ACPI: ibm-acpi: implement fan watchdog command

This patch implements a fan control safety watchdog, by request of the
authors of userspace fan control scripts.

When the watchdog timer expires, the equivalent action of a "fan enable"
command is executed.  The watchdog timer is reset at every reception of a
fan control command that could change the state of the fan itself.

This command is meant to be used by userspace fan control daemons, to make
sure the fan is never left set to an unsafe level because of userspace
problems.

Users of the X31/X40/X41 "speed" command are on their own, the current
implementation of "speed" is just too incomplete to be used safely,
anyway.  Better to never use it, and just use the "level" command instead.

The watchdog is programmed using echo "watchdog <number>" > fan, where
number is the number of seconds to wait before doing an "enable", and zero
disables the watchdog.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>

1 files changed, 20 insertions, 0 deletions

diff --git a/Documentation/ibm-acpi.txt b/Documentation/ibm-acpi.txt
index cbd3a603a7ef..0132d363feb5 100644
--- a/Documentation/ibm-acpi.txt
+++ b/Documentation/ibm-acpi.txt
@@ -670,6 +670,26 @@ example:
 
         modprobe ibm_acpi hotkey=enable,0xffff video=auto_disable
 
+The ibm-acpi kernel driver can be programmed to revert the fan level
+to a safe setting if userspace does not issue one of the fan commands:
+"enable", "disable", "level" or "watchdog" within a configurable
+ammount of time.  To do this, use the "watchdog" command.
+
+        echo 'watchdog <interval>' > /proc/acpi/ibm/fan
+
+Interval is the ammount of time in seconds to wait for one of the
+above mentioned fan commands before reseting the fan level to a safe
+one.  If set to zero, the watchdog is disabled (default).  When the
+watchdog timer runs out, it does the exact equivalent of the "enable"
+fan command.
+
+Note that the watchdog timer stops after it enables the fan.  It will
+be rearmed again automatically (using the same interval) when one of
+the above mentioned fan commands is received.  The fan watchdog is,
+therefore, not suitable to protect against fan mode changes made
+through means other than the "enable", "disable", and "level" fan
+commands.
+
 
 Example Configuration
 ---------------------