RFC for a render API to support adaptive sync and VRR

[Daniel Vetter]

On Mon, Apr 09, 2018 at 04:00:21PM -0400, Harry Wentland wrote:
> Adding dri-devel, which I should've included from the start.

Top posting, because I'm lazy and was out sick ...

Few observations:
- Stéphane has a great point which seems to have been ignored thus far.
- Where's the VK extension for this - there must be one :-) Starting with
  a full implementation for that (based on radv or anv or something like
  that) might help.
- Imo if we do a conversion between the vk api and what we feed into the
  hw, then let's not do a midlayer mistake: That conversion should happen
  at the bottom, in the kernel driver, maybe assisted with some helpers.
  Not somewhere in-between, like in libdrm of all places!

Cheers, Daniel

> 
> 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
> > _______________________________________________
> > amd-gfx mailing list
> > amd-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/amd-gfx
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch