Decided to bite the bullet and attempt to install debian sid on my emac yesterday. After fighting KMS and the strange timings of the emac monitor for a while I have graphics working (with KMS and radeon).
The key is that you must manually specify a monitor EDID binary, but do it in the initramfs so it is available at boot.
A rough guide, hopefully I remembered all the steps:
Make sure you have an ethernet connection for internet during the install
Use https://cdimage.debian.org/cdimage/ports/10.0/powerpc/iso-cd/debian-10.0-powerpc-NETINST-1.iso
(I boot from USB by dd'ing to a USB stick at /dev/sdb with
dd if=debian-10.0-powerpc-NETINST-1.iso of=/dev/sdb
and then booting from OF with
boot ud:,\\yaboot
... YMMV, but make sure /dev/sdb is really your USB stick)
After installation boot by entering "Linux nomodeset" at the yaboot "boot:" prompt (just so you can boot to console and fix some things).
Login and change to root with "su". I also need to "export PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin" as root since for some reason this isn't set already (add it to /root/.bashrc).
Download debian-ports-archive-keyring_2018.01.05_all.deb from http://archive.ubuntu.com/ubuntu/pool/universe/d/debian-ports-archive-keyring/ and install it with "dpkg -i debian-ports-archive-keyring_2018.01.05_all.deb". You can do the download either from the newly booted machine or copy the file on a USB stick.
Edit /etc/apt/sources.list to look like:
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deb http://ftp.ports.debian.org/debian-ports/ unstable main
deb http://ftp.ports.debian.org/debian-ports/ unreleased main
deb http://ftp.ports.debian.org/debian-ports/ experimental main
deb [arch=all] http://ftp.debian.org/debian unstable main contrib non-free
deb https://www.deb-multimedia.org sid main
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Update all your packages with:
sudo apt-get update && sudo apt-get upgrade -y
Install some other packages (CA certs, ssh server, xfce4 desktop, new kernel, all firmware packages):
apt-get install ca-certificates xfce4 openssh-server linux-image-5.3.0-2-powerpc linux-headers-5.3.0-2-powerpc firmware-linux-free firmware-linux-nonfree
Grab the 1280x960.bin file attached to this post. Run:
mkdir -p /lib/firmware/edid
Copy the file into the newly created directory.
Create a file /usr/share/initramfs-tools/hooks/edid with the following contents:
(facebook is mangling the text here, so I've created a pastebin: https://pastebin.com/LAMkdTVf)
Give it executable permissions with:
chmod +x /usr/share/initramfs-tools/hooks/edid
Run "update-initramfs -u"
Finally reboot. At the yaboot "boot:" prompt enter "Linux drm.edid_firmware=edid/1280x960.bin".
You should find everything works great :)
Now you can permanently add the change. Edit /etc/yaboot.conf and after "initrd=/boot/initrd.img" line add:
append="drm.edid_firmware=edid/1280x960.bin"
Then run "ybin -v"