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

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Message-ID: <20240920144113.427606a7@booty>
Date: Fri, 20 Sep 2024 14:41:13 +0200
From: Luca Ceresoli <luca.ceresoli@...tlin.com>
To: Daniel Thompson <daniel.thompson@...aro.org>
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

Hello Daniel,

On Thu, 19 Sep 2024 14:43:23 +0200
Daniel Thompson <daniel.thompson@...aro.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