Re: [PATCH v2] hwmon: max6650: add thermal cooling device capability

[Jean-Francois Dagenais <jeff.dagenais@gmail.com>]

> On Apr 18, 2019, at 14:06, Guenter Roeck <linux@roeck-us.net> wrote:
> 
> On Thu, Apr 18, 2019 at 02:02:48PM -0400, Jean-Francois Dagenais wrote:
>> 
>>> On Apr 18, 2019, at 13:38, Guenter Roeck <linux@roeck-us.net> wrote:
>>> 
>>>> +#if IS_ENABLED(CONFIG_THERMAL)
>>> 
>>> This will result in missing symbols if THERMAL is built as module
>>> and this driver is built into the kernel. You'll have to adjust
>>> Kconfig dependencies accordingly. See other drivers for examples.
>> 
>> Right! Was not a problem for me, but I do remember seing the "funny"
>> ifdefs around.
>> 
> 
> I know, it is annoying. There was an effort to make THERMAL boolean
> instead of tristate, but it looks like that has stalled or was
> rejected.

Ok, so I had used pwm-fan.c as a reference and simply forgotten to copy over the
Kconfig bit which makes sure if the thermal is a module, then so is the chip
driver. I will put in v3.

> 
>>> 
>>>> +	data->cooling_dev =
>>>> +		thermal_of_cooling_device_register(client->dev.of_node,
>>>> +						   id->name, data,
>>>> +						   &max6650_cooling_ops);
>>>> +	if (IS_ERR(data->cooling_dev)) {
>>>> +		err = PTR_ERR(data->cooling_dev);
>>>> +		dev_err(&client->dev,
>>>> +			"Failed to register as cooling device (%d)\n", err);
>>>> +		return err;
>>> 
>>> Why would it be fatal for the driver if this fails ? It wasn't
>>> fatal before.
>> 
>> Mmmh, you are right. This assumes that all users of max6650 would now require to
>> be referred to in some thermal zone. Again, this was not a problem for my test
>> environment.

And about this bit... I had confused the thermal cooling register with the
thermal_of_sensor register. So if my static analysis of the code in
__thermal_cooling_device_register is correct, the function cannot really fail
unless something is terribly wrong in the kernel... like a malloc failure or
something.

If I am wrong, then pwm-fan.c should also be visited since my max6650_probe code
was copy-pasted from it.


On a related note, I am worried about everyone having to progressively add
thermal-zone support to each fan chip in drivers/hwmon. Would it not be possible
for hwmon to detect, through `enum hwmon_sensor_types` set to `hwmon_fan` in
`struct hwmon_channel_info` to be registered as cooling device in thermal ? Of
course for max665x to work, it would have to be modified to use
`devm_hwmon_device_register_with_info` instead of
`devm_hwmon_device_register_with_groups`

Another possiblity would be to create a new driver in drivers/thermal for
max665x as a thermal cooling device. Or even a
drivers/thermal/thermal-generic-hwmon-fan.c or something, where DTS would
instanciate such a cooling device which references a hwmon device's specific
pwm channel.

Phew! So many possibilities!