thermal: add available policies sysfs attribute

The Linux thermal framework support to change thermal governor
policy in userspace, but it can't show what available policies
supported.

This patch adds available_policies attribute to the thermal
framework, it can list the thermal governors which can be
used for a particular zone. This attribute is read only.

Signed-off-by: Wei Ni <[e-mail]>
Reviewed-by: Javi Merino <[e-mail]>
Signed-off-by: Zhang Rui <[e-mail]>

Documentation/thermal/sysfs-api.txt

···
     |---temp:			Current temperature
     |---mode:			Working mode of the thermal zone
     |---policy:			Thermal governor used for this zone
+    |---available_policies:	Available thermal governors for this zone
     |---trip_point_[0-*]_temp:	Trip point temperature
     |---trip_point_[0-*]_type:	Trip point type
     |---trip_point_[0-*]_hyst:	Hysteresis value for this trip point
···
 policy
 	One of the various thermal governors used for a particular zone.
 	RW, Required
+
+available_policies
+	Available thermal governors which can be used for a particular zone.
+	RO, Required
 
 trip_point_[0-*]_temp
 	The temperature above which trip point will be fired.
···
     |---temp:			37000
     |---mode:			enabled
     |---policy:			step_wise
+    |---available_policies:	step_wise fair_share
     |---trip_point_0_temp:	100000
     |---trip_point_0_type:	critical
     |---trip_point_1_temp:	80000

drivers/thermal/thermal_core.c

···
 	return sprintf(buf, "%s\n", tz->governor->name);
 }
 
+static ssize_t
+available_policies_show(struct device *dev, struct device_attribute *devattr,
+			char *buf)
+{
+	struct thermal_governor *pos;
+	ssize_t count = 0;
+	ssize_t size = PAGE_SIZE;
+
+	mutex_lock(&thermal_governor_lock);
+
+	list_for_each_entry(pos, &thermal_governor_list, governor_list) {
+		size = PAGE_SIZE - count;
+		count += scnprintf(buf + count, size, "%s ", pos->name);
+	}
+	count += scnprintf(buf + count, size, "\n");
+
+	mutex_unlock(&thermal_governor_lock);
+
+	return count;
+}
+
 #ifdef CONFIG_THERMAL_EMULATION
 static ssize_t
 emul_temp_store(struct device *dev, struct device_attribute *attr,
···
 static DEVICE_ATTR(mode, 0644, mode_show, mode_store);
 static DEVICE_ATTR(passive, S_IRUGO | S_IWUSR, passive_show, passive_store);
 static DEVICE_ATTR(policy, S_IRUGO | S_IWUSR, policy_show, policy_store);
+static DEVICE_ATTR(available_policies, S_IRUGO, available_policies_show, NULL);
 
 /* sys I/F for cooling device */
 #define to_cooling_device(_dev)	\
···
 	if (result)
 		goto unregister;
 
+	/* Create available_policies attribute */
+	result = device_create_file(&tz->device, &dev_attr_available_policies);
+	if (result)
+		goto unregister;
+
 	/* Update 'this' zone's governor information */
 	mutex_lock(&thermal_governor_lock);
 
···
 	if (tz->ops->get_mode)
 		device_remove_file(&tz->device, &dev_attr_mode);
 	device_remove_file(&tz->device, &dev_attr_policy);
+	device_remove_file(&tz->device, &dev_attr_available_policies);
 	remove_trip_attrs(tz);
 	thermal_set_governor(tz, NULL);