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

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Message-ID: <20250520080508.3a15a949@bootlin.com>
Date: Tue, 20 May 2025 08:05:08 +0200
From: Herve Codina <herve.codina@...tlin.com>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Lee Jones <lee@...nel.org>, Daniel Thompson <danielt@...nel.org>, Jingoo
 Han <jingoohan1@...il.com>, Helge Deller <deller@....de>, Tony Lindgren
 <tony@...mide.com>, Pavel Machek <pavel@....cz>, Jean-Jacques Hiblot
 <jjhiblot@...com>, Tomi Valkeinen <tomi.valkeinen@...com>, Saravana Kannan
 <saravanak@...gle.com>, Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 Daniel Thompson <daniel.thompson@...aro.org>,
 dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Alexander Sverdlin
 <alexander.sverdlin@...il.com>
Subject: Re: [PATCH v6] backlight: led-backlight: add devlink to supplier
 LEDs

Hi Luca,

On Mon, 19 May 2025 22:19:11 +0200
Luca Ceresoli <luca.ceresoli@...tlin.com> 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@...mens.com>
> Fixes: ae232e45acf9 ("backlight: add led-backlight driver")
> Signed-off-by: Luca Ceresoli <luca.ceresoli@...tlin.com>

Reviewed-by: Herve Codina <herve.codina@...tlin.com>

Best regards,
Hervé