[PATCH v2] drm/syncobj: ensure progress for syncobj queries

[Chia-I Wu]

On Tue, Oct 22, 2024 at 10:24 AM Chia-I Wu <olvaffe at gmail.com> wrote:
>
> On Tue, Oct 22, 2024 at 9:53 AM Christian König
> <christian.koenig at amd.com> wrote:
> >
> > Am 22.10.24 um 18:18 schrieb Chia-I Wu:
> > > Userspace might poll a syncobj with the query ioctl.  Call
> > > dma_fence_enable_sw_signaling to ensure dma_fence_is_signaled returns
> > > true in finite time.
> >
> > Wait a second, just querying the fence status is absolutely not
> > guaranteed to return true in finite time. That is well documented on the
> > dma_fence() object.
> >
> > When you want to poll on signaling from userspace you really need to
> > call poll or the wait IOCTL with a zero timeout. That will also return
> > immediately but should enable signaling while doing that.
> >
> > So just querying the status should absolutely *not* enable signaling.
> > That's an intentional separation.
> I think it depends on what semantics DRM_IOCTL_SYNCOBJ_QUERY should have.
>
> If DRM_IOCTL_SYNCOBJ_QUERY is mainly for vulkan timeline semaphores,
> it is a bit heavy if userspace has to do a
> DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT before a query.

I filed a Mesa issue,
https://gitlab.freedesktop.org/mesa/mesa/-/issues/12094, and Faith
suggested a kernel-side fix as well.  Should we reconsider this?