drivers: thermal: make usage of CONFIG_THERMAL_HWMON optional

When registering a new thermal_device, the thermal framework
will always add a hwmon sysfs interface.

This patch adds a flag to make this behavior optional. Now
when registering a new thermal device, the caller can
optionally inform if hwmon interface is desirable. This can
be done by means of passing a thermal_zone_params.no_hwmon == true.

In order to keep same behavior as of today, all current
calls will by default create the hwmon interface.

Cc: David Woodhouse <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: Zhang Rui <[e-mail]>
Suggested-by: Wei Ni <[e-mail]>
Signed-off-by: Eduardo Valentin <[e-mail]>

Documentation/thermal/sysfs-api.txt

···
     This is an optional feature where some platforms can choose not to
     provide this data.
     .governor_name: Name of the thermal governor used for this zone
+    .no_hwmon: a boolean to indicate if the thermal to hwmon sysfs interface
+               is required. when no_hwmon == false, a hwmon sysfs interface
+               will be created. when no_hwmon == true, nothing will be done.
+               In case the thermal_zone_params is NULL, the hwmon interface
+               will be created (for backward compatibility).
     .num_tbps: Number of thermal_bind_params entries for this zone
     .tbp: thermal_bind_params entries
 

drivers/thermal/thermal_core.c

···
 
 	mutex_unlock(&thermal_governor_lock);
 
-	result = thermal_add_hwmon_sysfs(tz);
-	if (result)
-		goto unregister;
+	if (!tz->tzp || !tz->tzp->no_hwmon) {
+		result = thermal_add_hwmon_sysfs(tz);
+		if (result)
+			goto unregister;
+	}
 
 	mutex_lock(&thermal_list_lock);
 	list_add_tail(&tz->node, &thermal_tz_list);

include/linux/thermal.h

···
 /* Structure to define Thermal Zone parameters */
 struct thermal_zone_params {
 	char governor_name[THERMAL_NAME_LENGTH];
+
+	/*
+	 * a boolean to indicate if the thermal to hwmon sysfs interface
+	 * is required. when no_hwmon == false, a hwmon sysfs interface
+	 * will be created. when no_hwmon == true, nothing will be done
+	 */
+	bool no_hwmon;
+
 	int num_tbps;	/* Number of tbp entries */
 	struct thermal_bind_params *tbp;
 };