Re: [PATCH v5 02/20] drm/msm: Fix drm/sched point of no return rules

[Rob Clark <robdclark@gmail.com>]

On Fri, Aug 6, 2021 at 9:42 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> On Fri, Aug 6, 2021 at 12:58 AM Rob Clark <robdclark@gmail.com> wrote:
> >
> > On Thu, Aug 5, 2021 at 3:47 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > >
> > > Originally drm_sched_job_init was the point of no return, after which
> > > drivers must submit a job. I've split that up, which allows us to fix
> > > this issue pretty easily.
> > >
> > > Only thing we have to take care of is to not skip to error paths after
> > > that. Other drivers do this the same for out-fence and similar things.
> > >
> > > Fixes: 1d8a5ca436ee ("drm/msm: Conversion to drm scheduler")
> > > Cc: Rob Clark <robdclark@chromium.org>
> > > Cc: Rob Clark <robdclark@gmail.com>
> > > Cc: Sean Paul <sean@poorly.run>
> > > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > > Cc: "Christian König" <christian.koenig@amd.com>
> > > Cc: linux-arm-msm@vger.kernel.org
> > > Cc: dri-devel@lists.freedesktop.org
> > > Cc: freedreno@lists.freedesktop.org
> > > Cc: linux-media@vger.kernel.org
> > > Cc: linaro-mm-sig@lists.linaro.org
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > ---
> > >  drivers/gpu/drm/msm/msm_gem_submit.c | 15 +++++++--------
> > >  1 file changed, 7 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
> > > index 6d6c44f0e1f3..d0ed4ddc509e 100644
> > > --- a/drivers/gpu/drm/msm/msm_gem_submit.c
> > > +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
> > > @@ -52,9 +52,6 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
> > >                 return ERR_PTR(ret);
> > >         }
> > >
> > > -       /* FIXME: this is way too early */
> > > -       drm_sched_job_arm(&job->base);
> > > -
> > >         xa_init_flags(&submit->deps, XA_FLAGS_ALLOC);
> > >
> > >         kref_init(&submit->ref);
> > > @@ -883,6 +880,9 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> > >
> > >         submit->user_fence = dma_fence_get(&submit->base.s_fence->finished);
> > >
> > > +       /* point of no return, we _have_ to submit no matter what */
> > > +       drm_sched_job_arm(&submit->base);
> > > +
> > >         /*
> > >          * Allocate an id which can be used by WAIT_FENCE ioctl to map back
> > >          * to the underlying fence.
> > > @@ -892,17 +892,16 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> > >         if (submit->fence_id < 0) {
> > >                 ret = submit->fence_id = 0;
> > >                 submit->fence_id = 0;
> > > -               goto out;
> > >         }
> > >
> > > -       if (args->flags & MSM_SUBMIT_FENCE_FD_OUT) {
> > > +       if (ret == 0 && args->flags & MSM_SUBMIT_FENCE_FD_OUT) {
> > >                 struct sync_file *sync_file = sync_file_create(submit->user_fence);
> > >                 if (!sync_file) {
> > >                         ret = -ENOMEM;
> > > -                       goto out;
> > > +               } else {
> > > +                       fd_install(out_fence_fd, sync_file->file);
> > > +                       args->fence_fd = out_fence_fd;
> > >                 }
> > > -               fd_install(out_fence_fd, sync_file->file);
> > > -               args->fence_fd = out_fence_fd;
> >
> > I wonder if instead we should (approximately) undo "drm/msm/submit:
> > Simplify out-fence-fd handling" so that the point that it could fail
> > is moved up ahead of the drm_sched_job_arm()?
>
> Hm yeah. Up to you how you want to paint this shed, I think either is fine.
>
> > Also, does the dma_fence_get() work before drm_sched_job_arm()?  From
> > a quick look, it looks like it won't, but I'm still playing catchup
> > and haven't had a chance to look at your entire series.  If it doesn't
> > work before drm_sched_job_arm(), then there is really no way to
> > prevent a error path between the fence-init and job-submit.
>
> Yes. I thought I've checked that I put the _arm() in the right spot,
> but I guess I screwed up and you need the fence before the point where
> I've put the job_arm()? And yes the error path cannot be avoided for
> out-fences, that's what I tried to explain in the commit message.
>
> > But, prior to your series, wouldn't a failure after
> > drm_sched_job_init() but before the job is submitted just burn a
> > fence-id, and otherwise carry on it's merry way?
>
> Maybe? I'm not sure whether the scheduler gets confused about the gap
> and freak out abou that. I'm fairly new to that code and learning
> (which is part why I'm working on it). Since you look up in
> fences/syncobj after job_init() it should be pretty easy to whip up a
> testcase and see what happens. Also as long as nothing fails you won't
> see an issue, that's for sure.

fair.. I'll try to come up with a test case.. pre-scheduler-conversion
it wasn't a problem to fail after the fence seqno was allocated (well,
I guess you might have problems if you had 2^31 failures in a row)

BR,
-R

> -Daniel
>
> > BR,
> > -R
> >
> > >         }
> > >
> > >         submit_attach_object_fences(submit);
> > > --
> > > 2.32.0
> > >
>
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch