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

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Message-ID: <ab816c34-ff98-911f-e53d-b91cd3be6f2b@igel.co.jp>
Date:   Wed, 23 Jun 2021 18:22:47 +0900
From:   Esaki Tomohito <etom@...l.co.jp>
To:     Pekka Paalanen <ppaalanen@...il.com>
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 2021/06/23 17:39, Pekka Paalanen wrote:
> 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.

Hi,

Thank you for your comments and advice.
I will consider leasing a plane.

Thanks,
Esaki