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Here are my specs:

OS - Ubuntu 20.04.1 LTS 64-bit

Device - iMac mid 2011 21.5 inch

Processor - Intel® Core™ i5-2400S CPU @ 2.50GHz × 4

Graphics - AMD® Turks / AMD® Turks

Screen - 1920 by 1080 pixels

More info: https://support.apple.com/kb/sp623?locale=en_GB

I have switched off auto adjust, and moving the slider does not adjust screen brightness. It remains the same. Luckily, it has been fixed at a kind of medium brightness, so it's neither too bright nor too dark, but I would obviously like to be able to adjust it. Is there a fix to get control functionality working, as opposed to having to use commands?

Beware that I'm not too command/linux savvy.

Thanks in advance to any answers!

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6 Answers 6

4

Try to add acpi_backlight=vendor as a boot parameter.

Run in a terminal:

sudo nano /etc/default/grub

and add the parameter like this (edit the line):

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"

Then save the file, run sudo update-grub and reboot.

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  • I'm sorry... I did say I'm not so good with commands, etc. So, I can run the initial sudo. But what do I need to edit on the secondary line. And do I write it in the same line? Or basically... sudo nano /etc/default/grub hit enter GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor" hit enter Like that?
    – awjm
    Nov 27, 2020 at 13:18
  • When you run the command, nano editor will open. Then edit the existing line, Press Ctrl+O to save and Press+X to exit.
    – Pilot6
    Nov 27, 2020 at 14:08
  • 1
    Hey. I tried the above. I'm pretty sure I followed it to the letter. However, still no control over brightness. The strange thing is... under the settings app, the brightness slider is gone from the power menu :/
    – awjm
    Nov 28, 2020 at 12:44
  • The brightness degraded and fixed to some distinct value. The brightness setting has gone from Settings>Power>Power Saving. Could you please shed more light? Is this the intended output? How can I adjust the brightness now? Apr 29, 2021 at 0:28
  • Works in Ubuntu Unity 22.10 running in an iMac from 2011. Oct 13, 2022 at 21:40
3

Using iMac12,1 with Ubuntu 23

Open the grub file from the directory /etc/default or run the command:

sudo nano /etc/default/grub

Edit the following line from:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pcie_aspm=force acpi_backlight=native"

Run the command:

sudo update-grub

Reboot

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  • 2
    Working with brightness control under volume control and Apple keys on keyboard
    – pinei
    Jul 19, 2023 at 19:37
  • 2
    This works perfectly on my iMac11,2. I have the i3 base model with an AMD HD 4670.
    – Béné
    Aug 27, 2023 at 15:06
  • This also perfectly works on an iMac 12,2.
    – Octopus
    Oct 26, 2023 at 9:20
2

I found this method to work very well:

echo 50 > /sys/class/backlight/radeon_bl0/brightness

(replace 50 with the value you want, of course)

Check your /sys/class/backlight folder for video cards. I have a 2011 iMac and the card is called radeon_bl0 for me, sometimes it's called radeon_bl1.

If you echo a value to that file that is lower than the value in /sys/class/backlight/radeon_bl0/max_brightness, it will set the value correctly.

A lot of good information can be found here as well: https://wiki.archlinux.org/title/backlight

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  • I have tried your workaround but the brightness remains unchanged. What I have checked is that the files brightness and actual_brightness both get updated with the iMac brightness buttons (F1, F2) My card is the same as yours, radeon_bl0. Do you have to launch some refresh command? Mar 16, 2022 at 7:14
  • In my iMac15,1 in ubuntu live I could change brightness with this command ``echo 89 > /sys/class/backlight/acpi_video0/brightness`. 89 is the highest valid value, 90 is "invalid". Feb 4, 2023 at 5:17
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This specific iMac revision (iMac 12,1 and 12,2) has its backlight control set up in an unusual way; it was affected by an oversight in the radeon driver until fairly recently. See https://lkml.org/lkml/2022/2/21/996 (February 2022). If the approach written by Gert M does not work, or more specifically if you do not have any directory starting with 'radeon' in /sys/class/backlight/, you might need to update your radeon driver to one that was built after this patch.

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Here is a workaround to change the brightness level for the apple i-mac using ubuntu 22.04

in terminal cd to /sys/class/backlight/radeon_bl0

then sudo nano max_brightness and make a note of the value (mine was 255)

then sudo nano brightness and change the value to 255 (or whatever brightness level you prefer)

WARNING if you set it to low you might not be able to see the screen.

This allows me to change the brightness levels since the sliders and system preferences don't change it at all.

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I'm currently running Xubuntu 23.04 on my mid-2011 iMac (iMac12,1). What worked for me were setting these Grub parameters:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=native"

Once that is set there should be a device called radeon_bl1 in /sys/class/backlight and the brightness control will function normally.

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