linux-kernel - Re: [PATCH 0/6] drm: Allow the damage helpers to handle buffer damage

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Message-ID: <87ttpo5hvs.fsf@minerva.mail-host-address-is-not-set>
Date:   Tue, 14 Nov 2023 19:06:47 +0100
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Cc:     Pekka Paalanen <pekka.paalanen@...labora.com>,
        dri-devel@...ts.freedesktop.org, Jonathan Corbet <corbet@....net>,
        Bilal Elmoussaoui <belmouss@...hat.com>,
        linux-doc@...r.kernel.org, Maxime Ripard <mripard@...nel.org>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        VMware Graphics Reviewers 
        <linux-graphics-maintainer@...are.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Sima Vetter <daniel.vetter@...ll.ch>,
        David Airlie <airlied@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Erico Nunes <nunes.erico@...il.com>
Subject: Re: [PATCH 0/6] drm: Allow the damage helpers to handle buffer damage

Thomas Zimmermann <tzimmermann@...e.de> writes:

Hello Thomas,

> Hi

[...]

>>> And why does it flicker? Is there old data stored somewhere?
>>>
>> 
>> It flickers because the framebuffer changed and so the damage tracking
>> is not used correctly to flush the damaged areas to the backing storage.
>
> I think I got it from the links in patch 5.  In out other drivers, 
> there's a single backing storage for each plane (for example in the 
> video memory). Here, there's a backing storage for each buffer. On page

Correct, that's what I understood too.

> flips, the plane changes its backing storage.  Our GEM buffer is up to 
> date, but the respective backing storage is missing all the intermediate 
> changes.
>
> If I'm not mistaken, an entirely different solution would be to 
> implement a per-plane back storage in these drivers.
>

I believe so but I'm not sure if that's possible since the virtio-gpu spec
defines that the VM should send a VIRTIO_GPU_CMD_RESOURCE_FLUSH to the VMM
in the host to do an update and the granularity for that is a framebuffer.

For that reason the only solution (other than forcing a full plane update
like this patch-set does) is to implement tracking suppor for buffer damage.

> Best regards
> Thomas
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat