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

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Message-ID: <b5d2b71ffc38eb1980ce72994a20e8f6ee7869bd.camel@siemens.com>
Date: Mon, 12 May 2025 12:13:54 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "luca.ceresoli@...tlin.com" <luca.ceresoli@...tlin.com>
CC: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>
Subject: Re: [PATCH v4 6/8] backlight: led-backlight: add devlink to supplier
 LEDs

Hi Luca,

On Mon, 2025-05-12 at 13:39 +0200, Alexander Sverdlin 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
> > 
> > Fix by adding a devlink between the consuming led-backlight device and the
> > supplying LED device.
> > 
> > Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>
> 
> I've tested the patch wit LP8864 LED as a provider for led_bl, removing the
> underlying I2C bus. The patch avoids the crash for me (by removing led_bl device as well),
> thanks for fixing it!
> 
> Tested-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

Would it make sense to add

Fixes: ae232e45acf9 ("backlight: add led-backlight driver")

?

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com