On Fri, Mar 04, 2016 at 11:03:49AM +0800, Leo Yan wrote: > Hi Eduardo, > > On Thu, Mar 03, 2016 at 08:29:44AM -0800, Eduardo Valentin wrote: > > Hi Leo, > > > > On Fri, Feb 26, 2016 at 11:43:43AM +0800, Leo Yan wrote: > > > The property "hysteresis" is mandatory for trip points, so if without > > > it the thermal zone cannot register successfully. But "hysteresis" is > > > ignored in the thermal subsystem and only inquired by several thermal > > > sensor drivers. > > > > I am not sure this a good direction to go. Remember that Linux > > implementation not necessarily has to be the implication of the DT > > binding. Hysteresis is a property that plays a significant role on > > thermal control systems, which in many cases avoid overshooting cooling > > actions. Having the DT writer to explicitly set it to 0 means that zone > > does not suffer of overshooting and does not need hysteresis. > > After review current code, the "hysteresis" is used to calculate > temperature falling threshold with a more conservative value; so that > finally avoid overshooting issue. > > Please confirm if is my understanding correct or not? > > > If the Linux thermal subsystem has a problem with handling hysteresis, I > > would rather fix Linux code than relaxing the DT binding. Or if you > > still believe hysteresis is really optional, I would prefer to see a > > better justification than "Linux ignores it". I see it the other way round, Is hysteresis a property that, without it, the thermal code can't configure itself so it fails to create the trip point? The current code goes "There is no hysteresis for this property, I don't know how to set up this trip point!". I think we can do better than this. > If we think about power allocator governor, PID's two parameters are > also used to dismiss overshooting issue: one is k_po (proportional > term), another is k_i (integral term). So that means after we apply > power allocator governor, we don't need parameter "hysteresis" due PID > algorithm can automatically dismiss potential errors. I disagree. We shouldn't base DT decisions based on only one governor in linux. Having said that, AFAICS all governors currently ignore hysteresis. Cheers, Javi -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
