[PATCH] drm: avoid spurious EBUSY due to nonblocking atomic modesets

[Daniel Vetter]

On Tue, Sep 22, 2020 at 3:36 PM Marius Vlad <marius.vlad at collabora.com> wrote:
>
> On Fri, Jan 31, 2020 at 07:34:00AM +0000, Daniel Stone wrote:
> > On Thu, 5 Jul 2018 at 11:21, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > > When doing an atomic modeset with ALLOW_MODESET drivers are allowed to
> > > pull in arbitrary other resources, including CRTCs (e.g. when
> > > reconfiguring global resources).
> > >
> > > But in nonblocking mode userspace has then no idea this happened,
> > > which can lead to spurious EBUSY calls, both:
> > > - when that other CRTC is currently busy doing a page_flip the
> > >   ALLOW_MODESET commit can fail with an EBUSY
> > > - on the other CRTC a normal atomic flip can fail with EBUSY because
> > >   of the additional commit inserted by the kernel without userspace's
> > >   knowledge
> > >
> > > For blocking commits this isn't a problem, because everyone else will
> > > just block until all the CRTC are reconfigured. Only thing userspace
> > > can notice is the dropped frames without any reason for why frames got
> > > dropped.
> > >
> > > Consensus is that we need new uapi to handle this properly, but no one
> > > has any idea what exactly the new uapi should look like. As a stop-gap
> > > plug this problem by demoting nonblocking commits which might cause
> > > issues by including CRTCs not in the original request to blocking
> > > commits.
> Gentle ping. I've tried out Linus's master tree and, and like Pekka,
> I've noticed this isn't integrated/added.

Defacto the uapi we have now is that userspace needs to ignore "spurious" EBUSY.

> Noticed this is fixing (also) DPMS when multiple outputs are in use.
> Wondering if we can just use a _ONCE() variant instead of WARN_ON(). I'm seeing
> the warning quite often.

This would be a driver bug I think. That really shouldn't happen for
normal page flips.
-Daniel

> >
> > Thanks for writing this up Daniel, and for reminding me about it some
> > time later as well ...
> >
> > Reviewed-by: Daniel Stone <daniels at collabora.com>
> >
> > Cheers,
> > Daniel
> > _______________________________________________
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> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel



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