On Mon, 14 Dec 2020 21:57:25 +0100 Christian König <christian.koenig@xxxxxxx> wrote: > Am 11.12.20 um 13:20 schrieb Pekka Paalanen: > > On Fri, 11 Dec 2020 11:28:36 +0100 > > Christian König <ckoenig.leichtzumerken@xxxxxxxxx> wrote: > > > >> Am 11.12.20 um 10:55 schrieb Pekka Paalanen: > >>> On Fri, 11 Dec 2020 09:56:07 +0530 > >>> Shashank Sharma <shashank.sharma@xxxxxxx> 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@xxxxxxx> 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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