Re: [Linaro-mm-sig] [PATCH] drm/msm: Add fence->wait() op

[Daniel Vetter]

On Wed, Jul 21, 2021 at 12:32 AM Rob Clark <robdclark@xxxxxxxxx> wrote:
>
> On Tue, Jul 20, 2021 at 1:55 PM Daniel Vetter <daniel@xxxxxxxx> wrote:
> >
> > On Tue, Jul 20, 2021 at 8:26 PM Rob Clark <robdclark@xxxxxxxxx> wrote:
> > >
> > > On Tue, Jul 20, 2021 at 11:03 AM Christian König
> > > <ckoenig.leichtzumerken@xxxxxxxxx> wrote:
> > > >
> > > > Hi Rob,
> > > >
> > > > Am 20.07.21 um 17:07 schrieb Rob Clark:
> > > > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > >
> > > > > Somehow we had neither ->wait() nor dma_fence_signal() calls, and no
> > > > > one noticed.  Oops.
> > > >
> > > >
> > > > I'm not sure if that is a good idea.
> > > >
> > > > The dma_fence->wait() callback is pretty much deprecated and should not
> > > > be used any more.
> > > >
> > > > What exactly do you need that for?
> > >
> > > Well, the alternative is to track the set of fences which have
> > > signalling enabled, and then figure out which ones to signal, which
> > > seems like a lot more work, vs just re-purposing the wait
> > > implementation we already have for non-dma_fence cases ;-)
> > >
> > > Why is the ->wait() callback (pretty much) deprecated?
> >
> > Because if you need it that means for your driver dma_fence_add_cb is
> > broken, which means a _lot_ of things don't work. Like dma_buf poll
> > (compositors have patches to start using that), and I think
> > drm/scheduler also becomes rather unhappy.
>
> I'm starting to page back in how this works.. fence cb's aren't broken
> (which is also why dma_fence_wait() was not completely broken),
> because in retire_submits() we call
> dma_fence_is_signaled(submit->hw_fence).
>
> But the reason that the custom wait function cleans up a tiny bit of
> jank is that the wait_queue_head_t gets signaled earlier, before we
> start iterating the submits and doing all that retire_submit() stuff
> (unpin/unref bo's, etc).  I suppose I could just split things up to
> call dma_fence_signal() earlier, and *then* do the retire_submits()
> stuff.

