Re: RFC for a render API to support adaptive sync and VRR

[Harry Wentland <harry.wentland-5C7GfCeVMHo@public.gmane.org>]

Adding dri-devel, which I should've included from the start.

On 2018-04-09 03:56 PM, Harry Wentland wrote:
> === What is adaptive sync and VRR? ===
> 
> Adaptive sync has been part of the DisplayPort spec for a while now and allows graphics adapters to drive displays with varying frame timings. VRR (variable refresh rate) is essentially the same, but defined for HDMI.
> 
> 
> 
> === Why allow variable frame timings? ===
> 
> Variable render times don't align with fixed refresh rates, leading to
> stuttering, tearing, and/or input lag.
> 
> e.g. (rc = render completion, dr = display refresh)
> 
> rc       B          C            D            E      F
> dr	A	B	C	C	D	E	F
> 
>                     ^             ^        
> 		  frame         missed     
> 		 repeated	display    
> 		  twice	        refresh   
> 
> 
> 
> === Other use cases of adaptive sync ====
> 
> Beside the variable render case, adaptive sync also allows adjustment of refresh rates without a mode change. One such use case would be 24 Hz video.
> 
> 
> 
> === A DRM render API to support variable refresh rates ===
> 
> In order to benefit from adaptive sync and VRR userland needs a way to let us know whether to vary frame timings or to target a different frame time. These can be provided as atomic properties on a CRTC:
>  * bool	variable_refresh_compatible
>  * int	target_frame_duration_ns (nanosecond frame duration)
> 
> This gives us the following cases:
> 
> variable_refresh_compatible = 0, target_frame_duration_ns = 0
>  * drive monitor at timing's normal refresh rate
> 
> variable_refresh_compatible = 1, target_frame_duration_ns = 0
>  * send new frame to monitor as soon as it's available, if within min/max of monitor's reported capabilities
> 
> variable_refresh_compatible = 0/1, target_frame_duration_ns = > 0
>  * send new frame to monitor with the specified target_frame_duration_ns
> 
> When a target_frame_duration_ns or variable_refresh_compatible cannot be supported the atomic check will reject the commit.
> 
> 
> 
> === Previous discussions ===
> 
> https://lists.freedesktop.org/archives/dri-devel/2017-October/155207.html
> 
> 
> 
> === Feedback and moving forward ===
> 
> I'm hoping to get some feedback on this or continue the discussion on how adaptive sync / VRR might look like in the DRM ecosystem. Once there are no major concerns or objections left we'll probably start creating some patches to sketch this out a bit better and see how it looks in practice.
> 
> 
> 
> Cheers,
> Harry
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> 
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