Updating iMac 2015 5K model to use AMDGPU kernel drivers in Ubuntu 26.04

Hello. I acquired a 2015 iMac with 5K display. Unfortunatly this Mac doesn’t support the latest version of MacOS, so I decided that I would install Ubuntu on it. The installation was straight forward. I have a blog on this site that details how to install Linux on Apple systems. Just following that Blog to install Ubuntu on an iMac.

So I have been running it pretty consistantly and haven’t had any issues until I noticed that if I try to stream video through Brave or Chrome broswers, or really any web browser, the performance was terrible. I did find that I could disable hardware accelleration in Brave and Chrome, but some sites it worked, most it did not.

After some online searching I found out that Ubuntu 26.04 by default on older AMD Graphics cards, which this system does have, it uses the older radeon kernel driver. So, as a test, I decided to try to force the newer amdgpu driver and use that instead.

To do this I had to black list the radeon driver and enable the amdgpu driver, and then I found a couple of other parameters to get this working smoothly.

First, you need to update the /etc/default/grub configuration. I like to use vi so:

sudo vi /etc/default/grub

Once I have the file open, look for the line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

We’re going to modify this line to blacklist the radeon driver and use the amdgpu driver. After splash, we need to enter the following:

radeon.cik_support=0 amdgpu.cik_support=1 amdgpu.dc=1 amdgpu.ppfeaturemask=0xfff7bffd

The full GRUB_CMDLINE_LINUX_DEFAULT should look like this:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.cik_support=0 amdgpu.cik_support=1 amdgpu.dc=1 amdgpu.ppfeaturemask=0xfff7bffd"

Save the configuration by pressing the ESC key and typing :wq to save and quit vi.

Next, reboot your computer. Once it starts up, you can run the following command to verify that you are using the amdgpu driver:

lspci -k | grep -E -A 3 "VGA|3D"

This will be the output:

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Amethyst [Radeon R9 M395/ M395X Mac Edition] (rev 01)
        Subsystem: Apple Inc. Device 014c
        Kernel driver in use: amdgpu
        Kernel modules: amdgpu

Now, if you disabled graphic accelleration in your web browser, you can re-enable it and try going to a site like YouTube and stream a video and you will notice it is working correctly and streaming normally.

I hope this helps.