Re: [PATCH v2 0/3] Experimental freesync video mode optimization

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Mon, 14 Dec 2020 21:57:25 +0100
Christian König <christian.koenig@amd.com> wrote:

> Am 11.12.20 um 13:20 schrieb Pekka Paalanen:
> > On Fri, 11 Dec 2020 11:28:36 +0100
> > Christian König <ckoenig.leichtzumerken@gmail.com> wrote:
> >  
> >> Am 11.12.20 um 10:55 schrieb Pekka Paalanen:  
> >>> On Fri, 11 Dec 2020 09:56:07 +0530
> >>> Shashank Sharma <shashank.sharma@amd.com> wrote:
> >>>     
> >>>> Hello Simon,
> >>>>
> >>>> Hope you are doing well,
> >>>>
> >>>> I was helping out Aurabindo and the team with the design, so I have
> >>>> taken the liberty of adding some comments on behalf of the team,
> >>>> Inline.
> >>>>
> >>>> On 11/12/20 3:31 am, Simon Ser wrote:  
> >>>>> Hi,
> >>>>>
> >>>>> (CC dri-devel, Pekka and Martin who might be interested in this as
> >>>>> well.)  
> >>> Thanks for the Cc! This is very interesting to me, and because it was
> >>> not Cc'd to dri-devel@ originally, I would have missed this otherwise.
> >>>     
> >>>>> On Thursday, December 10th, 2020 at 7:48 PM, Aurabindo Pillai <aurabindo.pillai@amd.com> wrote:
> >>>>>        
> >>>>>> This patchset enables freesync video mode usecase where the userspace
> >>>>>> can request a freesync compatible video mode such that switching to this
> >>>>>> mode does not trigger blanking.
> >>>>>>
> >>>>>> This feature is guarded by a module parameter which is disabled by
> >>>>>> default. Enabling this paramters adds additional modes to the driver
> >>>>>> modelist, and also enables the optimization to skip modeset when using
> >>>>>> one of these modes.  
> >>>>> Thanks for working on this, it's an interesting feature! However I'd like to
> >>>>> take some time to think about the user-space API for this.
> >>>>>
> >>>>> As I understand it, some new synthetic modes are added, and user-space can
> >>>>> perform a test-only atomic *without* ALLOW_MODESET to figure out whether it can
> >>>>> switch to a mode without blanking the screen.  
> >>>> The implementation is in those lines, but a bit different. The idea
> >>>> is to:
> >>>>
> >>>> - check if the monitor supports VRR,
> >>>>
> >>>> - If it does, add some new modes which are in the VRR tolerance
> >>>> range, as new video modes in the list (with driver flag).
> >>>>
> >>>> - when you get modeset on any of these modes, skip the full modeset,
> >>>> and just adjust the front_porch timing
> >>>>
> >>>> so they are not test-only as such, for any user-space these modes
> >>>> will be as real as any other probed modes of the list.  
> >>> But is it worth to allow a modeset to be glitch-free if the userspace
> >>> does not know they are glitch-free? I think if this is going in, it
> >>> would be really useful to give the guarantees to userspace explicitly,
> >>> and not leave this feature at an "accidentally no glitch sometimes"
> >>> level.
> >>>
> >>>
> >>> I have been expecting and hoping for the ability to change video mode
> >>> timings without a modeset ever since I learnt that VRR is about
> >>> front-porch adjustment, quite a while ago.
> >>>
> >>> This is how I envision userspace making use of it:
> >>>
> >>> Let us have a Wayland compositor, which uses fixed-frequency video
> >>> modes, because it wants predictable vblank cycles. IOW, it will not
> >>> enable VRR as such.  
> >> Well in general please keep in mind that this is just a short term
> >> solution for X11 applications.  
> > I guess someone should have mentioned that. :-)
> >
> > Do we really want to add more Xorg-only features in the kernel?  
> 
> Well, my preferred solution was to add the mode in userspace instead :)
> 
> > It feels like "it's a short term solution for X11" could be almost used
> > as an excuse to avoid proper uAPI design process. However, with uAPI
> > there is no "short term". Once it's in, it's there for decades. So why
> > does it matter if you design it for Xorg foremost? Are others forbidden
> > to make use of it? Or do you deliberately want to design it so that
> > it's not generally useful and it will indeed end up being a short term
> > feature? Planned obsolescence from the start?  
> 
> Yes exactly. We need something which works for now and can be removed 
> again later on when we have a better solution. Adding some extra modes 
> is not considered UAPI since both displays and drivers are doing this 
> for a couple of different purposes already.
> 
> Another main reason is that this approach works with existing media 
> players like mpv and kodi without changing them because we do pretty 
> much the same thing in the kernel which TVs do in their EDID.
> 
> E.g. when starting to play a video kodi will automatically try to switch 
> to a mode which has the same fps as the video.

Aha! That is very valuable information to put this proposal into
perspective. I'll leave you to it, then. :-)


Thanks,
pq

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