linux-kernel - Re: [PATCH] drm: add capability DRM_CAP_ASYNC

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Date:   Fri, 21 Dec 2018 10:47:27 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     Daniel Vetter <daniel@...ll.ch>,
        Alex Deucher <alexdeucher@...il.com>
Cc:     dnicoara@...omium.org,
        Stéphane Marchesin <marcheu@...gle.com>,
        Sean Paul <seanpaul@...gle.com>,
        Alexandros Frantzis <alexandros.frantzis@...labora.com>,
        David Airlie <airlied@...ux.ie>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Helen Koike <helen.koike@...labora.com>, kernel@...labora.com,
        "Kazlauskas, Nicholas" <nicholas.kazlauskas@....com>
Subject: Re: [PATCH] drm: add capability DRM_CAP_ASYNC_UPDATE

On 2018-12-20 6:16 p.m., Michel Dänzer wrote:
> On 2018-12-20 6:09 p.m., Daniel Vetter wrote:
>> On Thu, Dec 20, 2018 at 6:03 PM Alex Deucher <alexdeucher@...il.com> wrote:
>>> On Thu, Dec 20, 2018 at 11:54 AM Daniel Vetter <daniel@...ll.ch> wrote:
>>
>>>> Not sure about the gamma thing since we had opposite bugs on i915
>>>> about gamma not being vsynced and tearing terribly. Cursor is special
>>>> since it tends to be too small to notice tearing.
>>>
>>> Our cursor hw (and possibly gamma as well Nicholas?  Harry?) is double
>>> buffered, so we can update it any time for the most part and the
>>> changes won't take affect until the next vupdate period.
>>
>> Hm, I guess we could make the gamma update optionally async, and let
>> drivers deal. The issue is that the current async helper stuff won't
>> cope with gamma updates (it's aimed at plane updates only, not at crtc
>> property updates). Or we get userspace to do proper atomic updates. Or
>> we do some faking in the kernel, e.g. waiting with the gamma update
>> until the next atomic update happens. But that kinda breaks
>> ->atomic_check.
> 
> Would it be possible to merge gamma changes into a pending commit, if
> there is one, and create a new commit otherwise?
> 
> Otherwise the atomic API just moves this same problem to be solved by
> userspace.

Actually, I'm not sure this is even solvable in a Xorg driver. When it
gets called to update the gamma LUT:

1. If there's a pending atomic commit, it cannot amend that, can it?

2. It has no way of knowing when the next atomic commit would be
submitted e.g. for a page flip, so it kind of has to create its own
commit for the gamma update.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer