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Date:   Tue, 22 Jun 2021 10:57:57 +0300
From:   Pekka Paalanen <ppaalanen@gmail.com>
To:     Esaki Tomohito <etom@igel.co.jp>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        devicetree@vger.kernel.org, Takanari Hayama <taki@igel.co.jp>,
        linux-doc@vger.kernel.org, David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-renesas-soc@vger.kernel.org,
        Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Damian Hobson-Garcia <dhobsong@igel.co.jp>
Subject: Re: [PATH 0/4] [RFC] Support virtual DRM
Message-ID: <20210622105757.2b9dec32@eldfell>
In-Reply-To: <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>
References: <20210621062742.26073-1-etom@igel.co.jp>
        <9853d0a9-6053-db64-9c79-40b7e0689eec@suse.de>
        <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>
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On Tue, 22 Jun 2021 13:02:59 +0900
Esaki Tomohito <etom@igel.co.jp> wrote:

> Hi, Thomas
> Thank you for reply.
>=20
> On 2021/06/21 16:10, Thomas Zimmermann wrote:
> > Hi
> >=20
> > Am 21.06.21 um 08:27 schrieb Tomohito Esaki: =20
> >> 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. =20
> >=20
> > 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? =20
>=20
> 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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