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: > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
