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