Yeah reducing the latency there sounds like a good idea.
-Daniel

>
> BR,
> -R
>
> > It essentially exists only for old drivers where ->enable_signalling
> > is unreliable and we paper over that with a retry loop in ->wait and
> > pray no one notices that it's too butchered. The proper fix is to have
> > a driver thread to guarantee that ->enable_signalling works reliable,
> > so you don't need a ->wait.
> >
> > Can you type up a kerneldoc patch for dma_fence_ops->wait to hammer
> > this in please?
> > -Daniel
> >
> > >
> > > BR,
> > > -R
> > >
> > > > Regards,
> > > > Christian.
> > > >
> > > > >
> > > > > Note that this removes the !timeout case, which has not been used in
> > > > > a long time.
> > > >
> > > >
> > > > >
> > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > > > ---
> > > > >   drivers/gpu/drm/msm/msm_fence.c | 59 +++++++++++++++++++--------------
> > > > >   1 file changed, 34 insertions(+), 25 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/msm/msm_fence.c b/drivers/gpu/drm/msm/msm_fence.c
> > > > > index cd59a5918038..8ee96b90ded6 100644
> > > > > --- a/drivers/gpu/drm/msm/msm_fence.c
> > > > > +++ b/drivers/gpu/drm/msm/msm_fence.c
> > > > > @@ -38,11 +38,10 @@ static inline bool fence_completed(struct msm_fence_context *fctx, uint32_t fenc
> > > > >       return (int32_t)(fctx->completed_fence - fence) >= 0;
> > > > >   }
> > > > >
> > > > > -/* legacy path for WAIT_FENCE ioctl: */
> > > > > -int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
> > > > > -             ktime_t *timeout, bool interruptible)
> > > > > +static signed long wait_fence(struct msm_fence_context *fctx, uint32_t fence,
> > > > > +             signed long remaining_jiffies, bool interruptible)
> > > > >   {
> > > > > -     int ret;
> > > > > +     signed long ret;
> > > > >
> > > > >       if (fence > fctx->last_fence) {
> > > > >               DRM_ERROR_RATELIMITED("%s: waiting on invalid fence: %u (of %u)\n",
> > > > > @@ -50,33 +49,34 @@ int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
> > > > >               return -EINVAL;
> > > > >       }
> > > > >
> > > > > -     if (!timeout) {
> > > > > -             /* no-wait: */
> > > > > -             ret = fence_completed(fctx, fence) ? 0 : -EBUSY;
> > > > > +     if (interruptible) {
> > > > > +             ret = wait_event_interruptible_timeout(fctx->event,
> > > > > +                     fence_completed(fctx, fence),
> > > > > +                     remaining_jiffies);
> > > > >       } else {
> > > > > -             unsigned long remaining_jiffies = timeout_to_jiffies(timeout);
> > > > > -
> > > > > -             if (interruptible)
> > > > > -                     ret = wait_event_interruptible_timeout(fctx->event,
> > > > > -                             fence_completed(fctx, fence),
> > > > > -                             remaining_jiffies);
> > > > > -             else
> > > > > -                     ret = wait_event_timeout(fctx->event,
> > > > > -                             fence_completed(fctx, fence),
> > > > > -                             remaining_jiffies);
> > > > > -
> > > > > -             if (ret == 0) {
> > > > > -                     DBG("timeout waiting for fence: %u (completed: %u)",
> > > > > -                                     fence, fctx->completed_fence);
> > > > > -                     ret = -ETIMEDOUT;
> > > > > -             } else if (ret != -ERESTARTSYS) {
> > > > > -                     ret = 0;
> > > > > -             }
> > > > > +             ret = wait_event_timeout(fctx->event,
> > > > > +                     fence_completed(fctx, fence),
> > > > > +                     remaining_jiffies);
> > > > > +     }
> > > > > +
> > > > > +     if (ret == 0) {
> > > > > +             DBG("timeout waiting for fence: %u (completed: %u)",
> > > > > +                             fence, fctx->completed_fence);
> > > > > +             ret = -ETIMEDOUT;
> > > > > +     } else if (ret != -ERESTARTSYS) {
> > > > > +             ret = 0;
> > > > >       }
> > > > >
> > > > >       return ret;
> > > > >   }
> > > > >
> > > > > +/* legacy path for WAIT_FENCE ioctl: */
> > > > > +int msm_wait_fence(struct msm_fence_context *fctx, uint32_t fence,
> > > > > +             ktime_t *timeout, bool interruptible)
> > > > > +{
> > > > > +     return wait_fence(fctx, fence, timeout_to_jiffies(timeout), interruptible);
> > > > > +}
> > > > > +
> > > > >   /* called from workqueue */
> > > > >   void msm_update_fence(struct msm_fence_context *fctx, uint32_t fence)
> > > > >   {
> > > > > @@ -114,10 +114,19 @@ static bool msm_fence_signaled(struct dma_fence *fence)
> > > > >       return fence_completed(f->fctx, f->base.seqno);
> > > > >   }
> > > > >
> > > > > +static signed long msm_fence_wait(struct dma_fence *fence, bool intr,
> > > > > +             signed long timeout)
> > > > > +{
> > > > > +     struct msm_fence *f = to_msm_fence(fence);
> > > > > +
> > > > > +     return wait_fence(f->fctx, fence->seqno, timeout, intr);
> > > > > +}
> > > > > +
> > > > >   static const struct dma_fence_ops msm_fence_ops = {
> > > > >       .get_driver_name = msm_fence_get_driver_name,
> > > > >       .get_timeline_name = msm_fence_get_timeline_name,
> > > > >       .signaled = msm_fence_signaled,
> > > > > +     .wait = msm_fence_wait,
> > > > >   };
> > > > >
> > > > >   struct dma_fence *
> > > >
> >
> >
> >
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch



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