[PATCH v2 2/3] drm: Add variable refresh property to DRM CRTC

[Michel Dänzer]

On 2018-10-05 7:48 p.m., Kazlauskas, Nicholas wrote:
> On 10/05/2018 12:56 PM, Michel Dänzer wrote:
>> On 2018-10-05 6:21 p.m., Kazlauskas, Nicholas wrote:
>>> On 10/05/2018 04:10 AM, Pekka Paalanen wrote:
>>>>
>>>> I also believe that it would be useful to expose vmin/vmax to userspace
>>>> as properties, so that display servers and apps can plan ahead on when
>>>> they will render. I suppose that can be left for later when someone
>>>> starts working on userspace taking advantage of it, so that the proper
>>>> userspace requirement for new UABI is fulfilled.
>>>
>>> Knowing the vmin/vmax could potentially be useful for testing but most
>>> applications shouldn't be trying to predict or adjust rendering based on
>>> these.
>>
>> FWIW, recent research indicates that this is necessary for perfectly
>> smooth animation without micro-stuttering, even with VRR.
> 
> This was brought up in previous RFCs about this but I'm not really
> convinced by the research methodology and the results from those threads.
> 
> Targeting the max vrefresh of the display (which is known without
> additional properties) should be best for prediction since anything
> lower would be inherently less smooth by definition.

Smooth animation isn't only about frame-rate, it's also about the timing
of each frame's presentation and its contents being consistent with each
other[0]. To ensure this, the application has to know the constraints
for when the next frame can be presented, and it has to be able to
control when the frame is presented.


[0] One example of this is https://www.youtube.com/watch?v=V4fgYS38aQg .
When this video is played back at 60 fps, the upper sphere moves
smoothly, but the lower sphere sometimes jumps back and forth slightly
(it's pretty subtle, might take a while to notice), because its position
isn't always consistent with the frame's presentation timing.

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