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.
For things like Wayland we probably want to approach this from a
completely different vector.
When the Wayland compositor starts, it will choose *some* video mode
for an output. It may or may not be what a KMS driver calls "preferred
mode", because it depends on things like user preferences. The
compositor makes the initial modeset to this mode.
I think the general idea we settled on is that we specify an earliest
display time for each frame and give feedback to the application when a
frame was actually displayed.
That is indeed something completely different, and feels like it would
be several years in the future, while the proposed scheme or the
min/max properties could be done fairly quickly. But I'm not in a hurry.
X11 already supports that with the DRI3 extension. Also see VDPAU
present and the Vulkan extension for reference application API
implementations, so that stuff is already present.
It's just not wired up correctly with KMS and we don't have anything
similar in Wayland as far as I know.
Setting the earliest display time for a flip does not fully cover what
video mode timing adjustment or min/max frame time or refresh rate
property would offer: prediction of when the next flip can happen.
Choosing display times requires knowing the possible display times, so
something more is needed. The min/max properties would fit in.
Well you need to communicate to the application what the minimum and
maximum refresh rates are by some extension of the mode description.
Regards,
Christian.
This approach should also be able to handle multiple applications with
different refresh rates. E.g. just think of a video playback with 25 and
another one with 30 Hz in two windows when the max refresh rate is
something like 120Hz.
But that's just an extension to what I already described and completely
possible with the proposed and the min/max approaches.
Thanks,
pq
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