On 10/15/2018 09:57 AM, Pekka Paalanen wrote:
On Fri, 12 Oct 2018 08:58:23 -0400
"Kazlauskas, Nicholas" <nicholas.kazlauskas@xxxxxxx> wrote:
On 10/12/2018 07:20 AM, Koenig, Christian wrote:
Am 12.10.2018 um 11:21 schrieb Pekka Paalanen:
On Fri, 12 Oct 2018 07:35:57 +0000
"Koenig, Christian" <Christian.Koenig@xxxxxxx> wrote:
Am 12.10.2018 um 09:23 schrieb Pekka Paalanen:
On Wed, 10 Oct 2018 09:35:50 -0400
"Kazlauskas, Nicholas" <nicholas.kazlauskas@xxxxxxx> wrote:
The patches I've put out target this use case mostly because of the
benefit for a relatively simple interface. VRR can and has been used in
more ways that this, however.
An example usecase that differs from this would actually be video
playback. The monitor's refresh rate likely doesn't align with the video
content rate. An API that exposes direct control over VRR (like the
range, refresh duration, presentation timestamp) could allow the
application to specify the content rate directly to deliver a smoother
playback experience. For example, if you had a 24fps video and the VRR
range was 40-60Hz you could target 48Hz via some API here.
The way that has been discussed to be implemented is that DRM page flips
would carry a target timestamp, which the driver will then meet as good
as it can. It is better to define an absolute target timestamp than a
frequency, because a timestamp can be used to synchronize with audio
and more. Mario Kleiner can tell you all about scientific use cases
that require accurate display timing, not just frequency.
To summarize what information should be provided by the driver stack to
make applications happy:
1. The minimum time a frame can be displayed, in other words the maximum
frame rate.
2. The maximum time a frame can be displayed, in other words the minimum
frame rate.
3. How much change of frame timing is allowed between frames to avoid
luminescence flickering.
Number 1 and 2 can also be used to signal the availability of VRR to
applications, e.g. if they are identical we don't support VRR at all.
Hi Christian,
"the maximum time a frame can be displayed" is perhaps not an
unambiguous definition. A frame can be shown indefinitely in any case.
Good point. Please also note that I'm not an expert on the display stuff
in general.
My background comes more from implementing the VDPAU mediaplayer
interface in mesa.
So just throwing some ideas in here from the point of view of an
userspace developer which wants to write a media player :)
Excellent!
The CRTC will simply start scanning out the same frame again if there
is no new one. I understand what you want to say, but perhaps some
different words will be in order.
I do wonder if applications really want to know the maximum acceptable
slew rate in timings... maybe that should be left for the drivers to
apply. What I'm thinking is that we have the page flip timestamp with
the page flip events to tell when the new FB became active. That
information could be extended with a time range on when the very next
flip could take place. Applications are already computing that
prediction from the flip timestamp and fixed refresh rate, but it might
be nice to give them the driver's opinion explicitly. Maybe the
tolerable slew rate is not a constant.
Well it depends. I agree that the kernel should probably enforce the
slew rate to avoid flickering for the user.
That's what I was hoping if the monitor hardware does not do that
already, but now it sounds like it's not possible. Another failed
assumption from my side.
But it might be beneficial for the application to know things like this.
Applications should know when they could likely flip, my question is
how to tell them. Is the acceptable slew rate a constant for a video
mode, or does it depend on the previous refresh interval.
What the application definitely needs to know is when a frame was
actually displayed. E.g. application says I want to display this at
point X and at some point later the kernel returns saying the frame was
displayed at point N where in the ideal case N=X.
You already get this timestamp with the DRM page flip events. We also
have Wayland and X11 extensions to get it to apps.
Additional to that let's please use 64bit values and nanoseconds for
every value we use here, and NOT fps, line numbers or similar :)
Seconded!
I'd really favour absolute timestamps even.
Regards,
Christian.
Flickering will really depend on the panel itself. A wider ranger is
more likely to exhibit the issue, but factors like topology, pixel
density and other display technology can affect this.
It can be hard for a driver to guess at all of this. For many panels
it'd be difficult to notice unless it's consistently jumping between the
min and max range.
Do you mean that there is no way to know and get that information from
the monitor itself? Nothing in EDID that could even be used as a
default and quirk the monitors that got it wrong?
The variable refresh range is exposed, but that's about it. There's
certainly no simple algorithm you can feed monitor information into to
determine how bad the luminance flickering is going to look to the user.
Opening up an API that restricts how much the driver can change the
refresh rate in a VRR scenario seems a bit extreme, but there's probably
some applications that could benefit from this. I'd certainly want to
see the actual use case first, though. This still feels more like a
driver policy to me.
I don't think anyone suggested that, certainly I did not. The driver
should get that information from the monitor hardware so that it can
drive it correctly. If the hardware driver doesn't know, then how could
the DRM core or userspace know any better? My whole premise was that the
driver knows.
This was referring to the "How much change of frame timing is allowed
between frames to avoid luminescence flickering" comment. I assumed that
this implied restriction of the min/max VRR ranges from the driver.
Sounds like VRR hardware was designed not only for a consistent refresh
rate but also temporary glitches being ok.
I suppose this will result in choosing a very pessimistic allowed slew
rate in the driver that covers 95% of VRR monitors and handle the rest
with quirks. That could still work.
I agree with the nanosecond based timestamp API making the most logical
sense here from an API perspective. This does overlap a little bit with
the target vblank property that's already on the CRTC, perhaps.
KMS UABI already has a target vblank property defined, or are you
talking about your CRTC hardware?
Target vblank counter value makes even less sense with VRR than it ever
did with a fixed refresh rate. :-) >
The target vblank could be determined based on the timestamp. The driver
is likely to exceed the target presentation timestamp by a fair bit if
it was to just do this, however. VRR could be used in this case to get
closer to the timestamp. A naive implementation could iterate over every
rate in the range and take the one with the minimum difference, for
example.
Could you elaborate on that, who could be doing what exactly to achieve
what?
Thanks,
pq
These comments were mostly discussing about how a kernel driver might
approach a target presentation timestamp.
A target presentation timestamp isn't related to VRR directly. Without
VRR this could still be implemented by a kernel driver, but it would
effectively be the client asking the driver to pick the right target
vblank. There's already some support for this in DRM with the
target_vblank CRTC property, but it isn't the easiest thing to use.
The driver can do much better at getting closer to the client's target
presentation timestamp by adjusting the refresh rate accordingly on a
VRR capable monitor.
Nicholas Kazlauskas
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