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

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Date:   Tue, 22 Jun 2021 10:57:57 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Esaki Tomohito <etom@...l.co.jp>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        devicetree@...r.kernel.org, Takanari Hayama <taki@...l.co.jp>,
        linux-doc@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>
Subject: Re: [PATH 0/4] [RFC] Support virtual DRM

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.


Thanks,
pq

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