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

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Message-ID: <20210623113922.1e603139@eldfell>
Date:   Wed, 23 Jun 2021 11:39:22 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Esaki Tomohito <etom@...l.co.jp>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        devicetree@...r.kernel.org, Takanari Hayama <taki@...l.co.jp>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        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 Wed, 23 Jun 2021 15:56:05 +0900
Esaki Tomohito <etom@...l.co.jp> wrote:

> Hi,
> Thank you all for your comments.
> 
> On 2021/06/22 17:12, Pekka Paalanen wrote:
> > On Tue, 22 Jun 2021 13:03:39 +0900
> > Esaki Tomohito <etom@...l.co.jp> wrote:
> >   
> >> 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.  
> > 
> > Hi,
> > 
> > indeed. Why not use DRM leases instead?
> >   
> 
> In this use case, I understand that this is not possible with DRM lease,
> am I wrong?
> I understand that it’s not possible to lease a plane and update planes
> on the same output independently from different processes in current DRM
> lease.
> 
> If this is correct, what do you think of adding support for plane leases
> to the DRM lease to handle this case?

Hi,

I would love to see support added for leasing individual planes,
especially to replace the virtual DRM proposal which seems to be
eradicating everything that atomic modesetting and nuclear pageflip
have built over the many years.

However, please note that "on the same output independently" is
physically impossible. Semantically, the planes define what a CRTC
scans out, and the CRTC defines the scanout timings. Therefore it is not
possible to update individual planes independently, they will all
always share the timings of the CRTC.

That combined with KMS not allowing multiple updates to be queued at
the same time for the same CRTC (atomic commits and legacy pageflips
returning EBUSY) makes the plane updates very much inter-dependent.

If you want to avoid EBUSY and have planes update on the vblank you
intended, you really need a userspace compositor to pull everything
together *before* submitting anything to the kernel.


Thanks,
pq

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