Re: [PATCH] drm/radeon: Update pitch for page flip

[Daniel Vetter]

On Tue, Aug 03, 2021 at 10:49:39AM -0400, Alex Deucher wrote:
> On Tue, Aug 3, 2021 at 4:34 AM Michel Dänzer <michel@xxxxxxxxxxx> wrote:
> >
> > On 2021-08-02 4:51 p.m., Alex Deucher wrote:
> > > On Mon, Aug 2, 2021 at 4:31 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >>
> > >> On Mon, Aug 02, 2021 at 10:12:47AM +0200, Christian König wrote:
> > >>> Am 02.08.21 um 09:43 schrieb Zhenneng Li:
> > >>>> When primary bo is updated, crtc's pitch may
> > >>>> have not been updated, this will lead to show
> > >>>> disorder content when user changes display mode,
> > >>>> we update crtc's pitch in page flip to avoid
> > >>>> this bug.
> > >>>> This refers to amdgpu's pageflip.
> > >>>
> > >>> Alex is the expert to ask about that code, but I'm not sure if that is
> > >>> really correct for the old hardware.
> > >>>
> > >>> As far as I know the crtc's pitch should not change during a page flip, but
> > >>> only during a full mode set.
> > >>>
> > >>> So could you elaborate a bit more how you trigger this?
> > >>
> > >> legacy page_flip ioctl only verifies that the fb->format stays the same.
> > >> It doesn't check anything else (afair never has), this is all up to
> > >> drivers to verify.
> > >>
> > >> Personally I'd say add a check to reject this, since testing this and
> > >> making sure it really works everywhere is probably a bit much on this old
> > >> hw.
> > >
> > > If just the pitch changed, that probably wouldn't be much of a
> > > problem, but if the pitch is changing, that probably implies other
> > > stuff has changed as well and we'll just be chasing changes.  I agree
> > > it would be best to just reject anything other than updating the
> > > scanout address.
> >
> > FWIW, that means page flipping cannot be used in some cases which work fine by changing the pitch, which can result in tearing: https://gitlab.freedesktop.org/xorg/xserver/-/issues/839 (which says the i915 driver handles this as well).
> >
> 
> Ok.  In that case, @Zhenneng can you update all of the pitch in all of
> the page_flip functions in radeon rather than just the evergreen one?

atomic drivers handle this fairly ok-ish, since we have a proper
atomic_check there.

For legacy kms I just wouldn't bother, too many corners to validate. But
also if you're bored, just do it :-)
-Daniel

> 
> Thanks,
> 
> Alex
> 
> 
> >
> > --
> > Earthling Michel Dänzer               |               https://redhat.com
> > Libre software enthusiast             |             Mesa and X developer

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