.@ Tony Finch – blog

I have an old Mac Mini which I am repurposing as a Linux box. Last month I upgraded its hardware and posted pictures on Twitter. This weekend I upgraded the software.

However there is a compatibility problem that I have not managed to solve: Linux isn’t able to drive the display hardware as well as it should be able to.

the problem

Linux is only able to drive a display when booted with radeon.modeset=0.

The consequences of this setting are:

Without the radeon.modeset=0 setting the display goes blank apart from an unblinking _; Linux boots OK but X fails to start.

the setup

causes

The Linux kernel radeon driver needs a Video BIOS to support modesetting; without that the graphics stack is crippled. It logs:

[drm:radeon_get_bios [radeon]] *ERROR* Unable to locate a BIOS ROM

The Mac has Video BIOS emulation as part of its UEFI BIOS Compatibility Support Module (CSM). The Mac’s boot ROM enables its UEFI CSM when starting an OS in legacy mode (aka BIOS compatibility mode), e.g. as part of Windows / Boot Camp support.

There is a summary of this issue on the Debian Wiki

So I need to get the Mac to boot Linux in legacy mode. How?

partitions

There is a description of the Mac’s boot mode logic on the Arch forums which says that it uses legacy mode if the disk has an MBR, or if it has a hybrid GPT/MBR with the active boot flag set in one of the MBR partitions.

I inspected both disks with fdisk under Linux, which shows they are both set up according to this scheme.

Windows boots OK, and I believe that like Linux, it also requires legacy mode to work properly.

oddly blessed

MacOS has a bless command that configures the boot process, including options for legacy booting. So maybe I need to bless it with the right incantation?

There is an oddity that the Mac boots by default into Linux (possibly because the SSD is the “first” disk according to the hardware), but when I boot while pressing “option” to get the device selection menu, it only lists the operating systems on the old disk.

This matches the behaviour of systemsetup -liststartupdisks, which only lists Mac OS and Windows. systemsetup -getstartupdisk prints nothing.

There is a (related?) oddity that bless --info --verbose complains that my boot configuration is inconsistent, but the XML plist in the output is too inscrutable for me to see the problem.

[eta] And in Mac OS System Preferences -> Startup Disk, the panel for choosing a volume remains empty. If I try to interact with System Preferences any further, it beachballs and seems to require a Force Quit or a very long wait.

dunno

This is all rather annoying, but I don’t want to spend much more time on it. I’ll leave the Mac Mini running linux headless, so I can get the crappy old monitor and the rest of the debugging clutter off my desk, and leave the DisplayPort on my 4k monitor unused.

… later …

more faff

Julian Field suggested resetting the NVRAM, which to be fair is the Mac equivalent of “have you tried turning it off and on again?”. There is some evidence that I have NVRAM problems so I tried it.

But it did not fix any of the problems, and it made some of them worse: bless became unable to read the efi-boot-device NVRAM variable, and (as I expected) the Mac no longer booted into Linux by default.

Booting into recovery mode and choosing the Mac OS startup disk (Linux was not an option) sorted out the NVRAM issue, and bless no longer complained about inconsistencies. After I disabled System Integrity Protection, bless was also able to change things.

But I was still stuck booting into Mac OS not Linux.

workaround

So I installed the rEFInd boot manager which is much more useful than the Mac’s built-in boot menu. It automatically found the Mac OS, Windows, and Recovery partitions on the old RRD, plus GRUB in the EFI system partition on the SSD, and the Linux kernel itself.

Booting via grub works as before (with modesetting disabled on the kernel command line). Booting Linux directly from rEFInd works even better: the UEFI CSM is enabled and the radeon driver is able to do modesetting, and drive the 4k monitor.

Amusingly, it seems that the Boot ROM (and rEFInd) are not able to drive the 4k monitor, so now it stays blank until Linux is up and running.

But now it works!