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

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Message-ID: <20240919124323.GB28725@aspen.lan>
Date: Thu, 19 Sep 2024 14:43:23 +0200
From: Daniel Thompson <daniel.thompson@...aro.org>
To: Luca Ceresoli <luca.ceresoli@...tlin.com>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Andrzej Hajda <andrzej.hajda@...el.com>,
	Neil Armstrong <neil.armstrong@...aro.org>,
	Robert Foss <rfoss@...nel.org>,
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
	Jonas Karlman <jonas@...boo.se>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
	Derek Kiernan <derek.kiernan@....com>,
	Dragan Cvetic <dragan.cvetic@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>, Lee Jones <lee@...nel.org>,
	Jingoo Han <jingoohan1@...il.com>, Helge Deller <deller@....de>,
	Paul Kocialkowski <contact@...lk.fr>,
	Hervé Codina <herve.codina@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-i2c@...r.kernel.org,
	linux-fbdev@...r.kernel.org,
	Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Subject: Re: [PATCH v4 6/8] backlight: led-backlight: add devlink to supplier
 LEDs

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?


Daniel.