Re: [RFC 2/3] drm/atomic: Call dma_fence_boost() when we've missed a vblank

From: Daniel Vetter
Date: Tue Jun 01 2021 - 12:11:41 EST

On Tue, Jun 01, 2021 at 08:46:14AM -0700, Rob Clark wrote:
> On Tue, Jun 1, 2021 at 7:18 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >
> > On Sun, May 30, 2021 at 07:33:57AM -0700, Rob Clark wrote:
> > > On Thu, May 20, 2021 at 9:29 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > > >
> > > > On Wed, May 19, 2021 at 11:38:53AM -0700, Rob Clark wrote:
> > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > >
> > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > > ---
> > > > > drivers/gpu/drm/drm_atomic_helper.c | 11 +++++++++++
> > > > > 1 file changed, 11 insertions(+)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > > > > index 560aaecba31b..fe10fc2e7f86 100644
> > > > > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > > > > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > > > > @@ -1435,11 +1435,15 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
> > > > > int i, ret;
> > > > >
> > > > > for_each_new_plane_in_state(state, plane, new_plane_state, i) {
> > > > > + u64 vblank_count;
> > > > > +
> > > > > if (!new_plane_state->fence)
> > > > > continue;
> > > > >
> > > > > WARN_ON(!new_plane_state->fb);
> > > > >
> > > > > + vblank_count = drm_crtc_vblank_count(new_plane_state->crtc);
> > > > > +
> > > > > /*
> > > > > * If waiting for fences pre-swap (ie: nonblock), userspace can
> > > > > * still interrupt the operation. Instead of blocking until the
> > > > > @@ -1449,6 +1453,13 @@ int drm_atomic_helper_wait_for_fences(struct drm_device *dev,
> > > > > if (ret)
> > > > > return ret;
> > > > >
> > > > > + /*
> > > > > + * Check if we've missed a vblank while waiting, and if we have
> > > > > + * signal the fence that it's signaler should be boosted
> > > > > + */
> > > > > + if (vblank_count != drm_crtc_vblank_count(new_plane_state->crtc))
> > > > > + dma_fence_boost(new_plane_state->fence);
> > > >
> > > > I think we should do a lot better here:
> > > > - maybe only bother doing this for single-crtc updates, and only if
> > > > modeset isn't set. No one else cares about latency.
> > > >
> > > > - We should boost _right_ when we've missed the frame, so I think we
> > > > should have a _timeout wait here that guesstimates when the vblank is
> > > > over (might need to throw in a vblank wait if we missed) and then boost
> > > > immediately. Not wait a bunch of frames (worst case) until we finally
> > > > decide to boost.
> > >
> > > I was thinking about this a bit more.. How about rather than calling
> > > some fence->op->boost() type thing when we are about to miss a vblank
> > > (IMO that is also already too late), we do something more like
> > > fence->ops->set_deadline() before we even wait?
> >
> > Hm yeah that sounds like a clean idea.
> >
> > Even more, why not add the deadline/waiter information to the callback
> > we're adding? That way drivers can inspect it whenever they feel like and
> > don't have to duplicate the tracking. And it's probably easier to
> > tune/adjust to the myriads of use-cases (flip target miss, userspace wait,
> > wakeup boost maybe too ...).
>
> You mean, enumerate the types of deadline?
>
> For userspace waits, we might have a timeout, but not really
> (currently) any more information than that? The vblank deadline is
> the only type of deadline that seems pretty clear to me.
>
> I suppose we could do something like:
>
> dma_fence_set_deadline(fence, &(struct dma_fence_deadline){
> .type = DMA_FENCE_DEADLINE_VBLANK,
> .time = next_vblank_ktime,
> });
>
> to make it a bit more extensible to add more deadline types or
> additional optional information

Nah not enumerate the types of deadlines, but the types of waits. Some of
which might have a deadline (like page flip), some wont (like userspace
waiting or poll() on a dma-fd or whatever).

What I had in mind is roughly


diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index 6ffb4b2c6371..e7c239145273 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -116,6 +116,8 @@ typedef void (*dma_fence_func_t)(struct dma_fence *fence,
struct dma_fence_cb {
struct list_head node;
dma_fence_func_t func;
+ enume dma_fence_wait_type wait_type;
+ struct ktime deadline; /* fixme how do we indicate no deadline? */
};

/**

With that waiters, and irrespective of whether they use dma_fence_wait or
have something else like the dma-buf fd poll stuff, can indicate to the
driver what kind of wait with what kind of deadline this is.

Maybe we should make this a sub-struct, so that it can also be passed to
dma_fence_wait().
-Daniel

>
> BR,
> -R
>
> >
> > I like this direction a lot more than what we discussed with post-miss
> > hints thus far.
> >
> > > It's probably a bit impossible for a gpu driver to really predict how
> > > long some rendering will take, but other cases like video decoder are
> > > somewhat more predictable.. the fence provider could predict given the
> > > remaining time until the deadline what clk rates are required to get
> > > you there.
> >
> > Well if we do have a deadline the driver can note that in its scheduler
> > and arm a driver to kick the clocks. Or maybe use past history to do this
> > upfront.
> > -Daniel
> >
> > >
> > > BR,
> > > -R
> > >
> > >
> > > >
> > > > Otherwise I really like this, I think it's about the only real reason i915
> > > > isn't using atomic helpers.
> > > >
> > > > Also adding Matt B for this topic.
> > > > -Daniel
> > > >
> > > > > +
> > > > > dma_fence_put(new_plane_state->fence);
> > > > > new_plane_state->fence = NULL;
> > > > > }
> > > > > --
> > > > > 2.30.2
> > > > >
> > > >
> > > > --
> > > > Daniel Vetter
> > > > Software Engineer, Intel Corporation
> > > > http://blog.ffwll.ch
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch