On Tue, Jul 23, 2019 at 03:13:37PM +0200, Daniel Vetter wrote:
> 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.
>
> v3:
>
> Ville noticed that on 32bit we might be breaking up the load/stores,
> now that the vblank counter has been switched over to be 64 bit. Fix
> that up by switching to atomic64_t. This this happens so rarely in
> practice I figured no need to cc: stable ...
>
> Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> Cc: Keith Packard <keithp@xxxxxxxxxx>
> References: 570e86963a51 ("drm: Widen vblank count to 64-bits [v3]")
> Cc: Rodrigo Siqueira <rodrigosiqueiramelo@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Reviewed-by: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/drm_vblank.c | 45 ++++++++++++++++++++++++++++++++----
> include/drm/drm_vblank.h | 15 ++++++++++--
> 2 files changed, 54 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 603ab105125d..03e37bceac9c 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -107,7 +107,7 @@ static void store_vblank(struct drm_device *dev, unsigned int pipe,
>
> write_seqlock(&vblank->seqlock);
> vblank->time = t_vblank;
> - vblank->count += vblank_count_inc;
> + atomic64_add(vblank_count_inc, &vblank->count);
> write_sequnlock(&vblank->seqlock);
> }
>
> @@ -273,7 +273,8 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
>
> DRM_DEBUG_VBL("updating vblank count on crtc %u:"
> " current=%llu, diff=%u, hw=%u hw_last=%u\n",
> - pipe, vblank->count, diff, cur_vblank, vblank->last);
> + pipe, atomic64_read(&vblank->count), diff,
> + cur_vblank, vblank->last);
>
> if (diff == 0) {
> WARN_ON_ONCE(cur_vblank != vblank->last);
> @@ -295,11 +296,23 @@ 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 = atomic64_read(&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 +777,14 @@ 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.
> */
> @@ -801,7 +822,7 @@ static u64 drm_vblank_count_and_time(struct drm_device *dev, unsigned int pipe,
>
> do {
> seq = read_seqbegin(&vblank->seqlock);
> - vblank_count = vblank->count;
> + vblank_count = atomic64_read(&vblank->count);
> *vblanktime = vblank->time;
> } while (read_seqretry(&vblank->seqlock, seq));
>
> @@ -818,6 +839,14 @@ 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 +1820,14 @@ 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 9fe4ba8bc622..c16c44052b3d 100644
> --- a/include/drm/drm_vblank.h
> +++ b/include/drm/drm_vblank.h
> @@ -109,9 +109,20 @@ 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;
> + atomic64_t count;
> /**
> * @time: Vblank timestamp corresponding to @count.
> */
> --
> 2.22.0
--
Ville Syrjälä
Intel
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