backlight: Add a display name to the core, and a function to set it

The naming of backlight devices is not terribly useful for
associating a backlight controller with a display (assuming
it is attached to one).

Add a sysfs node that will return a display name that can be set
by other subsystems.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Author: 6by9

drivers/video/backlight/backlight.c

@@ -285,6 +285,15 @@ static ssize_t max_brightness_show(struct device *dev,
 }
 static DEVICE_ATTR_RO(max_brightness);
 
+static ssize_t display_name_show(struct device *dev,
+				 struct device_attribute *attr, char *buf)
+{
+	struct backlight_device *bd = to_backlight_device(dev);
+
+	return sprintf(buf, "%s\n", bd->props.display_name);
+}
+static DEVICE_ATTR_RO(display_name);
+
 static ssize_t actual_brightness_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
@@ -365,6 +374,7 @@ static struct attribute *bl_device_attrs[] = {
 	&dev_attr_max_brightness.attr,
 	&dev_attr_scale.attr,
 	&dev_attr_type.attr,
+	&dev_attr_display_name.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(bl_device);
@@ -662,6 +672,17 @@ static int of_parent_match(struct device *dev, const void *data)
 	return dev->parent && dev->parent->of_node == data;
 }
 
+int backlight_set_display_name(struct backlight_device *bd, const char *name)
+{
+	if (!bd)
+		return -EINVAL;
+
+	strscpy_pad(bd->props.display_name, name, sizeof(bd->props.display_name));
+
+	return 0;
+}
+EXPORT_SYMBOL(backlight_set_display_name);
+
 /**
  * of_find_backlight_by_node() - find backlight device by device-tree node
  * @node: device-tree node of the backlight device

include/linux/backlight.h

@@ -270,6 +270,13 @@ struct backlight_properties {
 	 * @scale: The type of the brightness scale.
 	 */
 	enum backlight_scale scale;
+
+#define BL_DISPLAY_NAME_LEN 32
+	/**
+	 * @display_name: Optional name that can be registered to associate a
+	 * backlight device with a display device.
+	 */
+	char display_name[BL_DISPLAY_NAME_LEN];
 };
 
 /**
@@ -478,12 +485,20 @@ of_find_backlight_by_node(struct device_node *node)
 
 #if IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
 struct backlight_device *devm_of_find_backlight(struct device *dev);
+int backlight_set_display_name(struct backlight_device *bd, const char *name);
 #else
 static inline struct backlight_device *
 devm_of_find_backlight(struct device *dev)
 {
 	return NULL;
 }
+
+static inline int backlight_set_display_name(struct backlight_device *bd,
+					     const char *name)
+{
+	return 0;
+}
+
 #endif
 
 #endif