[Intel-gfx] [PATCH] drm/vblank: Document and fix vblank count barrier semantics
Rodrigo Siqueira
rodrigosiqueiramelo at gmail.com
Tue Sep 3 15:17:12 UTC 2019
Hi, thanks for the explanation.
I noticed that I forgot to add my r-b.
Reviewed-by: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
On 09/03, Daniel Vetter wrote:
> On Tue, Sep 03, 2019 at 08:47:03AM -0400, Rodrigo Siqueira wrote:
> > Hi Daniel,
> >
> > All the series look really good for me. I just have some few questions
> > here.
> >
> > On 07/23, 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
> >
> > I have to admit that I'm a little bit confused about the "missed vblank
> > hilarity we've been seeing". Could you elaborate a little bit more about
> > this problem in the commit message?
>
> We've had various reports on various drivers that hw vblanks seem to get
> lost and the driver stuck on vblank waits. I think most of those where
> just driver bugs, but could be also that there's some issues in the vblank
> core.
>
> > Additionally, how about break this commit in two? One dedicated to the barriers
> > and the atomic64, and the other related to the documentation?
> >
> > > 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 at linux.intel.com>
> > > Cc: Keith Packard <keithp at keithp.com>
> > > References: 570e86963a51 ("drm: Widen vblank count to 64-bits [v3]")
> > > Cc: Rodrigo Siqueira <rodrigosiqueiramelo at gmail.com>
> > > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > > ---
> > > 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();
> >
> > I think I did not get all the idea behind the smp_rmb() in this function. FWIU,
> > smp_xxx are used for preventing race conditions in a multiprocessor system,
> > right? In this sense, I can presume that this change can bring benefits for
> > VKMS or any other virtual driver; on the other hand, this will not bring any
> > advantage on real drivers like i915 and amdgpu since these devices are not
> > related with smp stuff, right?
>
> smp or not smp is about the cpu your driver is running on, not anything to
> do with the device hardware itself. And nowadays there's simply no
> single-threaded processors anymore, everything has at least 2 cores (even
> the tiniest soc). So yeah, this matters for everyone.
>
> smp_* functions only get compiled out to nothing if you have CONFIG_UP
> (which means only 1 cpu core with only 1 SMT thread is supported).
>
> And yeah correctly placing smp barriers is Real Hard Stuff (tm).
> -Daniel
>
>
> > Thanks
> >
> > > +
> > > + 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
> > >
> >
> > --
> > Rodrigo Siqueira
> > Software Engineer, Advanced Micro Devices (AMD)
> > https://siqueira.tech
>
>
>
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
--
Rodrigo Siqueira
Software Engineer, Advanced Micro Devices (AMD)
https://siqueira.tech
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