leds: led-class: Add generic [devm_]led_get()

Add a generic [devm_]led_get() method which can be used on both devicetree
and non devicetree platforms to get a LED classdev associated with
a specific function on a specific device, e.g. the privacy LED associated
with a specific camera sensor.

Note unlike of_led_get() this takes a string describing the function
rather then an index. This is done because e.g. camera sensors might
have a privacy LED, or a flash LED, or both and using an index
approach leaves it unclear what the function of index 0 is if there is
only 1 LED.

This uses a lookup-table mechanism for non devicetree platforms.
This allows the platform code to map specific LED class_dev-s to a specific
device,function combinations this way.

For devicetree platforms getting the LED by function-name could be made
to work using the standard devicetree pattern of adding a -names string
array to map names to the indexes.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120114524.408368-5-hdegoede@redhat.com

drivers/leds/led-class.c

@@ -23,6 +23,8 @@
 #include "leds.h"
 
 static struct class *leds_class;
+static DEFINE_MUTEX(leds_lookup_lock);
+static LIST_HEAD(leds_lookup_list);
 
 static ssize_t brightness_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
@@ -317,6 +319,88 @@ struct led_classdev *__must_check devm_of_led_get(struct device *dev,
 }
 EXPORT_SYMBOL_GPL(devm_of_led_get);
 
+/**
+ * led_get() - request a LED device via the LED framework
+ * @dev: device for which to get the LED device
+ * @con_id: name of the LED from the device's point of view
+ *
+ * @return a pointer to a LED device or ERR_PTR(errno) on failure.
+ */
+struct led_classdev *led_get(struct device *dev, char *con_id)
+{
+	struct led_lookup_data *lookup;
+	const char *provider = NULL;
+	struct device *led_dev;
+
+	mutex_lock(&leds_lookup_lock);
+	list_for_each_entry(lookup, &leds_lookup_list, list) {
+		if (!strcmp(lookup->dev_id, dev_name(dev)) &&
+		    !strcmp(lookup->con_id, con_id)) {
+			provider = kstrdup_const(lookup->provider, GFP_KERNEL);
+			break;
+		}
+	}
+	mutex_unlock(&leds_lookup_lock);
+
+	if (!provider)
+		return ERR_PTR(-ENOENT);
+
+	led_dev = class_find_device_by_name(leds_class, provider);
+	kfree_const(provider);
+
+	return led_module_get(led_dev);
+}
+EXPORT_SYMBOL_GPL(led_get);
+
+/**
+ * devm_led_get() - request a LED device via the LED framework
+ * @dev: device for which to get the LED device
+ * @con_id: name of the LED from the device's point of view
+ *
+ * The LED device returned from this function is automatically released
+ * on driver detach.
+ *
+ * @return a pointer to a LED device or ERR_PTR(errno) on failure.
+ */
+struct led_classdev *devm_led_get(struct device *dev, char *con_id)
+{
+	struct led_classdev *led;
+
+	led = led_get(dev, con_id);
+	if (IS_ERR(led))
+		return led;
+
+	return __devm_led_get(dev, led);
+}
+EXPORT_SYMBOL_GPL(devm_led_get);
+
+/**
+ * led_add_lookup() - Add a LED lookup table entry
+ * @led_lookup: the lookup table entry to add
+ *
+ * Add a LED lookup table entry. On systems without devicetree the lookup table
+ * is used by led_get() to find LEDs.
+ */
+void led_add_lookup(struct led_lookup_data *led_lookup)
+{
+	mutex_lock(&leds_lookup_lock);
+	list_add_tail(&led_lookup->list, &leds_lookup_list);
+	mutex_unlock(&leds_lookup_lock);
+}
+EXPORT_SYMBOL_GPL(led_add_lookup);
+
+/**
+ * led_remove_lookup() - Remove a LED lookup table entry
+ * @led_lookup: the lookup table entry to remove
+ */
+void led_remove_lookup(struct led_lookup_data *led_lookup)
+{
+	mutex_lock(&leds_lookup_lock);
+	list_del(&led_lookup->list);
+	mutex_unlock(&leds_lookup_lock);
+}
+EXPORT_SYMBOL_GPL(led_remove_lookup);
+
 static int led_classdev_next_name(const char *init_name, char *name,
 				  size_t len)
 {

include/linux/leds.h

@@ -39,6 +39,21 @@ enum led_default_state {
 	LEDS_DEFSTATE_KEEP	= 2,
 };
 
+/**
+ * struct led_lookup_data - represents a single LED lookup entry
+ *
+ * @list: internal list of all LED lookup entries
+ * @provider: name of led_classdev providing the LED
+ * @dev_id: name of the device associated with this LED
+ * @con_id: name of the LED from the device's point of view
+ */
+struct led_lookup_data {
+	struct list_head list;
+	const char *provider;
+	const char *dev_id;
+	const char *con_id;
+};
+
 struct led_init_data {
 	/* device fwnode handle */
 	struct fwnode_handle *fwnode;
@@ -211,6 +226,12 @@ void devm_led_classdev_unregister(struct device *parent,
 void led_classdev_suspend(struct led_classdev *led_cdev);
 void led_classdev_resume(struct led_classdev *led_cdev);
 
+void led_add_lookup(struct led_lookup_data *led_lookup);
+void led_remove_lookup(struct led_lookup_data *led_lookup);
+
+struct led_classdev *__must_check led_get(struct device *dev, char *con_id);
+struct led_classdev *__must_check devm_led_get(struct device *dev, char *con_id);
+
 extern struct led_classdev *of_led_get(struct device_node *np, int index);
 extern void led_put(struct led_classdev *led_cdev);
 struct led_classdev *__must_check devm_of_led_get(struct device *dev,