My old iMac (late 2015, retina 27 inch) sadly reached end of support on OSX a while back. It still has a beautiful screen and enough processor power to do most daily task. I brought it back out of storage to be a Claude co-work box. After a little dalliance with OpenCore (not bad as a Mac but unsafe through OS updates, some graphics driver issues, and a bit slow), it is now running Ubuntu 26.04.

The big problem with Ubuntu was power management. Once the machine went to sleep or screen dimmed it often didn’t wake up again, or if it did, it took just over 60 seconds to respond.

I’ve turned off sleep since I want the machine to be awake all the time to be ready to respond to dispatch commands from Claude co-work. However, I do want to have the screen dim (this really means screen off) happening so that the screen does not get burned in and also so that a password is required to access the machine.

After a quite a bit of debugging and help from Claude, here is the fix:

In /etc/default/grub
Find line like: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Add the two extra parameters to make it:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=native amdgpu.dpm=0"

sudo update-grub

<REBOOT>

amdgpu.dpm=0 disables broken power management (GPU then runs at fixed clocks). 
acpi_backlight=native switches on the native backlight driver, which helps the panel come back reliably.

If you have a similar old Apple machine, indicative of this problem / fix is repeated amdgpu error messages in kernel log:


sudo dmesg -w

amdgpu 0000:01:00.0: last message was failed ret is 0
[repeated many times]