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

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Date:   Tue, 22 Jun 2021 13:03:39 +0900
From:   Esaki Tomohito <etom@...l.co.jp>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Kieran Bingham <kieran.bingham+renesas@...asonboard.com>,
        dri-devel@...ts.freedesktop.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Damian Hobson-Garcia <dhobsong@...l.co.jp>,
        Takanari Hayama <taki@...l.co.jp>
Subject: Re: [PATH 0/4] [RFC] Support virtual DRM

Hi, Enrico Weigelt
Thank you for reply.

On 2021/06/22 1:05, Enrico Weigelt, metux IT consult wrote:
> On 21.06.21 08:27, Tomohito Esaki wrote:
> 
> Hi,
> 
>> 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.
> 
> Are you attempting to create an simple in-kernel compositor ?

I think the basic idea is the same as DRMlease.
We want to separate the resources from the master in units of planes,
so we proposed virtual DRM.
I think the advantage of vDRM is that you can use general DRM APIs
in userland.

> I don't think that's not the way to go, at least not by touching each
> single display driver, and not hardcoding the planes in DT.

Thank you for comment. I will reconsider about DT.

> What's the actual use case you're doing that for ? Why not using some
> userland compositor ?

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.
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. On the other hand, if we use vDRM, the
image will be displayed at the next VSYNC, so it will be displayed after
1 VSYNC at most.

Also, since the compositor is a single point of failure, we may not want
to make it dependent on it.

Best regards
Tomohito Esaki