So, intrepid me (not really) I upgrade my Stretch to 9.4 and (re)booted this
morning without success (boot hangs at or with the line "sp5100_tco I/0
address 0x0cd6 already in use").
Now, I'm impatient by nature, and after five or ten minutes staring at the
"sp5100" line I rebooted into recovery mode, whereupon a fsck was
performed. At which point I entered the "systemctl default" command and
the machine booted up fine into my LXDE desktop.
I blacklisted the 'sp5100_tco module.' Apparently there is a conflict
between that module and the i2c_piix4 module. It appears I can have one
or the other but not both at the same time.
I'm unclear about the implications of my blacklisting sp5100 but after
having done so I rebooted successfully with no hangs.
BTW, I've had the sp5100 warning message at boot ever since I converted
to systemd. But with only a ten second delay or so. Not sufficient to try my
So here I am without a watchdog.
Bah, the latest news, the latest news is not the last.