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

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Date:   Fri, 25 Jun 2021 10:55:20 +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 20:41, Pekka Paalanen wrote:
> On Wed, 23 Jun 2021 18:22:47 +0900
> Esaki Tomohito <etom@...l.co.jp> wrote:
> 
>> 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.
> 
> Hi,
> 
> I wish you considered a userspace compositor first, once more, with
> passion.
> 
> It does not need to be Weston, and it does not need to use Wayland.
> Just a userspace daemon that owns the whole display device and somehow
> talks to whatever else wants stuff on screen.
> 
> I have not seen any evidence that leasing individual planes would do
> you any good. I can easily see it doing you harm. I'm only saying that
> it would be better than the virtual DRM proposal if you absolutely have
> to go there. Please, consider not going there at all.
> 
> "On the same output independently" is not possible for the very simple
> reason that the pixel data needs to be streamed serially to a monitor.
> 

Hi,

Thank you for your advice.
Once again, I'll consider a userspace compositor first.

Best regards
Esaki