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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Tue, 20 Jul 2021 22:55:39 +0200
Message-ID: <CAKMK7uF1=Y6_9znGoWG8GrteXBBRmyW8C3bFE+eJQqOj0A1buA@mail.gmail.com>
Subject: Re: [Linaro-mm-sig] [PATCH] drm/msm: Add fence->wait() op
To:     Rob Clark <robdclark@gmail.com>
Cc:     =?UTF-8?Q?Christian_K=C3=B6nig?= <ckoenig.leichtzumerken@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Rob Clark <robdclark@chromium.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@lists.freedesktop.org>, David Airlie <airlied@linux.ie>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>, Sean Paul <sean@poorly.run>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>
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On Tue, Jul 20, 2021 at 8:26 PM Rob Clark <robdclark@gmail.com> wrote:
>
> On Tue, Jul 20, 2021 at 11:03 AM Christian K=C3=B6nig
> <ckoenig.leichtzumerken@gmail.com> wrote:
> >
> > Hi Rob,
> >
> > Am 20.07.21 um 17:07 schrieb Rob Clark:
> > > From: Rob Clark <robdclark@chromium.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.

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@chromium.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/ms=
m_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_fen=
ce_context *fctx, uint32_t fenc
> > >       return (int32_t)(fctx->completed_fence - fence) >=3D 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 =3D fence_completed(fctx, fence) ? 0 : -EBUSY;
> > > +     if (interruptible) {
> > > +             ret =3D wait_event_interruptible_timeout(fctx->event,
> > > +                     fence_completed(fctx, fence),
> > > +                     remaining_jiffies);
> > >       } else {
> > > -             unsigned long remaining_jiffies =3D timeout_to_jiffies(=
timeout);
> > > -
> > > -             if (interruptible)
> > > -                     ret =3D wait_event_interruptible_timeout(fctx->=
event,
> > > -                             fence_completed(fctx, fence),
> > > -                             remaining_jiffies);
> > > -             else
> > > -                     ret =3D wait_event_timeout(fctx->event,
> > > -                             fence_completed(fctx, fence),
> > > -                             remaining_jiffies);
> > > -
> > > -             if (ret =3D=3D 0) {
> > > -                     DBG("timeout waiting for fence: %u (completed: =
%u)",
> > > -                                     fence, fctx->completed_fence);
> > > -                     ret =3D -ETIMEDOUT;
> > > -             } else if (ret !=3D -ERESTARTSYS) {
> > > -                     ret =3D 0;
> > > -             }
> > > +             ret =3D wait_event_timeout(fctx->event,
> > > +                     fence_completed(fctx, fence),
> > > +                     remaining_jiffies);
> > > +     }
> > > +
> > > +     if (ret =3D=3D 0) {
> > > +             DBG("timeout waiting for fence: %u (completed: %u)",
> > > +                             fence, fctx->completed_fence);
> > > +             ret =3D -ETIMEDOUT;
> > > +     } else if (ret !=3D -ERESTARTSYS) {
> > > +             ret =3D 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), int=
erruptible);
> > > +}
> > > +
> > >   /* called from workqueue */
> > >   void msm_update_fence(struct msm_fence_context *fctx, uint32_t fenc=
e)
> > >   {
> > > @@ -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 =3D to_msm_fence(fence);
> > > +
> > > +     return wait_fence(f->fctx, fence->seqno, timeout, intr);
> > > +}
> > > +
> > >   static const struct dma_fence_ops msm_fence_ops =3D {
> > >       .get_driver_name =3D msm_fence_get_driver_name,
> > >       .get_timeline_name =3D msm_fence_get_timeline_name,
> > >       .signaled =3D msm_fence_signaled,
> > > +     .wait =3D msm_fence_wait,
> > >   };
> > >
> > >   struct dma_fence *
> >


--=20
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch