RFC for a render API to support adaptive sync and VRR

[Cyr, Aric]

> -----Original Message-----
> From: Wentland, Harry
> Sent: Tuesday, April 10, 2018 11:08
> To: Michel Dänzer <michel at daenzer.net>; Koenig, Christian <Christian.Koenig at amd.com>; Manasi Navare
> <manasi.d.navare at intel.com>
> Cc: Haehnle, Nicolai <Nicolai.Haehnle at amd.com>; Daniel Vetter <daniel.vetter at ffwll.ch>; Daenzer, Michel
> <Michel.Daenzer at amd.com>; dri-devel <dri-devel at lists.freedesktop.org>; amd-gfx mailing list <amd-gfx at lists.freedesktop.org>;
> Deucher, Alexander <Alexander.Deucher at amd.com>; Cyr, Aric <Aric.Cyr at amd.com>; Koo, Anthony <Anthony.Koo at amd.com>
> Subject: Re: RFC for a render API to support adaptive sync and VRR
> 
> On 2018-04-10 03:37 AM, Michel Dänzer wrote:
> > On 2018-04-10 08:45 AM, Christian König wrote:
> >> Am 09.04.2018 um 23:45 schrieb Manasi Navare:
> >>> Thanks for initiating the discussion. Find my comments below:
> >>> On Mon, Apr 09, 2018 at 04:00:21PM -0400, Harry Wentland wrote:
> >>>> On 2018-04-09 03:56 PM, Harry Wentland wrote:
> >>>>>
> >>>>> === 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.
> >>>>>
> >>> What I would like is two sets of properties on a CRTC or preferably on
> >>> a connector:
> >>>
> >>> KMD properties that UMD can query:
> >>> * vrr_capable -  This will be an immutable property for exposing
> >>> hardware's capability of supporting VRR. This will be set by the
> >>> kernel after
> >>> reading the EDID mode information and monitor range capabilities.
> >>> * vrr_vrefresh_max, vrr_vrefresh_min - To expose the min and max
> >>> refresh rates supported.
> >>> These properties are optional and will be created and attached to the
> >>> DP/eDP connector when the connector
> >>> is getting intialized.
> >>
> >> Mhm, aren't those properties actually per mode and not per CRTC/connector?
> >>
> >>> Properties that you mentioned above that the UMD can set before kernel
> >>> can enable VRR functionality
> >>> *bool vrr_enable or vrr_compatible
> >>> target_frame_duration_ns
> >>
> >> Yeah, that certainly makes sense. But target_frame_duration_ns is a bad
> >> name/semantics.
> >>
> >> We should use an absolute timestamp where the frame should be presented,
> >> otherwise you could run into a bunch of trouble with IOCTL restarts or
> >> missed blanks.
> >
> > Also, a fixed target frame duration isn't suitable even for video
> > playback, due to drift between the video and audio clocks.

Why?  Even if they drift, you know you want to show your 24Hz video frame for 41.6666ms and adaptive sync can ensure that with reasonable accuracy.  
All we're doing is eliminating the need for frame rate converters from the application and offloading that to hardware.

> > Time-based presentation seems to be the right approach for preventing
> > micro-stutter in games as well, Croteam developers have been researching
> > this.
> >
> 
> I'm not sure if the driver can ever give a guarantee of the exact time a flip occurs. What we have control over with our HW is frame
> duration.
> 
> Are Croteam devs trying to predict render times? I'm not sure how that would work. We've had bad experience in the past with
> games that try to do framepacing as that's usually not accurate and tends to lead to more problems than benefits.

For gaming, it doesn't make sense nor is it feasible to know how exactly how long a render will take with microsecond precision, very coarse guesses at best.  The point of adaptive sync is that it works *transparently* for the majority of cases, within the capability of the HW and driver.  We don't want to have every game re-write their engine to support this, but we do want the majority to "just work".

The only exception is the video case where an application may want to request a fixed frame duration aligned to the video content.  This requires an explicit interface for the video app, and our proposal is to keep it simple:  app knows how long a frame should be presented for, and we try to honour that.

-Aric