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To:     Alex Deucher <alexdeucher@gmail.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Zhenneng Li <lizhenneng@kylinos.cn>,
        Alex Deucher <alexander.deucher@amd.com>,
        "Pan, Xinhui" <Xinhui.Pan@amd.com>,
        David Airlie <airlied@linux.ie>,
        amd-gfx list <amd-gfx@lists.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>
References: <20210802074310.1526526-1-lizhenneng@kylinos.cn>
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From:   =?UTF-8?Q?Michel_D=c3=a4nzer?= <michel@daenzer.net>
Subject: Re: [PATCH] drm/radeon: Update pitch for page flip
Message-ID: <6a34fcc3-0aa3-85ff-21c4-86b9b5a48fd5@daenzer.net>
Date:   Tue, 3 Aug 2021 10:34:06 +0200
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On 2021-08-02 4:51 p.m., Alex Deucher wrote:
> On Mon, Aug 2, 2021 at 4:31 AM Daniel Vetter <daniel@ffwll.ch> 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