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

[Marius Vlad]

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.

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.

> 
> 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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