Multi-head setup - use only one display for vc

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Website · Post by **Zucca** » Mon Jan 14, 2019 6:49 am

My google-fu failed.

I know there is a way to disable displays via kernel command line.
I have three different resolution monitors on my desk and on the them is rotated. To look less messy I'd like to disable all but one, until user inside X/wayland -session turns the other(s) on.

I think it was some kms feature...

xdarma · Post by **xdarma** » Mon Jan 14, 2019 7:19 am

Maybe this comment can be useful?
Permanently disable second screen

Website · Post by **Zucca** » Mon Jan 14, 2019 5:20 pm

Do you happen to remember how to list all the display connectors from kernel's perspective. I remember that xrandr lists aren't neccessary same.

I guesss I could find them from /sys maybe ... Hmmm.

Website · Post by **Zucca** » Wed Jan 16, 2019 9:47 pm

Ok. I tried messing with video= on kernel command line and I lost all the native resolutions of my screens.
I guess the setting must conflict with my amdgpu drivers. Maybe I should try to command amdgpu instead of video?

Ant P. · Post by **Ant P.** » Thu Jan 17, 2019 5:39 am

I'm also interested in the answer to this, but haven't figured it out so far.
(two displays, want fbcon to fill the larger one but still use both for X11. Disabling one on the cmdline seems to permanently remove it until reboot.)

Website · Post by **Zucca** » Thu Jan 17, 2019 11:30 am

I changed the topic title to fit better and hopefully lure in someone who knows the answer.

In meantime I had an idea: Replace vc with kmscon as early as possible in the boot process. This may need initramfs. I just don't have any idea how to do it. Or if it's even possible. Then add disabling of the other display(s) in the mix...

I'll try to RTFM of kmscon...

sao98021 · Post by **sao98021** » Thu Jan 17, 2019 11:31 am

meh i didn't see that you already messed around with 'video', so scratch that. except i guess the resolution issue, i suppose you could also set it manually with <xres> x <yres>: resolution

it would look like

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video=<conn>:<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]

i'm not sure if this would be of any value to you, but i stumbled across something similar, except the guy wanted his laptop monitor disabled.

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I managed to find a fix for my problem. I simply added this to my /etc/default/grub file:

GRUB_CMDLINE_LINUX_DEFAULT="<default paramaters> video=LVDS-1:d"
Then did a good ol' fashin update-grub.

This disabled the laptop's LCD on boot, and allowed the external monitor to automatically set its resolution, which worked without a hitch.

you can use 'video' multiple times

there is also

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Unfortunately, GRUB loads very early in the boot process before any drivers. It relies on the BIOS to put its menus up on the screen, so you're totally at the mercy of the hardware at that point.

he worded it like that because apparently, "Which screen gets BIOS output during the boot process is up to the graphics card."... so that may be a problem, but that thread came to the conclusion that its not a yes or no answer, it's a hard 'maybe'

there is also some issue doing this with displays using HDMI, but it seems to be more related to using TV's as monitors, which is what i do.

Website · Post by **Zucca** » Thu Jan 17, 2019 12:09 pm

Yeah. Using video= messes things up. :(
I actually managed to get it working on vc BUT

other diplays were on until OpenRC configured virtual console
setting resolution via video= (4k in my case) resulted to blank screen, so leaving resolution unspecified gave me a resolution which had thumb sized fonts
after entering X there wasn't any resolution higher than 1024*768 to choose from xfce's display configuration (amdgpu got disabled?)

sao98021 · Post by **sao98021** » Thu Jan 17, 2019 12:27 pm

Zucca wrote:Yeah. Using video= messes things up.
I actually managed to get it working on vc BUT
other diplays were on until OpenRC configured virtual console
setting resolution via video= (4k in my case) resulted to blank screen, so leaving resolution unspecified gave me a resolution which had thumb sized fonts
after entering X there wasn't any resolution higher than 1024*768 to choose from xfce's display configuration (amdgpu got disabled?)

what happens if you set the resolution in video= to something lower than 4k, like 1980x1080, or 2560×1440? (which is the next one lower than 4k((3840×2160))? forgive me if you've already tried.. if it yields the same result then...only things i can think of is a conflict somewhere either with a conf, or amdgpu like you said... or maybe something finicky with the display ports + specific monitors, i know they all don't behave the same, or perhaps (and i feel a little stupid for suggesting to you) maybe something in the kernel version or configuration dealing with framebuffer, xrandr

Website · Post by **Zucca** » Thu Jan 17, 2019 5:40 pm

I didn't bother to test. Even if the resolution setting would work on lower resolutions, being not able to turn other displays back on is a no-go. And I assume changing resolution on my primary display would not somehow enable turning on other displays later on. So I think disabling diplay outputs via video= is permanent, until reboot.