Problems with LCD screen brightness in Thinkpad T420i

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iShareFreedom
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Joined: 12/20/2021

One of my computers, the T420i in Trisquel (mate) has a problem that has prevented me from correctly setting the brightness of the screen. The issue is open in the Trisquel Gitlab: https://gitlab.trisquel.org/trisquel/package-helpers/-/issues/175

If anyone can help, their help is well received.

prospero
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Joined: 05/20/2022

Would you mind sharing the output of this command:

lspci -k | grep -EA3 'VGA|3D|Display'

iShareFreedom
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Joined: 12/20/2021

00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
Subsystem: Lenovo 2nd Generation Core Processor Family Integrated Graphics Controller
Kernel driver in use: i915
Kernel modules: i915

thats the output I add it too in to the gitlab

prospero
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Joined: 05/20/2022

I would use that information to search for existing brightness related issue reports about that specific hardware, module and driver, and see if any of the suggested fix/workaround solves the problem.

For example, a fast search seems to suggest that booting with the i915.enable_dpcd_backlight=1 grub parameter sometimes helped. It may or may not help in your specific case, but you can always try.

iShareFreedom
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Joined: 12/20/2021

Where exactly is I supposed to add that text?

Magic Banana

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I am a translator!

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Joined: 07/24/2010

Since it is a parameter for Linux, in the GRUB_CMDLINE_LINUX_DEFAULT variable that /etc/default/grub defines. In a terminal:
$ sudo sed -i "s/GRUB_CMDLINE_LINUX_DEFAULT=\"/&i915.enable_dpcd_backlight=1 /" /etc/default/grub
$ sudo update-grub

Then reboot.

iShareFreedom
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Joined: 12/20/2021

With the command you gave me the configuration file, it's like this in Trisquel:

GRUB_CMDLINE_LINUX_DEFAULT="i915.enable_dpcd_backlight=1 quiet splash"

Is this what you expect?

Magic Banana

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Joined: 07/24/2010

Yes, it is. I gave you a geeky way to achieve this. An alternative way is to use your favorite text editor, with administrative privileges.

iShareFreedom
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Joined: 12/20/2021

Unfortunately, making this change does not solve the problem because moving to a TTY involves losing the above-established brightness and returning to the graphic server if you increase the brightness will automatically go down.

iShareFreedom
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Joined: 12/20/2021

The problem is solved, thank you. Although I think it is not perfect since the brightness percentages, when raised and lowered, do not correspond to the real brightness level.