[PATCH] amd/display: remove ChromeOS workaround

[Simon Ser]

Hi again,

On Tuesday, October 19th, 2021 at 10:25, Paul Menzel <pmenzel at molgen.mpg.de> wrote:

> Dear Simon,
>
>
> Am 19.10.21 um 10:10 schrieb Simon Ser:
> > On Tuesday, October 19th, 2021 at 01:21, Paul Menzel <pmenzel at molgen.mpg.de> wrote:
> >
> >> Am 19.10.21 um 01:06 schrieb Simon Ser:
> >>> On Tuesday, October 19th, 2021 at 01:03, Paul Menzel wrote:
> >>>
> >>>> Excuse my ignorance. Reading the commit message, there was a Linux
> >>>> kernel change, that broke Chrome OS userspace, right? If so, and we do
> >>>> not know if there is other userspace using the API incorrectly,
> >>>> shouldn’t the patch breaking Chrome OS userspace be reverted to adhere
> >>>> to Linux’ no-regression rule?
> >>>
> >>> No. There was a ChromeOS bug which has been thought to be an amdgpu bug. But
> >>> fixing that "bug" breaks other user-space.
> >>
> >> Thank you for the explanation. I guess the bug was only surfacing
> >> because Chrome OS device, like Chromebooks, are only using AMD hardware
> >> since a short while (maybe last year).
> >>
> >> Reading your message *amdgpu: atomic API and cursor/overlay planes* [1]
> >> again, it says:
> >>
> >>> Up until now we were using cursor and overlay planes in gamescope [3],
> >>> but some changes in the amdgpu driver [1] makes us unable to use planes
> >>
> >> So this statement was incorrect? Which changes are that? Or did Chrome
> >> OS ever work correctly with an older Linux kernel or not?
> >
> > The sequence of events is as follows:
> >
> > - gamescope can use cursor and overlay planes.
> > - ChromeOS-specific commit lands, fixing some ChromeOS issues related to video
> >    playback. This breaks gamescope overlays.
>
> I guess, I am confused, which Chrome OS specific commit that is. Is it
> one of the reverted commits below? Which one?
>
> 1.  ddab8bd788f5 ("drm/amd/display: Fix two cursor duplication
> when using overlay")
> 2.  e7d9560aeae5 ("Revert "drm/amd/display: Fix overlay validation by
> considering cursors"")

ddab8bd788f5 ("drm/amd/display: Fix two cursor duplication when using overlay")
is the commit which introduced the validate_overlay logic fixing ChromeOS and
breaking gamescope.

Later, 33f409e60eb0 ("drm/amd/display: Fix overlay validation by considering
cursors") relaxed validate_overlay. This breaks ChromeOS and partially fixes
gamescope (when the overlay is used and the cursor plane is unused).

Finally, e7d9560aeae5 ("Revert "drm/amd/display: Fix overlay validation by
considering cursors"") has reverted that change, fixing ChromeOS (again) and
breaking gamescope completely again.

> > - Discussion to restrict the ChromeOS-specific logic to ChromeOS, or to revert
> >    it, either of these fix gamescope.
> >
> > Given this, I don't see how the quoted statement is incorrect? Maybe I'm
> > missing something?
>
> Your reply from August 2021 to commit ddab8bd788f5 (drm/amd/display: Fix
> two cursor duplication when using overlay) from April 2021 [2]:
>
> > Hm. This patch causes a regression for me. I was using primary + overlay
> > not covering the whole primary plane + cursor before. This patch breaks it.
> >
> > This patch makes the overlay plane very useless for me, because the primary
> > plane is always under the overlay plane.
>
> So, I would have thought, everything worked fine before some Linux
> kernel commit changed behavior, and regressed userspace.

I've tried to explain the full story above. My user-space went from working to
broken to partially broken to broken. The quoted reply is a complaint that the
commit flipped gamescope from partially broken to completely broken. At the
time I didn't realize that ddab8bd788f5 caused some pain too.

Does that clear things up?