linux-kernel - Re: [PATCH] backlight: lm3639: Unconditionally call led_classdev

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Message-ID: <20180921231047.GA25998@flashbox>
Date:   Fri, 21 Sep 2018 16:10:47 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     lee.jones@...aro.org, daniel.thompson@...aro.org,
        jingoohan1@...il.com, b.zolnierkie@...sung.com,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] backlight: lm3639: Unconditionally call
 led_classdev_unregister

On Fri, Sep 21, 2018 at 03:48:50PM -0700, Nick Desaulniers wrote:
> On Fri, Sep 21, 2018 at 1:23 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Clang warns that the address of a pointer will always evaluated as true
> > in a boolean context.
> >
> > drivers/video/backlight/lm3639_bl.c:403:14: warning: address of
> > 'pchip->cdev_torch' will always evaluate to 'true'
> > [-Wpointer-bool-conversion]
> >         if (&pchip->cdev_torch)
> >         ~~   ~~~~~~~^~~~~~~~~~
> > drivers/video/backlight/lm3639_bl.c:405:14: warning: address of
> > 'pchip->cdev_flash' will always evaluate to 'true'
> > [-Wpointer-bool-conversion]
> >         if (&pchip->cdev_flash)
> >         ~~   ~~~~~~~^~~~~~~~~~
> > 2 warnings generated.
> >
> > These statements have been present since 2012, introduced by
> > commit 0f59858d5119 ("backlight: add new lm3639 backlight
> > driver"). Given that they have been called unconditionally since
> > then presumably without any issues, removing the always true if
> > statements to fix the warnings without any real world changes.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/119
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >
> > Alternatively, it's possible the address wasn't supposed to be taken or
> > the dev in these structs should be checked instead. I don't have this
> > hardware to make that call so I would appreciate some review and
> > opinions on what was intended here.
> >
> > Thanks!
> >
> >  drivers/video/backlight/lm3639_bl.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/video/backlight/lm3639_bl.c b/drivers/video/backlight/lm3639_bl.c
> > index cd50df5807ea..086611c7bc03 100644
> > --- a/drivers/video/backlight/lm3639_bl.c
> > +++ b/drivers/video/backlight/lm3639_bl.c
> > @@ -400,10 +400,8 @@ static int lm3639_remove(struct i2c_client *client)
> >
> >         regmap_write(pchip->regmap, REG_ENABLE, 0x00);
> >
> > -       if (&pchip->cdev_torch)
> > -               led_classdev_unregister(&pchip->cdev_torch);
> > -       if (&pchip->cdev_flash)
> > -               led_classdev_unregister(&pchip->cdev_flash);
> > +       led_classdev_unregister(&pchip->cdev_torch);
> > +       led_classdev_unregister(&pchip->cdev_flash);
> 
> led_classdev_unregister() requires that its arg is non-null (as it
> dereferences it without any kind of check).  It's not clear that
> i2c_get_clientdata() can never return a null pointer, so I think all
> references to pchip in this function should instead be guarded with a
> null check.  Would you mind making that change and sending a v2?
> 

Hi Nick,

I did a quick grep throughout the tree and I didn't see any place where
there were null checks for i2c_get_clientdata, leading me to believe
that such a check isn't necessary although I am nowhere close to an expert
into this stuff. I'm not sure I follow the rest of the request though,
where should the check be? Before regmap_write?

Furthermore, the probe function seems to make sure all of these get
initialized properly, doesn't remove imply that probe was successful?

Thank you for the comment and review!
Nathan

> >         if (pchip->bled)
> >                 device_remove_file(&(pchip->bled->dev), &dev_attr_bled_mode);
> >         return 0;
> > --
> > 2.19.0
> >
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers