This post has been superseded by an updated version, available here.
After bashing my head against the metaphorical wall for a week or so, I have finally managed to install Linux on a 27” Retina iMac with Skylake. This is what I’ve learned, and it might help you if you’re trying to do the same thing.
The usual procedure applies here, if you’ve installed Linux on an Intel Mac before. This method doesn’t require you to install any additional software like rEFInd, although you certainly can if you want to. It might be easier if you plan on triple-booting with Windows as well.1
Bottom line, this is going to work best with Linux kernel 4.6 or later. This is due to both Skylake support that exists in that kernel, and the AMDGPU open-source driver, which works very nicely with the R9 graphics cards that retina iMacs ship with. The easiest way to accomplish this is to use a distribution that’s already shipping with kernel 4.6 or greater. I’ve had success with Arch and Fedora with the following:
- Prepare a USB flash drive using the distribution’s live ISO.
- Hold down the
Optionkey when booting the Mac, and select your flash drive.
Edit the GRUB entry for the live installation environment with the following kernel parameters:
irqpoll radeon.modeset=1 no_timer_check2
You may notice that Ctrl-X doesn’t work in GRUB. I don’t know why that is, and would love to know the reason. Press F10 to boot from the GRUB editing screen - some distributions mention F10 in their instructions, and some don’t. It should work universally, though.
- Install Linux as usual to any empty non-partitioned space on your hard drive.3 It’s okay (and required, if you’re not using rEFInd) to make a small EFI partition for Linux to install the bootloader to. In the future, when switching between operating sytems, you can hold down the
Optionkey when booting to pick from your available partitions.
- Reboot from the installer into your fresh Linux installation. You’ll need to edit the GRUB entry using the parameter from step 3 again (for the last time, I swear!)
- Once you’ve booted into Linux, edit your GRUB configuration files to include the parameters from step 3 by default. Regenerate the GRUB files using your distro’s tools, and reboot one last time to make sure it works. If it doesn’t work, you can always edit the entry at boot time again to get back into the system.
Distributions with old kernels
The following notes are especially useful for people who want to install Ubuntu – but the basic concept is the same for any distribution you like that doesn’t yet come with kernel 4.6 or newer. Basically, you’re doing the same thing as above, except with some different kernel parameters to get the OS to boot. Once the OS is installed, you’ll be installing kernel 4.6 or greater, and then using the parameters from step 3 above.
Edit the GRUB entry with parameters
acpi=off nomodeset when booting from the installation ISO. These are bare-bones parameters that only show 1 core and will deliver low performance, but this isn’t an issue for the installation process. Full performance will be restored later.
After installation and rebooting, install kernel 4.6. Most distributions provide some way of obtaining mainline Linux kernels that are pre-packaged for the distro’s package manager. For example, you can grab 4.6.3 (the newest kernel at time of writing) from Ubuntu’s mainline kernel PPA and install it with apt. Once it’s installed, edit the GRUB config with those parameters from step 3 above, and you should be all set.
Screw you, Canonical
Unity won’t work without the GPU drivers installed. This means that the normal Ubuntu installer ISO will not work. Point blank. Don’t bother. Unity uses Compiz as its rendering engine, and it won’t load unless it’s happy with the GPU driver situation. This means the installer itself won’t load either. Good job, guys.
To install Ubuntu, grab the Xubuntu installer. (Any desktop environment that doens’t use Compiz by default will work.) After you’ve installed the OS, feel free to install the DE of your choice.
As of today (2016-07-11), this information should be correct. Future kernel versions will hopefully remove the need for some of these settings. Until that time, good luck!
It’s generally recommended to keep a small partition on your drive with Mac OS left installed on it, since firmware updates are provided only via Mac executables. ↩
no_timer_checkwas the secret sauce that got the iMac booting with all 4 CPU cores. Using
nolapicas a kernel flag also boots successfully, but with only 1 core visible to the OS. The
irqpolloption is necesary due to some observed keyboard weirdness. Finally, using
radeon.modeset=1allows the internal DisplayPort system that drives the LCD to negotiate display resolution and other such details. Setting it to
0is useful for troubleshooting purposes, and will force a lower resolution. ↩
You did shrink your Mac OS partition to make free space for Linux before you started, right? ↩