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Subject: Re: [PATH 0/4] [RFC] Support virtual DRM
To:     Esaki Tomohito <etom@igel.co.jp>
Cc:     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>
References: <20210621062742.26073-1-etom@igel.co.jp>
 <9853d0a9-6053-db64-9c79-40b7e0689eec@suse.de>
 <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>
From:   Thomas Zimmermann <tzimmermann@suse.de>
Message-ID: <bd64733a-d45c-a985-d99d-0fa70bacb001@suse.de>
Date:   Tue, 22 Jun 2021 11:12:16 +0200
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Esaki Tomohito <etom@igel.co.jp>
Cc: 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>
Message-ID: <bd64733a-d45c-a985-d99d-0fa70bacb001@suse.de>
Subject: Re: [PATH 0/4] [RFC] Support virtual DRM
References: <20210621062742.26073-1-etom@igel.co.jp>
 <9853d0a9-6053-db64-9c79-40b7e0689eec@suse.de>
 <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>
In-Reply-To: <85593f2f-5aa9-6023-ecba-c5275a468b71@igel.co.jp>

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Hi

Am 22.06.21 um 06:02 schrieb Esaki Tomohito:
> Hi, Thomas
> Thank you for reply.
>=20
> 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 solv=
ed
>> in userspace?
>=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.
> 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, th=
e
> image will be displayed at the next VSYNC, so it will be displayed afte=
r
> 1 VSYNC at most.

Other commenters already addressed these points.

>=20
> Also, since the compositor is a single point of failure, we may not wan=
t
> to make it dependent on it.

The kernel is also a single point of failure.

TBH I don't think this feature should be merged until there's a clear=20
use case that cannot be solved in userspace idiomatically.

Best regards
Thomas

>=20
> Best regards
> Tomohito Esaki
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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