hwmon: (lm90) Add SMBus alert support

Tested successfully with an ADM1032 chip on its evaluation board. It
should work fine with all other chips as well.

At this point this is more of a proof-of-concept, we don't do anything
terribly useful on SMBus alert: we simply log the event. But this could
later evolve into libsensors signaling so that user-space applications
can take an appropriate action.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: Trent Piepho <tpiepho@freescale.com>

Documentation/hwmon/lm90

@@ -173,6 +173,18 @@ The lm90 driver will not update its values more frequently than every
 other second; reading them more often will do no harm, but will return
 'old' values.
 
+SMBus Alert Support
+-------------------
+
+This driver has basic support for SMBus alert. When an alert is received,
+the status register is read and the faulty temperature channel is logged.
+
+The Analog Devices chips (ADM1032 and ADT7461) do not implement the SMBus
+alert protocol properly so additional care is needed: the ALERT output is
+disabled when an alert is received, and is re-enabled only when the alarm
+is gone. Otherwise the chip would block alerts from other chips in the bus
+as long as the alarm is active.
+
 PEC Support
 -----------