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Thread: External monitor refresh rate

  1. #1
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    Default External monitor refresh rate

    Hi,

    I have a Dell XPS 9560 to which I connect a LG 27UD88 monitor over USB-C.
    50% of the time it uses a refresh rate of 30Hz instead of 60Hz.
    I can change it to 60Hz by going to Displays and selecting 60Hz instead of auto.
    However this is a bit anoying as I have to do this multiple times a day.

    xrandr shows the following:
    Code:
    DP-1 connected 3840x2160+3840+0 (normal left inverted right x axis y axis) 600mm x 340mm
       3840x2160     60.00 +  30.00*
    So it seems to have 60.00 as a default.

    Xorg.0.log shows the following when plugging the monitor in:
    Code:
    [ 31122.191] (II) modeset(0): EDID vendor "SHP", prod id 5238
    [ 31122.191] (II) modeset(0): Printing DDC gathered Modelines:
    [ 31122.192] (II) modeset(0): Modeline "3840x2160"x0.0  533.25  3840 3888 3920 4000  2160 2163 2168 2222 -hsync -vsync (133.3 kHz eP)
    [ 31123.100] (II) modeset(0): Allocate new frame buffer 7680x2160 stride
    
    I'm running OpenSUSE Tumbleweed 20180318 with KDE 5.12.3.
    I'm using Xorg with the Intel graphical chip (instead of the NVIDIA).

    What could be the reason for the system to pick the non default refresh rate?
    Is Xorg responsible for picking the refresh rate or is it another system?

  2. #2
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    Default Re: External monitor refresh rate

    Have you tried setting the desired refresh rate via System Settings > Display and Monitor? That should help with creating a persistent configuration that takes effect when the external display is connected. (In the past I've also used a KDE autostart script to achieve similar.)

    It is the driver that interrogates the display device capabilities, and the X-server that sets the display mode.
    Last edited by deano_ferrari; 27-Mar-2018 at 00:02.

  3. #3
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    Default Re: External monitor refresh rate

    Thanks for the fast answer.
    Yes I used System Settings - Display and monitor (that's called Displays when I open it).
    After that it seems to persist for a little while.
    I can unplug and plug the monitor back in while keeping 60Hz.
    But after some time it reverts back to 30Hz while plugging the monitor in.

  4. #4
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    Default Re: External monitor refresh rate

    You could examine what is being logged in /var/log/Xorg.0.log when the external display is connected perhaps.

  5. #5
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    Default Re: External monitor refresh rate

    A possible workaround for you might be to employ a custom udev rule (to detect when the external display is connected), which calls a script with xrandr commands.

    For example
    https://unix.stackexchange.com/quest...from-udev-rule

    Note that in order to for the xrandr commands to work, you need to pass the DISPLAY and appropriate (active suer) Xauthority environment variable as well. So, something like....

    Code:
    env DISPLAY=:0 XAUTHORITY=/tmp/xauth-1000-_0 /usr/bin/xrandr.....

  6. #6
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    Default Re: External monitor refresh rate

    The output from Xorg.log while attaching the monitor as it went to 30Hz is already in my initial post.
    This is the log from when it goes to 60Hz:
    Code:
    [ 49870.250] (II) modeset(0): EDID vendor "SHP", prod id 5238
    [ 49870.250] (II) modeset(0): Printing DDC gathered Modelines:
    [ 49870.250] (II) modeset(0): Modeline "3840x2160"x0.0  533.25  3840 3888 3920 4000  2160 2163 2168 2222 -hsync -vsync (133.3 kHz eP)
    [ 49871.138] (II) modeset(0): Allocate new frame buffer 7680x2160 stride
    
    So it seems there is no difference.
    The custom udev rule is something I will look into.
    However it's much nicer if we could find the cause of the issue so it works out of the box for other users as well.

  7. #7
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    Default Re: External monitor refresh rate

    Well the modeline you report in the opening post and last post is the same
    Code:
     Modeline "3840x2160"x0.0  533.25  3840 3888 3920 4000  2160 2163 2168 2222 -hsync -vsync (133.3 kHz eP)
    and the values are consistent with a 60Hz refresh rate (and reduced blanking interval)...
    Code:
    ~> cvt 3840 2160 -r
    # 3840x2160 59.97 Hz (CVT 8.29M9-R) hsync: 133.25 kHz; pclk: 533.00 MHz
    Modeline "3840x2160R"  533.00  3840 3888 3920 4000  2160 2163 2168 2222 +hsync -vsync
    I don't see a 30Hz refresh rate modeline reported.

  8. #8
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    Default Re: External monitor refresh rate

    Quote Originally Posted by joepadmiraal View Post
    However it's much nicer if we could find the cause of the issue so it works out of the box for other users as well.
    This is bleeding edge technology. I note a lot of posts online discussing users struggling to get 60Hz refresh with these 4K monitors. There may be limitations with the GPU impacting here.

  9. #9
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    Default Re: External monitor refresh rate

    Also don't rule out energy saving modes of these new monitors.
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  10. #10
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    Default Re: External monitor refresh rate

    There may be limitations with the GPU impacting here.
    I don't think this is the case as when I was using Ubuntu with Gnome on Wayland before it did always use 60Hz.
    I do have an Ubuntu 16 installation on this same machine which runs on Xorg so I can do some testing with that to rule out KDE and OpenSUSE.


    Also don't rule out energy saving modes of these new monitors.
    The Xorg log shows it's getting DDC information from the monitor which results in the given modeline.
    If I parsed the modeline correctly, the monitor seems to report 59.9966246624662Hz.
    As the monitor reports this, I don't think it has anything to do with power savings.

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