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Date:   Thu, 22 Jul 2021 11:08:27 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König 
        <ckoenig.leichtzumerken@...il.com>
Cc:     Rob Clark <robdclark@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>, David Airlie <airlied@...ux.ie>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Christian König <christian.koenig@....com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Sean Paul <sean@...rly.run>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH] drm/msm: Add fence->wait() op

On Thu, Jul 22, 2021 at 10:42:26AM +0200, Christian König wrote:
> Am 21.07.21 um 21:03 schrieb Daniel Vetter:
> > On Wed, Jul 21, 2021 at 09:34:43AM -0700, Rob Clark wrote:
> > > On Wed, Jul 21, 2021 at 12:59 AM Daniel Vetter <daniel@...ll.ch> wrote:
> > > > On Wed, Jul 21, 2021 at 12:32 AM Rob Clark <robdclark@...il.com> wrote:
> > > > > On Tue, Jul 20, 2021 at 1:55 PM Daniel Vetter <daniel@...ll.ch> wrote:
> > > > > > On Tue, Jul 20, 2021 at 8:26 PM Rob Clark <robdclark@...il.com> wrote:
> > > > > > > On Tue, Jul 20, 2021 at 11:03 AM Christian König
> > > > > > > <ckoenig.leichtzumerken@...il.com> wrote:
> > > > > > > > Hi Rob,
> > > > > > > > 
> > > > > > > > Am 20.07.21 um 17:07 schrieb Rob Clark:
> > > > > > > > > From: Rob Clark <robdclark@...omium.org>
> > > > > > > > > 
> > > > > > > > > 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
> > > > 
> > > Hmm, no, turns out that isn't the problem.. or, well, it is probably a
> > > good idea to call drm_fence_signal() earlier.  But it seems like
> > > waking up from wait_event_* is faster than wake_up_state(wait->task,
> > > TASK_NORMAL).  I suppose the wake_up_state() approach still needs for
> > > the scheduler to get around to schedule the runnable task.
> 
> As far as I know wake_up_state() tries to run the thread on the CPU it was
> scheduled last, while wait_event_* makes the thread run on the CPU who
> issues the wake by default.
> 
> And yes I've also noticed this already and it was one of the reason why I
> suggested to use a wait_queue instead of the hand wired dma_fence_wait
> implementation.

The first versions had used wait_queue, but iirc we had some issues with
the callbacks and stuff and that was the reasons for hand-rolling. Or
maybe it was the integration of the lockless fastpath for
dma_fence_is_signalled().

> > > So for now, I'm going back to my own wait function (plus earlier
> > > drm_fence_signal())
> > > 
> > > Before removing dma_fence_opps::wait(), I guess we want to re-think
> > > dma_fence_default_wait().. but I think that would require a
> > > dma_fence_context base class (rather than just a raw integer).
> > Uh that's not great ... can't we fix this instead of papering over it in
> > drivers? Aside from maybe different wakeup flags it all is supposed to
> > work exactly the same underneath, and whether using a wait queue or not
> > really shouldn't matter.
> 
> Well it would have been nicer if we used the existing infrastructure instead
> of re-inventing stuff for dma_fence, but that chance is long gone.
> 
> And you don't need a dma_fence_context base class, but rather just a flag in
> the dma_fence_ops if you want to change the behavior.

If there's something broken we should just fix it, not force everyone to
set a random flag. dma_fence work like special wait_queues, so if we
differ then we should go back to that.
-Daniel

> 
> Regards,
> Christian.
> 
> > -Daniel
> > 
> > > BR,
> > > -R
> > > 
> > > > > 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@...omium.org>
> > > > > > > > > ---
> > > > > > > > >    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
> 

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

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