linux-kernel - Re: [PATH 0/4] [RFC] Support virtual DRM

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Date:   Wed, 23 Jun 2021 10:04:47 +0200
From:   Michel Dänzer <michel@...nzer.net>
To:     Pekka Paalanen <ppaalanen@...il.com>,
        Esaki Tomohito <etom@...l.co.jp>
Cc:     devicetree@...r.kernel.org, Takanari Hayama <taki@...l.co.jp>,
        linux-doc@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-renesas-soc@...r.kernel.org,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>
Subject: Re: [PATH 0/4] [RFC] Support virtual DRM

On 2021-06-22 9:57 a.m., Pekka Paalanen wrote:
> On Tue, 22 Jun 2021 13:02:59 +0900
> Esaki Tomohito <etom@...l.co.jp> wrote:
> 
>> Hi, Thomas
>> Thank you for reply.
>>
>> On 2021/06/21 16:10, Thomas Zimmermann wrote:
>>> Hi
>>>
>>> Am 21.06.21 um 08:27 schrieb Tomohito Esaki:  
>>>> Virtual DRM splits the overlay planes of a display controller into
>>>> multiple
>>>> virtual devices to allow each plane to be accessed by each process.
>>>>
>>>> This makes it possible to overlay images output from multiple
>>>> processes on a
>>>> display. For example, one process displays the camera image without
>>>> compositor
>>>> while another process overlays the UI.  
>>>
>>> I briefly looked over your patches. I didn't understand how this is
>>> different to the functionality of a compositor? Shouldn't this be solved
>>> in userspace?  
>>
>> I think when latency is important (e.g., AR, VR, for displaying camera
>> images in IVI systems), there may be use cases where the compositor
>> cannot be used.
> 
> Hi,
> 
>> Normally, when the image is passed through the compositor, it is
>> displayed after 2 VSYNC at most, because the compositor combines the
>> image with VSYNC synchronization.
> 
> This is not a universal fact. You can write a Wayland compositor that
> consistently reaches app-to-screen latency of less than one monitor
> refresh cycle, while also using KMS planes.
> 
> I believe Weston succeeds in this already if you write the Wayland
> application accordingly.

For a specific example, https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1620 allows app-to-screen latency as low as ~6 ms (including a fixed 2 ms buffer to avoid skipped frames). mutter doesn't use KMS planes yet, but if anything I'd expect that to help rather than hurt for latency (if the compositor doesn't need to draw anything).


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