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

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello Daniel,

On Thu, 19 Sep 2024 14:43:23 +0200
Daniel Thompson <daniel.thompson@linaro.org> wrote:

> 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@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