[PATCH v4 6/8] backlight: led-backlight: add devlink to supplier LEDs

[Daniel Thompson]

On Tue, Sep 17, 2024 at 10:53:10AM +0200, Luca Ceresoli wrote:
> led-backlight is a consumer of one or multiple LED class devices, but no
> devlink is created for such supplier-producer relationship. One consequence
> is that removal ordered 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>;
>     };
>
> 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

This looks like the object became invalid whilst we were holding a reference
to it. Is that reasonable? Put another way, is using devlink here fixing a
bug or merely hiding one?


Daniel.