I found an install of LinuxPPC R4 to be reasonably straightforward up to the
point of switching from floppy boot to direct boot from the internal disk.

For this discussion I will assume that the install is at this point and that 
the Quik loader installation was chosen at the end of the RedHat install. 
Generally single quotes are for clarity and shouldn't be entered as shown.

Up to this point the setting for boot-device was 'fd:installer.coff'

Set boot-device to 'scsi/sd@0:0'
Set boot-file to ' /boot/vmlinux root=/dev/sda5' // Note leading space!

This specifies to boot from the internal HD at SCSI ID 0:partition 0. This is 
where the boot loader lives, Linux itself is pointed to by the boot-file value,
in this case to /boot/vmlinux on partition 5 of the first device of the SCSI 
chain (/dev/sda5). The fifth partition is the one that I named 'root' when I 
partioned my drive. If in doubt, fire up the LinuxDisks utility under MacOS 
and find the partion and location of the vmlinux file to determine these 
settings.

Now for the good stuff:
The issue is that the Open Firmware 1.0.5, when set to use the internal HD as 
the boot device, tries it before it's spun up, gives up and leaves the system 
dead in the water.
To fix this, connect a dumb terminal or emulator on the modem port of the target
machine. I used a MacPlus running ZTerm connected with a Mac printer cable.
Set the terminal to 38400 baud, 8 bits, 1 stop bit. You should try this with a 
terminal program at each end to make sure you have the terminal settings right.
Set auto-boot? to OFF and restart the machine. input-device and output-device 
must also be ttya, but this is the default.
The machine will print a message to the terminal mentioning Open Firmware 1.0.5,
the screen will stay black and no HD access will be heard. You will be facing a
prompt that looks like '0>' on the terminal.
Test your variable settings by typing 'boot'<cr>. If everything is correct,
linux should boot (since the drive has had time to spin up).

At this terminal you can accomplish a super-set of what you can do under the 
'Boot Variables' control panel from MacOS.
'printenv' will list the current and default settings of variables,
'setenv' <variable-name> <value> sets the variables

Now, type 'nvedit' to place a patch routine in the non-volatile RAM.
***Credit due to Alan Mimms, whose contributions are noted under 'Making the 
7500/7600 initialize the screen' elsewhere in the FAQ****.
The down arrow (^N) on a MacPlus keyboard scrolls down through the existing 
contents a line at a time. When some clear space comes up enter:
: slowboot begin
." Waiting for boot-device to spin up..." cr d# 500 ms
boot-device ['] $boot catch drop
key? until ;
Option-C // (^C) to leave the editor
nvstore  // to write back to the non-volatile RAM
turn the computer off and on to get Open Firmware to probe the nvram

This code loops until a key is pressed. In the loop it prints the message,
a linefeed, waits a half second and then attempts to boot. Mind the whitespace
after the : the ." the d# and the 'until', it is significant.

Before commiting to editing the nvram, you can practice by just entering the
four lines from ': slowboot begin' to 'key? until ;' at the 0> prompt and
then typing 'slowboot' to execute. This will allow you to work out all the 
bugs in the boot-device,boot-file settings and any bugs in entering the patch.
As long as auto-boot? is left set to false, cycling the power will get you back
to the 0> prompt. I highly recommend this while testing.

Once slowboot boots Linux as desired, edit it into the nvram and set the
variable boot-command = 'slowboot'. At the 0> prompt this is done with:
setenv boot-command slowboot
Set auto-boot? to true:
setenv auto-boot? true
and reset the machine one last time. Done.

Of course, you may wish to reconsider and take the BootX route.

-wdlvw@magma.ca