[Bug 91831] New: Corrupted video output with 32bit kernel

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91831

            Bug ID: 91831
           Summary: Corrupted video output with 32bit kernel
           Product: xorg
           Version: git
          Hardware: x86 (IA32)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/AMDgpu
          Assignee: xorg-driver-ati at lists.x.org
          Reporter: nholland at tisys.org
        QA Contact: xorg-team at lists.x.org

I'm not entirely sure if this is the right place to mention this issue, but I'm
fairly confident that the right people will see it here:

I'm using a Kabini GPU on a 32-bit (kernel + userland) system using linux 4.2
and have just decided to give the amdgpu kernel drm driver and
xf86-video-amdgpu a try (previously I was using radeon). What I noticed is that
screen output in X11 is garbled using these drivers - i.e. pixels are messed up
/ missing / corrupt in various places all over the screen.

I've seen this issue before in the radeon driver, there's actually a (resolved)
bug report about it at https://bugs.freedesktop.org/show_bug.cgi?id=84627.

Back then, the bug was fixed in the kernel in
linux/drivers/gpu/drm/radeon/radeon_object.c. Here's the relevant section (from
radeon_bo_create()):

#ifdef CONFIG_X86_32
        /* XXX: Write-combined CPU mappings of GTT seem broken on 32-bit
         * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
         */
        bo->flags &= ~RADEON_GEM_GTT_WC;

I'm not into this stuff enough, but I believe that in contrast to the radeon
driver, the amdgpu driver does not disable write-combined CPU mappings on 32
bit kernels. The question is: Would it be possible and reasonable to change
amdgpu to follow radeon in this regard and thus - hopefully - fix the garbled
screen problem on 32 bit?

As a side note, the problem goes away when disabling hardware acceleration:

Option "NoAccel" "true"

Screen output looks good then. Still, this is probably not the best solution.

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