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

[Marius Vlad]

On Wed, Sep 23, 2020 at 12:58:30PM +0200, Daniel Vetter wrote:
> 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.
> >
> > 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.
> 
> On which driver/chip does this happen?
I've tried it out on i915.
> -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
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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