[PATCH v6] backlight: led-backlight: add devlink to supplier LEDs

[Luca Ceresoli]

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 at 62 {
        compatible = "nxp,pca9632";
        reg = <0x62>;

	// ...

        addon_led_pwm: led-pwm at 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 at 62 (supplier). Instead it is created between the
backlight-addon (consumer) and the parent of the pca9632 at 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 at 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.

Tested-by: Alexander Sverdlin <alexander.sverdlin at siemens.com>
Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
Signed-off-by: Luca Ceresoli <luca.ceresoli at bootlin.com>
---
Changes in v6:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v5: https://lore.kernel.org/r/20250519-led-backlight-add-devlink-to-supplier-class-device-v5-1-76d6fd4aeb2f@bootlin.com
---

Changes in v6:
- fix incorrect array index
- further improve commit message
- Link to v5: https://lore.kernel.org/r/20250519-led-backlight-add-devlink-to-supplier-class-device-v5-1-76d6fd4aeb2f@bootlin.com

Changes in v5:
- separated this patch as a standalone patch
- improved commit message, adding feedback from Alexander
- no code changes

This patch first appeared in the v4 of a longer series, even though the
issue is orthogonal:
https://lore.kernel.org/all/20240917-hotplug-drm-bridge-v4-6-bc4dfee61be6@bootlin.com/
---
 drivers/video/backlight/led_bl.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/video/backlight/led_bl.c b/drivers/video/backlight/led_bl.c
index d2db157b2c290adc3a159023e9e2394fc877388c..0ed585eb27903df43651c5365f479f7865cc8a95 100644
--- a/drivers/video/backlight/led_bl.c
+++ b/drivers/video/backlight/led_bl.c
@@ -209,6 +209,19 @@ static int led_bl_probe(struct platform_device *pdev)
 		return PTR_ERR(priv->bl_dev);
 	}
 
+	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]);

---
base-commit: a5806cd506af5a7c19bcd596e4708b5c464bfd21
change-id: 20250519-led-backlight-add-devlink-to-supplier-class-device-6dec4e04445a

Best regards,
-- 
Luca Ceresoli <luca.ceresoli at bootlin.com>