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:

quoted

Hi Leo,

On Fri, Feb 26, 2016 at 11:43:43AM +0800, Leo Yan wrote:

quoted

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?

quoted

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