VRR, Lower Latency Likely Coming For KDE's KWin Wayland Compositor

[Zan Lynx]

It would be nice if they got KDE actually usably working on Wayland before focusing all their resources on gaming. You know minor things like actually being able to click buttons.

VRR is not only for gaming. It is also useful for power saving and video playback.

Intel laptop GPUs already do VRR internally. Their PSR (Panel Self Refresh) mode runs the laptop or tablet panel at whatever its lowest refresh rate is, until the framebuffer changes. Doing it with explicit software control should allow things like Wayland compositors to run at minimum (30 or 40 Hz usually) and still update at 120 Hz when moving the mouse or other interactive uses.

Much like Android uses VRR. Many phone displays now run at 0 FPS (as far as Surface Flinger is concerned anyway. the hardware does even more power saving than Intel PSR.) and burst to 90 or 120 Hz while scrolling.

[oiaohm]

you confuse a bit terms. At least for Gsync compatible (what is simply certified Freesync over DP) or better monitors, they all adjust refresh rate. What is important is that they adjust refresh rate only in certain range. When refresh rate goes outside range, thing you said happens, internal refresh rate doesn't go too far below and if FPS is smaller then minimum refresh rate range, frames gets duplicated OR you go too high fps, and frames gets skipped until next good frame appears. Vsync and frame rate limiter secures that you won't get additional inconsitent latency from too high fps.

Gsync compatible even a full Gsync is not required to adjust the refresh PSR rate to be certified. Yes a monitor with a fixed PSR can claim to VRR. If you have to worked out why 3 frames slower VRR than the PSR works is simple that the PSR is not changing. So you need spaces to display the off time frames.

Yes being 3 frames per second slower means that 3 frames are getting display twice so you do in places have a extra frame of latency.

You are talking about VRR and PSR (panel self-refresh) in combination.

Exactly. When you start with original fixed refresh rates the PSR and the fixed refresh rate matched. With VRR early on they were tried to keep matched but different flicker problems kept on happening and it turns out changing the PSR constantly is not good for part lifespan due to causing greater thermal changes. Even new monitors with Nvidia chipset in them for gsync are normally don't have the feature of change the PSR any more.

Change the PSR on the fly turned out that its been a best laid plans of mice and men. On paper it seamed like a good idea in real world where you have to deal with thermals and other things its not such a good idea. Once PSR stopped being changed exactly the effect of VRR just moves the miss aligned frame problem out of the GPU and into the Monitor so to end users not fixing that problem to their eyes just changing where the problem is generated.

[oiaohm]

VRR is not only for gaming. It is also useful for power saving and video playback.

Intel laptop GPUs already do VRR internally. Their PSR (Panel Self Refresh) mode runs the laptop or tablet panel at whatever its lowest refresh rate is, until the framebuffer changes. Doing it with explicit software control should allow things like Wayland compositors to run at minimum (30 or 40 Hz usually) and still update at 120 Hz when moving the mouse or other interactive uses.

Much like Android uses VRR. Many phone displays now run at 0 FPS (as far as Surface Flinger is concerned anyway. the hardware does even more power saving than Intel PSR.) and burst to 90 or 120 Hz while scrolling.

PSR is the Panel Self Refresh rate. Surface Flinger reporting 0 FPS is the VRR reporting these two figures don't align. Allowing you to power down the GPU and have the LCD controller keep on drawing the screen does serous-ally save power.

Intel PSR adjustment is not pure VRR. What you will see is PSR being a set of fixed speeds like 30, 60, 120. Yes picking 40Hz as your wayland compositor slow mode could be really bad idea as in some cases that that can be basically locking you at 80/120 now not all LCD screens do 80.

Yes it possible on Intel to have a VRR of zero and a PSR of 240Hz at the same time because the LCD panel only mode is 240Hz. Fixed PSR rates still exist even in a VRR supporting monitor. Fixed PSR on LCD screens is simple to calibrate brightness configurations compared truly dynamically changing PSR. The complexity configuring dynamically changing PSR brightness resulting in either too dark or too bright at different times that in fact generate flicker. Yes when PSR goes up to 120Hz then you are rendering by VRR at 30 frames of second following a that the PSR can remain up at 120Hz for a while before deciding to switch back with Intel just in case another 120Hz VRR will be coming soon so reducing brightness adjustments and reducing flicker risk that in worse case you have human laying on ground having epilepsy attack.

PSR and VRR are these days running out of alignment with each other. Yes the flicker of early VRR monitors is in fact dangerous the correction of it results in PSR not running in alignment with the VRR. Yes the cheaper monitors still doing the older method will have to disappear in time they are not safe monitors.

