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

[Michel Dänzer]

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).


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