[PATCH] drm/amd/display: Use vrr friendly pageflip throttling in DC.

[Michel Dänzer]

On 2019-02-09 7:52 a.m., Mario Kleiner wrote:
> In VRR mode, keep track of the vblank count of the last
> completed pageflip in amdgpu_crtc->last_flip_vblank, as
> recorded in the pageflip completion handler after each
> completed flip.
> 
> Use that count to prevent mmio programming a new pageflip
> within the same vblank in which the last pageflip completed,
> iow. to throttle pageflips to at most one flip per video
> frame, while at the same time allowing to request a flip
> not only before start of vblank, but also anywhere within
> vblank.
> 
> The old logic did the same, and made sense for regular fixed
> refresh rate flipping, but in vrr mode it prevents requesting
> a flip anywhere inside the possibly huge vblank, thereby
> reducing framerate in vrr mode instead of improving it, by
> delaying a slightly delayed flip requests up to a maximum
> vblank duration + 1 scanout duration. This would limit VRR
> usefulness to only help applications with a very high GPU
> demand, which can submit the flip request before start of
> vblank, but then have to wait long for fences to complete.
> 
> With this method a flip can be both requested and - after
> fences have completed - executed, ie. it doesn't matter if
> the request (amdgpu_dm_do_flip()) gets delayed until deep
> into the extended vblank due to cpu execution delays. This
> also allows clients which want to regulate framerate within
> the vrr range a much more fine-grained control of flip timing,
> a feature that might be useful for video playback, and is
> very useful for neuroscience/vision research applications.
> 
> In regular non-VRR mode, retain the old flip submission
> behavior. This to keep flip scheduling for fullscreen X11/GLX
> OpenGL clients intact, if they use the GLX_OML_sync_control
> extensions glXSwapBufferMscOML(, ..., target_msc,...) function
> with a specific target_msc target vblank count.
> 
> glXSwapBuffersMscOML() or DRI3/Present PresentPixmap() will
> not flip at the proper target_msc for a non-zero target_msc
> if VRR mode is active with this patch. They'd often flip one
> frame too early. However, this limitation should not matter
> much in VRR mode, as scheduling based on vblank counts is
> pretty futile/unusable under variable refresh duration
> anyway, so no real extra harm is done.
> 
> According to some testing already done with this patch by
> Nicholas on top of my tests, IGT tests didn't report any
> problems. If fixes stuttering and flickering when flipping
> at rates below the minimum vrr refresh rate.
> 
> Fixes: bb47de736661 ("drm/amdgpu: Set FreeSync state using drm VRR
> properties")
> Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>
> Cc: <stable at vger.kernel.org>
> Cc: Nicholas Kazlauskas <nicholas.kazlauskas at amd.com>
> Cc: Harry Wentland <harry.wentland at amd.com>
> Cc: Alex Deucher <alexander.deucher at amd.com>
> Cc: Michel Dänzer <michel at daenzer.net>

I wonder if this couldn't be solved in a simpler / cleaner way by making
use of the target MSC passed to the page_flip_target hook.


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