Re: [PATCH v2 0/2] Add a generic virtual thermal sensor

[Alexandre Bailon]

On 9/22/21 10:10 AM, Daniel Lezcano wrote:
> On 20/09/2021 15:12, Alexandre Bailon wrote:
> > On 9/17/21 4:03 PM, Daniel Lezcano wrote:
> > > On 17/09/2021 15:33, Alexandre Bailon wrote:
> > > > Hi Daniel,
> > > >
> > > > On 9/17/21 2:41 PM, Daniel Lezcano wrote:
> > > > > On 17/09/2021 09:27, Alexandre Bailon wrote:
> > > > > > This series add a virtual thermal sensor.
> > > > > > It could be used to get a temperature using some thermal sensors.
> > > > > > Currently, the supported operations are max, min and avg.
> > > > > > The virtual sensor could be easily extended to support others
> > > > > > operations.
> > > > > >
> > > > > > Note:
> > > > > > Currently, thermal drivers must explicitly register their sensors to
> > > > > > make them
> > > > > > available to the virtual sensor.
> > > > > > This doesn't seem a good solution to me and I think it would be
> > > > > > preferable to
> > > > > > update the framework to register the list of each available sensors.
> > > > > Why must the drivers do that ?
> > > > Because there are no central place where thermal sensor are registered.
> > > > The only other way I found was to update thermal_of.c,
> > > > to register the thermal sensors and make them available later to the
> > > > virtual thermal sensor.
> > > >
> > > > To work, the virtual thermal need to get the sensor_data the ops from
> > > > the thermal sensor.
> > > > And as far I know, this is only registered in thermal_of.c, in the
> > > > thermal zone data
> > > > but I can't access it directly from the virtual thermal sensor.
> > > >
> > > > How would you do it ?
> > > Via the phandles when registering the virtual sensor ?
> > As far I know, we can't get the ops or the sensor_data from the phandle
> > of a thermal sensor.
> > The closest solution I found so far would be to aggregate the thermal
> > zones instead of thermal sensors.
> > thermal_zone_device has the data needed and a thermal zone could be find
> > easily using its name.
> Yeah, the concept of the thermal zone and the sensor are very close.
>
> There is the function in thermal_core.h:
>
>   -> for_each_thermal_zone()
>
> You should be able for each 'slave' sensor, do a lookup to find the
> corresponding thermal_zone_device_ops.
>
> > But, using a thermal_zone_device, I don't see how to handle module
> > unloading.
> I think try_module_get() / module_put() are adequate for this situation
> as it is done on an external module and we can not rely on the exported
> symbols.
I don't see how it would be possible to use these functions.
The thermal zone doesn't have the data required to use it.

Maybe a more easier way is to use the thermal_zone_device mutex.
If I get a lock before to use the thermal_zone_device ops, I have the 
guaranty that module won't be unloaded.

When a "thermal of sensor" is unloaded, it calls 
thermal_zone_of_sensor_unregister which takes a lock before
update ops.

Thanks,
Alexandre

>