How to bless boot partition for Ubuntu 22.04 on 2009 MacBook Pro

I have performed a full-disk install of Ubuntu 22.04 on an old MacBook Pro (MacBookPro5,2 from 2009, Core 2 Duo) machine completely wiping OS X in the process.

I'm now facing the known issue of the 30-second white screen delay on boot. I already figured out I have to use the OS X bless command to set Ubuntu as the boot partition. My question is what command-line arguments do I have to pass exactly? All examples on the internet make use of the --legacy option and talk about BIOS mode, but from what I could figure out my Ubuntu installation is using EFI (the /sys/firmware/efi directory exists).

Am I right in assuming then that I shouldn't use the --legacy option when blessing the partition? Also which partition do I have to bless? The /dev/sda1 one?

Here's the output of df -h when run in Ubuntu:

t30@macbook:~$ df -h
tmpfs           770M    1,7M  768M    1% /run
/dev/sda2       457G     12G  423G    3% /
tmpfs           3,8G       0  3,8G    0% /dev/shm
tmpfs           5,0M    4,0K  5,0M    1% /run/lock
/dev/sda1       511M    6,1M  505M    2% /boot/efi
tmpfs           770M     52K  770M    1% /run/user/1000

So would I just invoke bless like this (replacing /dev/sda1 with whatever it's mounted as under bless)?

bless --device /dev/sda1 --setBoot

So after trying out different variations, here are the results:

bless --device /dev/disk0s1 --setBoot doesn't appear to do anything. Still long timeout on white screen. System still booting afterwards.

bless --device /dev/disk0s2 --setBoot same as above.

bless --device /dev/disk0s1 --setBoot --legacy still long timeout on white screen. After that system goes to black screen and won't boot into Ubuntu.

There are only /dev/disk0s1 and /dev/disk0s2 listed in fdisk for /dev/disk0. I'm assuming /dev/disk0s1 is /boot/efi, but I have no idea.

tl;dr: One possible command would be bless --folder /Volumes/EFI --file /Volumes/EFI/EFI/ubuntu/grubx64.efi --setBoot.

Since the question also involves boot times, I have included the boot times and specs on my Mac. I would expect the OP's Mac to be slower than my Mac.

My Mac has the following specs:

• iMac (21.5-inch, Mid 2011)
• 2.5 GHz Intel Core i5
• 16 GB 1333 MHz DDR3
• 1 TB SATA II (3 Gb/s) internal HDD

The table below shows the operating systems currently installed on my Mac.

I took the following steps to test using the bless command to set Ubuntu as the default operating system to boot.

1. Started the Mac and held down the option key until the Mac Startup Manager icons appeared.
2. Choose to boot from the macOS High Sierra Recovery volume on the internal drive.
3. Mounted the only EFI volume (/dev/disk0s1) on /Volumes/EFI.
4. Entered the following command in a Terminal window.^†

   bless --folder /Volumes/EFI --file /Volumes/EFI/EFI/ubuntu/grubx64.efi --setBoot
   

5. Restarted the Mac, which booted GRUB, then Ubuntu.

^{†This is the command the OP posted in a comment.}

Table of Boot Timing is shown below. Here, GRUB is configured to wait 10 seconds and Ubuntu has automatic login turned on.

Below is the contents of the current /etc/default/grub file on my Mac.

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_TIMEOUT_STYLE=hidden
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
GRUB_TERMINAL=console
GRUB_DISABLE_OS_PROBER=true