backlight: led-bl: Add devlink to supplier LEDs

LED Backlight is a consumer of one or multiple LED class devices, but
devlink is currently unable to create correct supplier-producer links when
the supplier is a class device. It creates instead a link where the
supplier is the parent of the expected device.

One consequence is that removal order is not correctly enforced.

Issues happen for example with the following sections in a device tree
overlay:

// An LED driver chip
pca9632@62 {
compatible = "nxp,pca9632";
reg = <0x62>;

// ...

addon_led_pwm: led-pwm@3 {
reg = <3>;
label = "addon:led:pwm";
};
};

backlight-addon {
compatible = "led-backlight";
leds = <&addon_led_pwm>;
brightness-levels = <255>;
default-brightness-level = <255>;
};

In this example, the devlink should be created between the backlight-addon
(consumer) and the pca9632@62 (supplier). Instead it is created between the
backlight-addon (consumer) and the parent of the pca9632@62, which is
typically the I2C bus adapter.

On removal of the above overlay, the LED driver can be removed before the
backlight device, resulting in:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
...
Call trace:
led_put+0xe0/0x140
devm_led_release+0x6c/0x98

Another way to reproduce the bug without any device tree overlays is
unbinding the LED class device (pca9632@62) before unbinding the consumer
(backlight-addon):

echo 11-0062 >/sys/bus/i2c/drivers/leds-pca963x/unbind
echo ...backlight-dock >/sys/bus/platform/drivers/led-backlight/unbind

Fix by adding a devlink between the consuming led-backlight device and the
supplying LED device, as other drivers and subsystems do as well.

Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
Signed-off-by: Luca Ceresoli <[e-mail]>
Reviewed-by: Daniel Thompson (RISCstar) <[e-mail]>
Reviewed-by: Herve Codina <[e-mail]>
Tested-by: Alexander Sverdlin <[e-mail]>
Link: https://patch.msgid.link/20250519-led-[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/video/backlight/led_bl.c

···
 	}
 
 	for (i = 0; i < priv->nb_leds; i++) {
+		struct device_link *link;
+
+		link = device_link_add(&pdev->dev, priv->leds[i]->dev->parent,
+				       DL_FLAG_AUTOREMOVE_CONSUMER);
+		if (!link) {
+			dev_err(&pdev->dev, "Failed to add devlink (consumer %s, supplier %s)\n",
+				dev_name(&pdev->dev), dev_name(priv->leds[i]->dev->parent));
+			backlight_device_unregister(priv->bl_dev);
+			return -EINVAL;
+		}
+	}
+
+	for (i = 0; i < priv->nb_leds; i++) {
 		mutex_lock(&priv->leds[i]->led_access);
 		led_sysfs_disable(priv->leds[i]);
 		mutex_unlock(&priv->leds[i]->led_access);