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

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Message-ID: <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>
Date:   Tue, 22 Jun 2021 13:02:59 +0900
From:   Esaki Tomohito <etom@...l.co.jp>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        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, 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.
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