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@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/amd-gfx