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

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Message-ID: <20180926085928.hzwunjz3a5zbfpam@holly.lan>
Date:   Wed, 26 Sep 2018 09:59:28 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>, lee.jones@...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 Mon, Sep 24, 2018 at 02:41:04PM -0700, Nick Desaulniers wrote:
> On Fri, Sep 21, 2018 at 4:10 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > 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>

Based on conversation below...

Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>


> > > > ---
> > > >
> > > > 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.
> 
> This seems to be the case.  We should start using
> __attribute__((returns_nonnull)) (gated on gcc 5+).
> I *think* that the device's driver_data is actually set in
> drivers/video/backlight/backlight.c.  Looks like
> CONFIG_BACKLIGHT_LM3639 depends on CONFIG_BACKLIGHT_CLASS_DEVICE so I
> feel more confident in your patch.
> I would still prefer the maintainers to review though.

AFAICT it is impossible for the probe function to complete
successfully without having called i2c_set_clientdata()
and therefore it is impossible for pchip to be NULL
in the remove function.

Not sure it is possible to use return_nonnull though. It is not that
i2c_get_clientdata() can *never* return non-NULL, it is that in *this*
case (which is actually a fairly common one) it can never return
non-NULL.


Daniel.