Dual-Booting Linux on an Apple PowerBook G4

I wanted to have a powerpc system, and saw a nice 1.66GHz PowerBook G4 for $100 at a local pawn shop, so I snapped it up (after verifyig it could boot from CD-ROM), with the intent of dual booting Linux.

My narrow narrative of success so far:

Problems/solutions

Challenge: in Mac OS, the wifi doesn't seem to be able to see any networks.
Solution: use the ethernet port, which still works. (Or do a fresh install of Mac OS, see below.)

Challenge: the system has Mac OS X 10.4 installed, but did not come with install media, so I needed to find a trustable iso image for Mac OS X (ideally 10.5, the last version to support PowerPC).
Unfortunately, the only reputable ISO online is too large to fit in an 8GB thumb drive, and larger ones don't seem to work (see below).
Solution: buy a retail Mac OS X 10.5 DVD on ebay, $30.
Bonus: doing a clean install of Mac OS X 10.5.6 from DVD also fixed wifi! (I repartitioned while installing to leave half the disk free for Linux, too.)

Challenge: Ubuntu 16.04 PPC installs fine, but when X comes up, the mouse cursor shows but won't move.
Workaround: at the second bootloader prompt, boot the Linux kernel, but with the additional parameter systemd.unit=multi-user.target. You can then do a text-mode login. (i.e. With yaboot, after pressing Tab to see the list of configs, type the name of the config followed by the additional parameter, then hit Enter.)
Fix: This is Ubuntu bug 1571416. Either use a newer Linux (e.g. https://voidlinux-ppc.org/ ), or add radeon.agpmode=-1 to the kernel commandline when booting. If that works, you can make it permanent by editing /etc/yaboot.conf and running sudo ybin -v. This problem is also described in the Debian PowerPC FAQ.

Challenge: when running Ubuntu 16.04, Firefox is too slow to log in to Facebook.
Workaround: You could try an alternate browser. Arctic Fox can at least log in to facebook...

Wonkier tips

Challenge: the thumb drive activity light blinks after dd finishes when copying the ISO to the thumb drive. Is it done?
Solution: use conf=fdatasync, e.g. sudo dd if=osx\ leopard\ install.iso of=/dev/sdb bs=1024k status=progress conv=fdatasync

Challenge: the system doesn't seem to let you boot from USB drives.
Solution: Boot to Open Firmware, and then tell it to boot from USB. (See also Open Firmware -- An introduction for users.)

Challenge: finding the device name for the USB slot with the thumb drive
Solution: Insert the USB device before booting. Then enter Open Firmware, and find the USB device with a disk using the command dev usb0 ls, and repeat with higher numbers until you find one. You should only have to try 0 and 1.

Challenge: the above step does not find your thumb drive
Solution: make sure you inserted the thumb drive before booting. Also, try putting your boot image on a different USB thumb drive; smaller and older drives might be better supported.

Challenge: finding the filename to boot
Solution: if the above step found your thumb drive is named usb1/disk@1, list its blessed boot directory on the device with the command dir usb1/disk@1:,\\. That should list, among others, a file with TYPE tbxi or boot. For Mac OS, use the one with type tbxi (usually BootX); for Linux, if one is named yaboot, use that instead.

Challege: finding the Open Firmware command to boot that file Solution: if dir usb1/disk@1:,\\ found a file named yaboot, the boot command should be boot usb1/disk@1:,\\yaboot

I tested thumb drives by loading them with the Debian 8.10 LXDE iso, and booting them as above.
Thumb drives that passed this test:

Thumb drives that did not work for me:

Challenge: the OS X 10.5.4 image is 8413577216 bytes, too big to fit on an "8GB" thumb drive (the Sandisk Cruzer 8GB has size 8004304896), but does not work when loaded on a "16GB" thumb drive.
Solution: Use a real retail Mac OS X 10.5.x DVD from ebay.