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

["Sverdlin, Alexander" <alexander.sverdlin@siemens.com>]

Hi Lee, Daniel, Jingoo,

On Mon, 2025-05-19 at 22:19 +0200, Luca Ceresoli wrote:
> 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.
> 
> Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
> Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> Reviewed-by: Herve Codina <herve.codina@bootlin.com>

I've noticed that the patch in archived in the patchwork [1] but I wasn't
able to find it in any branch of the backlight tree [2].

Could it be that the patch somehow slipped through?
It does solve a real-world crash, could you please consider to apply it?

> ---
> 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(+)
> 
> 
> ---
> base-commit: a5806cd506af5a7c19bcd596e4708b5c464bfd21
> change-id: 20250519-led-backlight-add-devlink-to-supplier-class-device-6dec4e04445a
> 
> Best regards,
> 
> 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]);

[1] Link: https://patchwork.kernel.org/project/dri-devel/patch/20250519-led-backlight-add-devlink-to-supplier-class-device-v6-1-845224aeb2ce@bootlin.com/
[2] Link: git://git.kernel.org/pub/scm/linux/kernel/git/lee/backlight.git

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com