On Tue, Sep 29, 2015 at 09:33:30PM +0100, Andrew Morton wrote: > On Mon, 28 Sep 2015 23:28:34 +0200 Andrea Arcangeli <aarca...@redhat.com> > wrote: > > > During boot I get a div by zero Oops regression starting in v4.3-rc3. > > > > ... > > > > --- a/drivers/thermal/power_allocator.c > > +++ b/drivers/thermal/power_allocator.c > > @@ -144,6 +144,16 @@ static void estimate_pid_constants(struct > > thermal_zone_device *tz, > > switch_on_temp = 0; > > > > temperature_threshold = control_temp - switch_on_temp; > > + /* > > + * estimate_pid_constants() tries to find appropriate default > > + * values for thermal zones that don't provide them. If a > > + * system integrator has configured a thermal zone with two > > + * passive trip points at the same temperature, that person > > + * hasn't put any effort to set up the thermal zone properly > > + * so just give up. > > + */ > > + if (!temperature_threshold) > > + return; > > > > if (!tz->tzp->k_po || force) > > tz->tzp->k_po = int_to_frac(sustainable_power) / > > a) Are we sure this won't leave tz->tzp fields uninitialized?
They will be all zeros. That's good enough. > b) I'm not understanding that code at all. The "proportional" term > in a PID controller is supposed to be proportional to the (desired - > actual) difference (aka "the error"). > > But estimate_pid_constants() appears to be setting the > "proportional" term to be proportional to 1/error! estimate_pid_constants() calculate the constants that you use in the PID algorithm. Say: k_p * error + k_i * integral_of_error + k_d * diff_of_error This code is calculating a reasonable k_p, k_i and k_d when they are not provided by the platform. > Maybe a description of local `temperature_threshold' would help > clue me in. The `error' in the above definition is: target_temperature - current_temperature whereas `temperature_threshold' is: `target_temperature' - `switch_on_temperature' `switch_on_temperature' is the temperature above which the thermal governor starts operating and throttling cpus (or whatever cooling device is configured). The `switch_on_temperature' and `target_temperature' are defined using trip points. A platform that sets two trip points to the same temperature is not properly configured. With Andrea's patch we provide degraded behavior instead of crashing. I agree with that approach (hence my Reviewed-by, maybe it should be an Acked-by?). Cheers, Javi -- 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/
