[PATCH v5] backlight: led-backlight: add devlink to supplier LEDs

[PATCH v5] 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. 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>;
    };

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.

Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
Signed-off-by: Luca Ceresoli <luca.ceresoli@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..8fe645b5b75447d009e1045dcf0c62d3bbc5425e 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[0]->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[0]->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@bootlin.com>

Re: [PATCH v5] backlight: led-backlight: add devlink to supplier LEDs

[Herve Codina]

Hi Luca,

On Mon, 19 May 2025 17:16:39 +0200
Luca Ceresoli <luca.ceresoli@bootlin.com> wrote:

...

> diff --git a/drivers/video/backlight/led_bl.c b/drivers/video/backlight/led_bl.c
> index d2db157b2c290adc3a159023e9e2394fc877388c..8fe645b5b75447d009e1045dcf0c62d3bbc5425e 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[0]->dev->parent,

It should be priv->leds[i]->dev->parent instead of leds[0], shouldn't it?

> +				       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[0]->dev->parent));

ditto

> +			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]);
> 

Best regards,
Hervé

Re: [PATCH v5] backlight: led-backlight: add devlink to supplier LEDs

[Luca Ceresoli]

Hi Hervé,

On Mon, 19 May 2025 17:47:29 +0200
Herve Codina <herve.codina@bootlin.com> wrote:

> Hi Luca,
> 
> On Mon, 19 May 2025 17:16:39 +0200
> Luca Ceresoli <luca.ceresoli@bootlin.com> wrote:
> 
> ...
> 
> > diff --git a/drivers/video/backlight/led_bl.c b/drivers/video/backlight/led_bl.c
> > index d2db157b2c290adc3a159023e9e2394fc877388c..8fe645b5b75447d009e1045dcf0c62d3bbc5425e 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[0]->dev->parent,  
> 
> It should be priv->leds[i]->dev->parent instead of leds[0], shouldn't it?

Ah, well spotted, thanks!

My led-backlight device only uses one LED, so I hadn't noticed during
testing.

v6 on its way.

Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com