[Regression] cached max_state breaks ACPI processor cooling device

[Zhang, Rui]

Hi, All,

Starting from commit c408b3d1d9bb("thermal: Validate new state in
cur_state_store()") and commit a365105c685c("thermal: sysfs: Reuse
cdev->max_state"), the cdev->get_max_state() is evaluated only once
during cooling device registration.

This is done to fix the below Smatch static checker warning:
	drivers/thermal/thermal_sysfs.c:656 thermal_cooling_device_stats_update()
	warn: potential integer overflow from user 'stats->state *
stats->max_states + new_state'
reported here https://lore.kernel.org/all/Y0ltRJRjO7AkawvE@kili/.

But this actually breaks cooling devices which could have dynamic max
cooling state, like ACPI processor cooling device.

acpi_processor_driver_init
	driver_register(&acpi_processor_driver);
		acpi_processor_start
			acpi_processor_thermal_init
				thermal_cooling_device_register
					processor_get_max_state
	acpi_processor_cpufreq_init = true
The driver doesn't count cpufreq as cooling state until
acpi_processor_cpufreq_init is set later.

As a result, without the commits above,
/sys/class/thermal/cooling_device10/cur_state:0
/sys/class/thermal/cooling_device10/max_state:3
/sys/class/thermal/cooling_device10/type:Processor
after the commits above, 
/sys/class/thermal/cooling_device10/cur_state:0
/sys/class/thermal/cooling_device10/max_state:0
/sys/class/thermal/cooling_device10/type:Processor

In order to fix this, there are something worth clarification IMO.
1. should we support dynamic max_state or not?
   ACPI processor cooling state is a combination of processor p-states
   and t-states.
   t-states are static, but p-states can vary depends on processor 
   frequency driver loaded or not.
   I'm not sure if there is any other user like this, but still this
   is a valid use case of dynamic max_state.
2. how to handle dynamic max_state for cooling device statistics in
   sysfs?
   IMO, when max_state changes, the definition of previous cooling
   device is changed as well, thus we should abandon the previous
   statistics and restart counting.
3. anything else needs to be handled when max_state changes?
   Say, as the definition of each cooling state is changed when
   max_state changes, should we invalidate and re-update the
   thermal instances of this cdev in each thermal zone device?
4. how to detect/handle max cooling states changes?
   Should we do this as before, which invokes .get_max_state()
   everywhere and do updates when necessary , or
   a. cache max_state like we do today
   b. introduce a new API for max_state change
   c. invoke the new API in the cooling device driver explicitly when
      max_state changes
   d. update cached max_state value, statistics sysfs and
      thermal_instances in the API
   e. remove .get_max_state() callback as we register and update the
      max_state with a fixed value each time.

thanks,
rui