[polarathene]

PSR is the Panel Self Refresh rate. Surface Flinger reporting 0 FPS is the VRR reporting these two figures don't align. Allowing you to power down the GPU and have the LCD controller keep on drawing the screen does serous-ally save power.

Intel PSR adjustment is not pure VRR. What you will see is PSR being a set of fixed speeds like 30, 60, 120.

Any link on PSR refresh rates?

All I know about it is that the framebuffer doesn't need to be pointlessly updated with the same output from GPU, the display just reuses the existing framebuffer from it's internal memory until the GPU has something new. And that PSR2 improves on that by being able to do so with subregions instead of an entire display update (or framebuffer data, something like that), eg only your cursor moving around, or some text being typed while the remaining area of the display is unchanged.

I assumed refresh rates of the display was a separate matter unrelated to PSR.

[oiaohm]

All I know about it is that the framebuffer doesn't need to be pointlessly updated with the same output from GPU, the display just reuses the existing framebuffer from it's internal memory until the GPU has something new. And that PSR2 improves on that by being able to do so with subregions instead of an entire display update (or framebuffer data, something like that), eg only your cursor moving around, or some text being typed while the remaining area of the display is unchanged.

This is close.

https://www.anandtech.com/show/7208/understanding-panel-self-refresh

PSR is memory in the panel of the screen that does have a HZ speed effecting its operation. Intel PSR is how they control the PSR not the PSR itself because the PSR is in the panel not in the Intel chipset. So its not a framebuffer as such but stored state in the LCD. PSR2 adds subregions its still modify memory/state in the panel.

When that stored stated is updated pixels on the screen it self that the PSR/panel rate/clock/Hz is how fast this memory is being draw to pixels.

There is a fun little issue with LCD and even oled.

Lets take a basic problem that leads to flicker. I will use some bogus number here but they are based kind what happens. For the example we have a fiction 120hz panel that can be updated at different speeds what would be possible values to give same appearance brightness/functional brightness to end user the real numbers could look like 30Hz brightness level 70% 120 brightness level 100%. Why driving a panel at 30Hz needs lower brightness level here is a mechanical issue as a pixel be it LCD or led changes brightness its not instant the more often this happens the more light is blocked in the LCD case or in the LED pixel case the light is not generated results kind of the same.

The simple result is different clock speed of panel drawing requires different brightness output value to get the same brightness to end user. Gets more complex with resistance and capacitance in the panel you could end up with a brain bending. In fiction panel here matching based off of real world you could have 30Hz 70% brightness, 60Hz 80% brightness 90Hz 70% brightness 120Hz 100% brightness. Please note on the fiction example I am making the diffeerences huge real panels you would be talking like 0.1-10 percent difference. The brightness value to the panel is not uniform across the panel clock speed. Worst this you have like 1000 panels from the same vendor meant to be the same specification and have not one in fact match when you test this.

This issue means changing the panel clock that is really doing the output a lot is really simple to end up with a dangerous flicker. Remember 25 frames per second /25Hz savage flicker is enough to land a lot of people in hospital.

VRR does not change the fact you have a PSR with panel clock between you and the pixels in most cases these days. Yes a LCD may not be for a laptop but in a desktop screen and it still have a PSR and the PSR is what the VRR data is being written into.

VRR you are doing a usable 50 frames per second aligning those 50 frames to the 120hz/frames the monitor/panel will get that frame displayed to user faster than if the user was running the panel at 50hz. This does not mean the panel clock speed is changing with VRR or that you will be get appearance advantage over driving the panel at 120Hz.

Panel clock changing with VRR was tried the result is dangerous flicker that is very hard to prevent due to all tight manufacturing tolerances required worse after the panels that first did this feature over time as parts of the LCD screens wore in the correct values to perform this in fact change. So you have a monitor changing panel clock aligned with VRR and you are using it in VRR mode its really only a matter of time until monitor developers a flicker and if you medically weak to flicker you could find yourself in really big trouble.

Hard reality Panel /PSR clock will not be kept aligned with VRR because that is not safe. Yes a change every 5 seconds in panel clock speed or so is safe as the flcker caused by that is that slow that it does normally cause bad effects.

Mechanical/medical limits means some of the things that were claimed when VRR first appeared we just should not have. Yes early claim that VRR directly changes Panel clock so gets the frame displayed as fast as possible is not true in the newer safe monitors yes it was done on some of the early VRR monitors before we knew better. In the newer safe monitor VRR frame displayed on the next panel clock and the panel clock if it is changing will be changing on a interval base way where its holding particular HZ speeds for reasonable amounts of time so very much like shifting gears this removes/reduces the flicker problem. Little bit of visual judder does not result in person on floor having a medical episode.

Do note I said reduce flicker in the one part not all monitors are 100% flicker free by fixing the HZ for like 5 seconds or so intervals but if the flicker is slow enough it does not cause humans to have medical problems.

