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

Hello Daniel,

On Thu, 19 Sep 2024 14:43:23 +0200
Daniel Thompson [off-list ref] wrote:

On Tue, Sep 17, 2024 at 10:53:10AM +0200, Luca Ceresoli wrote:

quoted

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@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  

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?

Thanks for your comment.

Hervé and I just had a look at the code and there actually might be a
bug here, which we will be investigating (probably next week).

Still I think the devlink needs to be added to describe the
relationship between the supplier (LED) and consumer (backlight).

Luca

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