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

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Date:   Fri, 21 Dec 2018 10:55:58 +0100
From:   Michel Dänzer <michel@...nzer.net>
To:     "Kazlauskas, Nicholas" <Nicholas.Kazlauskas@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Alex Deucher <alexdeucher@...il.com>
Cc:     "dnicoara@...omium.org" <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" <kernel@...labora.com>
Subject: Re: [PATCH] drm: add capability DRM_CAP_ASYNC_UPDATE

On 2018-12-20 6:38 p.m., Kazlauskas, Nicholas wrote:
> On 12/20/18 12:09 PM, 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.
> 
> I haven't really investigated too much into the gamma stuttering issue, 
> but I think it's similar to the cursor update - a high volume of atomic 
> updates that ends up skipping over a vblank or two.

FWIW, I don't think the use-cases described in
https://bugs.freedesktop.org/108917 (Night Light / RedShift) involve a
particularly high volume of gamma updates. I was able to reproduce the
stuttering with ~10 gamma changes per second, but I suspect even a
single one could cause a frame drop. I assume the issue is that gamma
updates are done as separate atomic commits, which can delay other
commits for page flips.


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