Lets say us humans like learning by screwing it up.

One of the fun things is 5 second flicker will not give you major medical episode but can increase you risk of getting motion sickness while playing a game. So a modern safe VRR monitor is not 100 percent safe for everyone. So some people will want their monitor driven at 120Hz/240Hz...(what ever the max is) with no VRR so monitor does not clock up at down at all so they don't get sick playing games.

VRR is not as big of magic bullet as one would hope.

[tildearrow]

Much like Android uses VRR. Many phone displays now run at 0 FPS (as far as Surface Flinger is concerned anyway. the hardware does even more power saving than Intel PSR.) and burst to 90 or 120 Hz while scrolling.

Since when has this been the case? My phone from 2018 runs at 60Hz, and I have been unable to run it at 50Hz for PAL video playback.

[Zan Lynx]

I see that it is quite new. Android 11.

https://developer.android.com/guide/topics/media/frame-rate

[oiaohm]

Since when has this been the case? My phone from 2018 runs at 60Hz, and I have been unable to run it at 50Hz for PAL video playback.

That can be true for new 2021 phones. Yes there are new 2021 phones that only have a 60Hz panel worse some really cheap ones only have a 30Hz panel. So you are not alone with this problem and its a lot more common that one thinks.

I see that it is quite new. Android 11.

https://developer.android.com/guide/...dia/frame-rate

This is a good write up. This is written closer to real hardware. The reality is a lot of LCD/oled panels cannot do 50Hz at all even in desktop monitors.

In that case, running the display at 120Hz will allow both the 60Hz surface and 24Hz surface to run with no pulldown required.

This happens a lot more than what you would think where you request from a 120Hz monitor for a 50 Hz fixed rate and what really happened is the panel is running at 120Hz and its being monitor side using like VRR method to put 50Hz into 120Hz of course that is not 100 percent judder free but it small enough most humans don't notice. So yes you can have a monitor/tv exposing 50Hz mode on HDMI when in reality it cannot do that in the panel at all.

We are not seeing LCD or oled panels with dynamic panel clocks in new devices instead we are seeing LCD/oled panels with static clocks the general list of panel clocks is 30,60,90,120, 240 and 360Hz of course none of those suite PAL perfectly but the higher you go the better alignment you get. A panel costs more to make for each extra clock speed it has to be certified for this is why you will find 60/120Hz panels and 30/120Hz panels with no other modes. Yes getting a phone what claims to have a 120Hz screen and you find yourself dropping back into 30Hz mode for general operations does not make a nice phone to use.

What goes over a display port or a hdmi cable may have absolutely no alignment to what HZ the monitor/tv panel is really refreshing at these days.

[tildearrow]

That can be true for new 2021 phones. Yes there are new 2021 phones that only have a 60Hz panel worse some really cheap ones only have a 30Hz panel. So you are not alone with this problem and its a lot more common that one thinks.

This is not a low-end or mid-range phone. It is a flagship one.

[oiaohm]

This is not a low-end or mid-range phone. It is a flagship one.

https://en.wikipedia.org/wiki/List_of_mobile_phones_with_a_display_refresh_rate_greater_than_60Hz

High refresh rate rare screen feature in 2018 was rare is a more common one now. In 2018 there was a total of 4 phones that were Android that had a screen faster than 60Hz.. That list there still some 2021 flagship phones that still don't have greater that 60Hz display but that has come rare,

None of the displays you will find on any new phone is ideal for playing pal content. You will have judder/frame pacing issues caused by miss aligned screen refresh rate to video encoding frame rate. Of course the higher the monitor panel clock speed the less noticeable it will be.

I find I don't notice the judder with Pal videos with displays running at 120Hz or higher but I really do have a annoying visible to me issue on 90 and less(that are not PAL Hz speeds) with some PAL vidoes. Its fine things like people know that wheels can appear to spin backwards due to frame-rate judder effect on that is wheel is now trying to spin both directions.

For a long time I had only a 60Hz desktop monitor one of the big improvements moving to a 144Hz monitor was better video play back due to less of this problem. Of course I noticed a difference I went looking for why this lead me to all the information on what monitors real do these days.

Lot of people talk about high Hz monitors for game play you don't see many reviewers talking about how the high Hz monitors in a lot of cases result in better video play back for PAL encoded content due to how many 60Hz monitors the panel freq is really locked at 60Hz. Basically tildearrow its not just your phone with the 60Hz and bad PAL playback. PAL content playing back badly on monitors and TVs is horrible common as well and its going to become move common in lower end monitors/tv as chipsets do broad range clocking of panel keeps on disappearing being replaced by PSR work around to advertise 50Hz but then map 50 into 60hz.