platform/x86: dell-ddv: Add hwmon support

Thanks to bugreport 216655 on bugzilla triggered by the
dell-smm-hwmon driver, the contents of the sensor buffers
could be almost completely decoded.
Add an hwmon interface for exposing the fan and thermal
sensor values. Since the WMI interface can be quite slow
on some machines, the sensor buffers are cached for 1 second
to lessen the performance impact.
The debugfs interface remains in place to aid in reverse-engineering
of unknown sensor types and the thermal buffer.

Tested-by: Antonín Skala <[email protected]>
Tested-by: Gustavo Walbon <[email protected]>
Signed-off-by: Armin Wolf <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Hans de Goede <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>

Author: Wer-Wolf

drivers/platform/x86/dell/Kconfig

@@ -192,12 +192,12 @@ config DELL_WMI_DESCRIPTOR
 config DELL_WMI_DDV
 	tristate "Dell WMI sensors Support"
 	default m
-	depends on ACPI_BATTERY
 	depends on ACPI_WMI
+	depends on ACPI_BATTERY || HWMON
 	help
-	  This option adds support for WMI-based sensors like
-	  battery temperature sensors found on some Dell notebooks.
-	  It also supports reading of the battery ePPID.
+	  This option adds support for WMI-based fan and thermal sensors
+	  found on some Dell notebooks. It also supports various WMI-based battery
+	  extras like reading of the battery temperature and ePPID.
 
 	  To compile this drivers as a module, choose M here: the module will
 	  be called dell-wmi-ddv.