=== 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
