PowerMac G5 spurious sensor readings

[Matthew Rezny]

I have a G5 of the first model (PowerMac7,2) on which I've been using FreeBSD/ppc64 for over a year. Today, it suddenly rebooted. Not the first time by any means, but this is the first time I found the following log message:
Jan 17 17:32:19 powermac kernel: WARNING: Current temperature (MLB MAX6690 AMB:127.8 C) exceeds critical temperature (80.0 C)! Shutting down!

This is the first time I have seen such a message. After reboot, that sensor shows a temperature near 30C, which seems appropriate. The reading of 127.8C looks suspiciously like a max value. My only guess is there was a bad read that resulted in 
the sensor value going over the threshold. That raises a question in my mind as to whether there is any filtering or sanity checking of the data. Could a single bad read cause the threshold to be exceeded and trigger shutdown immediately, or would 
the excessive value have to be returned from that sensor multiple times for it to be believed an acted upon?

$ uname -a
FreeBSD powermac 9.1-RC1 FreeBSD 9.1-RC1 #0: Thu Aug 16 00:43:39 UTC 2012     root at anacreon.physics.wisc.edu:/usr/obj/usr/src/sys/GENERIC64  powerpc

The build is a bit old, though I wouldn't expect too much change to the code in question since then. I will update to 9.1-RELEASE or -STABLE in the next few days, but as this is a problem that has happened once in over a year, I wouldn't call it 
resolved just by a quick failure to reproduce after updating.

I was already planning to do an update after the box has completed it's current task. I noticed a problem with excessive output causing the console to hang. A couple days ago I found the machine apparently hung in that the keyboard and mouse were 
not responsive, but I found it was still alive on the network and I could ssh in to reboot. The only clues were no buffer space for dmesg to output anything before reboot, and a rather full /var/log/messages file which had exhausted the drive. 
Under the same workload (and after freeing some drive space), the problem reoccurred in a matter of hours, but this time with me watching. While running ddrescue against a drive with some bad sectors, read errors flood the console in spurts. When 
some dozens of read errors are displayed at once, the console scrolls whole pages by in a fraction of a second, and then goes dead. Messages that should go to console are not shown on screen but are in the log. Attempts to switch virtual console or 
to reboot are not successful, but ssh access continues to work and the box is clearly still processing other workloads. The only sign of life from the console are the messages about flushing buffers just before completion of the reboot commanded 
via ssh.