[Mesa-dev] Upstream support for FreeSync / Adaptive Sync

[Michel Dänzer]

On 17/10/17 11:34 AM, Nicolai Hähnle wrote:
> Hi all,
> 
> I just sent out a patch that enables FreeSync in Mesa for the somewhat
> hacked implementation of FreeSync that exists in our hybrid (amdgpu-pro)
> stack's DDX and kernel module. [0]
> 
> While this patch isn't meant for upstream, that's as good a time as any
> to raise the issue of how a proper upstream solution would look like. It
> needs to cut across the entire stack, and we should try to align the KMS
> interface with the X11 protocol and the Wayland protocol.
> 
> Prior art that I'm aware of includes:
> 
> 1. The Present protocol extension has a PresentOptionUST bit for
> requesting a specific present time, but the reality is that the
> implementation of that is even less than what the protocol docs claim. [1]

FWIW, I do think this could be a good way for clients to signal that
they want a frame to be displayed ASAP. It would also allow for e.g.
video players to naturally adapt the refresh rate to the video frame
rate (the VDPAU presentation API has a target timestamp for this).


> 3. Keith Packard's CRTC_{GET,QUEUE}_SEQUENCE is not specific to Adaptive
> Sync, but seems like something Adaptive Sync-aware applications would
> want to use. [2]

Not sure I can agree with that. Applications should use higher level
APIs, not low level ones like these directly. (Also, they're basically
just KMS variants of DRM_IOCTL_WAIT_VBLANK, not directly related to
adaptive sync)


> Common sense suggests that there need to be two side to FreeSync / VESA
> Adaptive Sync support:
> 
> 1. Query the display capabilities. This means querying minimum / maximum
> refresh duration, plus possibly a query for when the earliest/latest
> timing of the *next* refresh.
> 
> 2. Signal desired present time. This means passing a target timer value
> instead of a target vblank count, e.g. something like this for the KMS
> interface:
> 
>   int drmModePageFlipTarget64(int fd, uint32_t crtc_id, uint32_t fb_id,
>                               uint32_t flags, void *user_data,
>                               uint64_t target);
> 
>   + a flag to indicate whether target is the vblank count or the
> CLOCK_MONOTONIC (?) time in ns.

drmModePageFlip(Target) is part of the pre-atomic KMS API, but adapative
sync should probably only be supported via the atomic API, presumably
via output properties.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer