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I installed Ubuntu 24.04 on a new Thinkpad E14 and I am unable to get the display brightness control working. I tried almost all suggestions that were proposed here: Ubuntu 20.04 brightness adjust not working

This includes all possible permutations of the grub CMDLINE. The behavior I get is the following:

  1. With any grub CMDLINE that includes acpi_backlight=native, or acpi_backlight=none I get no backlight device.

  2. With acpi_backlight=vendor I get a backlight device from the thinkpad_acpi driver, but when I change the brightness I get an IO error:

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root@ks-laptop:/sys/devices/virtual/backlight/thinkpad_screen# cat brightness 
10
root@ks-laptop:/sys/devices/virtual/backlight/thinkpad_screen# cat max_brightness 
15
root@ks-laptop:/sys/devices/virtual/backlight/thinkpad_screen# echo 7 > brightness 
bash: echo: write error: Input/output error
root@ks-laptop:/sys/devices/virtual/backlight/thinkpad_screen# cat brightness 
7

No errors are logged in the kernel. The thinkpad_acpi driver seems to be loaded correctly. These are the logs:

[   73.218186] thinkpad_acpi: ThinkPad ACPI Extras v0.26
[   73.218192] thinkpad_acpi: http://ibm-acpi.sf.net/
[   73.218193] thinkpad_acpi: ThinkPad BIOS R2CET34W(1.16 ), EC R2CHT34W
[   73.218196] thinkpad_acpi: Lenovo ThinkPad E14 Gen 5, model 21JRCTO1WW
[   73.226016] thinkpad_acpi: radio switch found; radios are enabled
[   73.257197] thinkpad_acpi: rfkill switch tpacpi_bluetooth_sw: radio is unblocked
[   73.322034] thinkpad_acpi: ACPI native brightness control enabled
[   73.396744] thinkpad_acpi: secondary fan control detected & enabled
[   73.416288] thinkpad_acpi: battery 1 registered (start 95, stop 100, behaviours: 0x7)
[   73.459881] input: ThinkPad Extra Buttons as /devices/platform/thinkpad_acpi/input/input17

When I enable the debugging logs of the thinkpad_acpi driver, it logs the request to change the brightness, but it doesn't log why it fails.

  1. With acpi_backlight=video I get a backlight device where I can write brightness values, but they have no effect.

I am really running out of ideas. One suggestion that I've read was to install the closed-source AMD GPU drivers, but as there are no working drivers from AMD yet for 24.04 I cannot test this.

I am running out of ideas here. Any help is greatly appreciated.

Update 2024-05-05

I have been debugging this issue further. I booted an Ubuntu 22.04 where the backlight works out of the box. The driver in /sys/backlight references the i2c-9 bus. In total this version detects 12 i2c buses. On Ubuntu 24.04, however, there are only 5 i2c buses and no i2c-9. I haven't made progress yet on why this bus isn't detected.

user1786499
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1 Answers1

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I finally fixed it. I noticed that in the Ubuntu 24.04 live system the display brightness control was actually working, so I just re-installed the entire system and now brightness control works.

Why it didn't work after the first installation? I don't know. It could be because I tried to install the AMD drivers for 23.10, which didn't work and I uninstalled them again. Maybe that left something broken? No idea...

user1786499
  • 21