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

Commit 20f85ef89d94 ("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 20f85ef89d94
("drm/i915/backlight: use unique backlight device names")
v3: fixed the ref count leak(Jani N)

Fixes: 20f85ef89d94 ("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

diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
index 262b2fd..bdb2294 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_backlight.c
@@ -972,26 +972,24 @@ 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);
-
-       /*
-        * Using the same name independent of the drm device or connector
-        * prevents registration of multiple backlight devices in the
-        * driver. However, we need to use the default name for backward
-        * 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) {
+       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
+                * driver. However, we need to use the default name for backward
+                * compatibility. Use unique names for subsequent backlight devices as a
+                * fallback when the default name already exists.
+                */
                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);
        }
+       bd = backlight_device_register(name, connector->base.kdev, connector,
+                                      &intel_backlight_device_ops, &props);
 
        if (IS_ERR(bd)) {
                drm_err(&i915->drm,