I decided to test the daily images released in the leadup to the GA of Ubuntu 20.04. When I started, things worked very well on my test laptop, being a 2011 MacBook Pro. After a few days, an update seemed to break the startup sequence on my aging test rig. The problem with that particular laptop is that one of the 2 built in graphics cards has broken. The standard Intel graphics accelerator is still working fine, but the discreet graphics card is a Radeon effort which gave up the ghost back in 2018. Up until now, Linux has been perfectly happy to ignore the broken graphics card, but as of this update that behaviour has changed.
The following instructions were adapted from this askUbuntu page.
On boot
On the released version of Ubuntu 20.04, the install media fails to boot now too. That means that you need to a way to override the kernel’s desire to attach to the Radeon card. You can do this by adding the following to the kernel parameters in the grub menu:
Once you’re in
Once you’re booted you need to make the following changes to your grub config in /etc/default/grub:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.modeset=0"
Then run:
sudo update-grub
sudo reboot
Where it gets awkward
Under normal circumstances, if the changes to /etc/default/grub don’t work or you haven’t had a chance to make them (e.g. like on the boot after the install), you can just use the method of changing the linux kernel parameters in the grub menu. However, on this particular Mac, the grub menu doesn’t present itself.
This really only leaves you with one option: try and boot something that gets you to a command line and then make changes to the grub config directly. I managed to do this with the ubuntu install media.
Step 1 is mount the local drive that contains your /boot directory:
mount /dev/sda1 /mnt
Step 2 is edit the grub configs. There can be 2: /boot/grub/grub.cfg and /boot/efi/grub/grub.cfg There are warnings at the top of these files telling you not to edit them by hand, but this is a temporary change. Look for instances of ‘quiet splash’ and add ‘radeon.modeset=0’
For example:
menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-d4ac9aad-3e0e-417b-9684-22182d6d1f62' { recordfail load_video gfxmode $linux_gfx_mode insmod gzio if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi insmod part_gpt insmod ext2 set root='hd0,gpt2' if [ x$feature_platform_search_hint = xy ]; then search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt2 --hint-efi=hd0,gpt2 --hint-baremetal=ahci0,gpt2 d4ac9aad-3e0e-417b-9684-22182d6d1f62 else search --no-floppy --fs-uuid --set=root d4ac9aad-3e0e-417b-9684-22182d6d1f62 fi linux /boot/vmlinuz-5.4.0-37-generic root=UUID=d4ac9aad-3e0e-417b-9684-22182d6d1f62 ro quiet splash radeon.modeset=0 $vt_handoff
initrd /boot/initrd.img-5.4.0-37-generic
Step 3 is to reboot and hope that it works. If it does, loop back to making an edit in /etc/default/grub
