Re: [PATCH 2/3] drm: Add basic helper to allow precise pageflip timestamps in vrr.

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On Wed, Feb 13, 2019 at 10:56 AM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>
> Quoting Daniel Vetter (2019-02-13 09:50:55)
> > On Tue, Feb 12, 2019 at 10:32:31PM +0100, Mario Kleiner wrote:
> > > I think all kms drivers try to call drm_crtc_handle_vblank() at start
> > > of vblank to give Mesa the most time for frontbuffer rendering for
> > > classic X. But vblank events are also used for scheduling bufferswaps
> > > or other stuff for redirected windowed rendering, or via api's like
> > > OML_sync_controls glXWaitForMscOML, so there might be other things
> > > affected by a more delayed vblank handling.
> >
> > The frontbuffer rendering is very much X driver specific, and I think
> > -amdgpu/radeon is the only one that requires this. No i915 driver ever
> > used the vblank interrupt to schedule frontbuffer blits, we use some
> > CS-side stalls.
>
> Fwiw, the Present midlayer does use vblank scheduling for inplace copy
> updates. Not that I wish to encourage anyone to use frontbuffer
> rendering.
> -Chris

Yes, that's what i meant. Under DRI2 at least AMD, Intel and nouveau
have throttling based on CS stalls to avoid tearing and do throttling.
DRI3/Present last time i checked just waited for a vblank event and
then triggered the blit - something that causes tearing even when
triggered at start of vblank.

-mario
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