drm/i915/display: avoid warnings when registering dual panel backlight

Commit 20f85ef8 ("drm/i915/backlight: use unique backlight device
names") added support for multiple backlight devices on dual panel
systems, but did so with error handling on -EEXIST from
backlight_device_register(). Unfortunately, that triggered a warning in
dmesg all the way down from sysfs_add_file_mode_ns() and
sysfs_warn_dup().

Instead of optimistically always attempting to register with the default
name ("intel_backlight", which we have to retain for backward
compatibility), check if a backlight device with the name exists first,
and, if so, use the card and connector based name.

v2: reworked on top of the patch commit 20f85ef8
("drm/i915/backlight: use unique backlight device names")
v3: fixed the ref count leak(Jani N)

Fixes: 20f85ef8 ("drm/i915/backlight: use unique backlight device names")
Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808035750.3111046-1-arun.r.murthy@intel.com

drivers/gpu/drm/i915/display/intel_backlight.c

@@ -972,9 +972,9 @@ int intel_backlight_device_register(struct intel_connector *connector)
 	if (!name)
 		return -ENOMEM;
 
-	bd = backlight_device_register(name, connector->base.kdev, connector,
-				       &intel_backlight_device_ops, &props);
-
+	bd = backlight_device_get_by_name(name);
+	if (bd) {
+		put_device(&bd->dev);
 		/*
 		 * Using the same name independent of the drm device or connector
 		 * prevents registration of multiple backlight devices in the
@@ -982,16 +982,14 @@ int intel_backlight_device_register(struct intel_connector *connector)
 		 * compatibility. Use unique names for subsequent backlight devices as a
 		 * fallback when the default name already exists.
 		 */
-	if (IS_ERR(bd) && PTR_ERR(bd) == -EEXIST) {
 		kfree(name);
 		name = kasprintf(GFP_KERNEL, "card%d-%s-backlight",
 				 i915->drm.primary->index, connector->base.name);
 		if (!name)
 			return -ENOMEM;
-
+	}
 	bd = backlight_device_register(name, connector->base.kdev, connector,
 				       &intel_backlight_device_ops, &props);
-	}
 
 	if (IS_ERR(bd)) {
 		drm_err(&i915->drm,