[ltp] WARNING for everyone doing ThinkPad fan control

[Henrique de Moraes Holschuh]

I have just added this warning to ThinkWiki's fan control page:

=== WARNING: ================================================
When designing fan-control applets, never ignore a valid thermal sensor.

Even if it seems to be stuck at a certain temperature, you must take that
sensor into account.  While it will probably make the fan spin faster than
if the sensor was ignored, that's exactly what would happen when the fan is
under EC control, and could very well be the reason for the "stuck" value in
the first place.

Always play it safe.  If a battery pack seems to want the fan to always run
faster, the only safe thing to do is to make it run faster.
=============================================================

The facts as I know them:

1.  As far as I could find out, there are no battery packs with defective
thermal sensors in the field:  Lenovo would have exchanged those right away
if they were defective, and the Windows utility doesn't flag them as
defective, either.  I got some information that implies the Windows utility
is correct (the packs are not defective).

2.  Some battery packs have been observed to have a sensor "stuck" at 50C.
It is plain impossible for a Linux bug to get *just* that reading incorrect.

3.  The EC won't turn the fan off (and in fact, probably not let it go below
level 3) if a sensor is at 50C.

4.  The Windows utility is not showing the temperature as 50C (there are two
sensors per pack, one near the cells, the other in the charger/monitor), so
it looks like that sensor is not the more important thermal sensor of the
battery pack.

Do the math.

Don't use fan control to run the machine any hotter than the EC would.
There is a damn good reason why the drivers require EXPLICIT configuration
by the user to even ENABLE fan control.

You have been warned.   

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh