drm: apple: Adaptive Sync/ProMotion

[chadmed]

Tested:

• Adaptive Sync over DisplayPort (Dell S2721DGF, [48, 165] Hz)
• Apple ProMotion on 14" and 16" MacBook Pros (tested [24, 120] Hz)

Not tested:

• Adaptive Sync over HDMI (I have no HDMI 2.x displays)

What works:

• "Modesetting" in and out of Adaptive Sync
• mpv... mostly
• Games running via Proton via Steam via FEX via muvm
• Native AArch64 games
• Forcing Adaptive Sync on and sitting at the desktop at minimum refresh rate

What doesn't

• Firefox will always render at the display's refresh rate
• On the internal MBP displays only, mpv's gpu and gpu-next outputs leak file descriptors and halt the GPU when in fullscreen. Other backends work fine, other applications work fine, gpu and gpu-next work fine when not in fullscreen...

What needs work

• The VRR timestamps are best guesses only. We know they are always within a few frames of each other, meaning that they are likely something to do with swap submission and presentation time, but what exactly remains unclear.
• Wrapping set_digital_out_mode in a dcpep callback seems kind of silly? There's probably a smarter way to do this.