2.6 upgrade: no root partition

[Gergo Szakal]

I have just upgraded to 2.6. Compiled everything as per the default
settings, installed and after the reboot, I get the well-known
ffs_mountroot: can't find rootvp message. Unfortunately I cannot issue
'?' since I am having an USB keyboard which does not work in this
situation. I'll get a PS/2 one ASAP but until then can someone give me
a hint? This is a very old i810-equipped board with an ATA drive that
has been working for years with DF since 1.4. i can log into the box via
the old kernel so I can provide dmesg and other things you need.

Thanks in advance.