On Mon, May 18, 2015 at 02:09:44PM +0200, Sascha Hauer wrote:

On Mon, May 18, 2015 at 12:06:50PM +0300, Mikko Perttunen wrote:

quoted

One interesting thing I noticed was that at least the bang-bang
governor only acts if the temperature is properly smaller than (trip
temp - hysteresis). So perhaps we should specify the non-tripping
range as [low, high)? Or we could change bang-bang.

I wonder how we can protect against such off-by-one errors anyway.
Generally a hardware might operate on raw values rather than directly
in temperature values in ?C. This means a driver for this must have
celsius_to_raw and raw_to_celsius conversion functions. Now it can
happen that due to rounding errors celsius_to_raw(Tcrit) returns a raw
value that when converted back to celsius is different from the
original value in ?C. This would mean the hardware triggers an interrupt
for a trip point and the thermal core does not react because get_temp
actually returns a different temperature than previously programmed as
interrupt trigger. This way we would lose hot (or cold) events.

This also highlights another fact: there's a race between interrupt
generation and temperature reading (->get_temp()). I would expect any
hardware interrupt thermal sensor would also have a latched temperature
reading to correspond with it, and there would be no guarantee that this
latched temperature will match the polled reading seen once you reach
thermal_zone_device_update(). So a hardware driver might report a
thermal update, but the temperature reported to the core won't
necessarily match what interrupt was meant for.

I have a patch that adds a thermal_zone_device_update_temp() API, so
drivers can report the temperature along with the interrupt
notification. (Such a patch also helps so that the driver can choose to
round down on cold events and up on hot events, resolving your rounding
issue too.)

Brian