank counter value.
+ *
+ * Note that for a given vblank counter value drm_crtc_handle_vblank()
+ * and drm_crtc_vblank_count() or drm_crtc_vblank_count_and_time()
+ * provide a barrier: Any writes done before calling
+ * drm_crtc_handle_vblank() will be visible to callers of the later
+ * functions, iff the vblank count is the same or a later one.
+ *
+ * See also &drm_vblank_crtc.count.
*/
u64 drm_crtc_vblank_count_and_time(struct drm_crtc *crtc,
ktime_t *vblanktime)
@@ -1791,6 +1819,14 @@ EXPORT_SYMBOL(drm_handle_vblank);
*
* This is the native KMS version of drm_handle_vblank().
*
+ * Note that for a given vblank counter value drm_crtc_handle_vblank()
+ * and drm_crtc_vblank_count() or drm_crtc_vblank_count_and_time()
+ * provide a barrier: Any writes done before calling
+ * drm_crtc_handle_vblank() will be visible to callers of the later
+ * functions, iff the vblank count is the same or a later one.
+ *
+ * See also &drm_vblank_crtc.count.
+ *
* Returns:
* True if the event was successfully handled, false on failure.
*/
diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
index e528bb2f659d..5ec623740158 100644
--- a/include/drm/drm_vblank.h
+++ b/include/drm/drm_vblank.h
@@ -110,7 +110,18 @@ struct drm_vblank_crtc {
seqlock_t seqlock;
/**
- * @count: Current software vblank counter.
+ * @count:
+ *
+ * Current software vblank counter.
+ *
+ * Note that for a given vblank counter value drm_crtc_handle_vblank()
+ * and drm_crtc_vblank_count() or drm_crtc_vblank_count_and_time()
+ * provide a barrier: Any writes done before calling
+ * drm_crtc_handle_vblank() will be visible to callers of the later
+ * functions, iff the vblank count is the same or a later one.
+ *
+ * IMPORTANT: This guarantee requires barriers, therefor never access
+ * this field directly. Use drm_crtc_vblank_count() instead.
*/
u64 count;
/**
--
2.22.0
_______________________________________________
dri-devel mailing list
dri-devel@xxxxxxxxxxxxxxxxxxxxx
https://lists.freedesktop.org/mailman/listinfo/dri-devel
