linux-kernel - Re: [PATCH] backlight: pwm_bl: Fix uninitialized variable

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Date:   Wed, 18 Jul 2018 10:53:35 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Marcel Ziswiler <marcel.ziswiler@...adex.com>
Cc:     "daniel.thompson@...aro.org" <daniel.thompson@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jingoohan1@...il.com" <jingoohan1@...il.com>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "b.zolnierkie@...sung.com" <b.zolnierkie@...sung.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "patches@...aro.org" <patches@...aro.org>
Subject: Re: [PATCH] backlight: pwm_bl: Fix uninitialized variable

On Wed, 18 Jul 2018, Marcel Ziswiler wrote:

> On Wed, 2018-07-18 at 09:09 +0100, Lee Jones wrote:
> > On Mon, 16 Jul 2018, Daniel Thompson wrote:
> > 
> > > Currently, if the DT does not define num-interpolated-steps then
> > > num_steps is undefined and the interpolation code will deploy
> > > randomly.
> > > Fix this.
> > > 
> > > Fixes: 573fe6d1c25c ("backlight: pwm_bl: Linear interpolation
> > > between
> > > brightness-levels")
> > > Reported-by: Marcel Ziswiler <marcel.ziswiler@...adex.com>
> > > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
> > > Signed-off-by: Marcel Ziswiler <marcel.ziswiler@...adex.com>
> > 
> > This line is confusing.  Did you guys author this patch together?
> 
> Yes, I reported it and we came to a conclusion together.

It sounds like you need to have all of the tags (except this one). :)

 Reported-by:  for reporting the issue
 Suggested-by: for suggesting a resolution
 Acked-by:     for reviewing it
 Tested-by:    for testing it

Signed-off-by usually means you either wrote a significant amount of
the diffstat or you were part of the submission path.

> > My guess is that this line should be dropped and the RB and TB tags
> > should remain?  If it was reviewed too, perhaps an AB too?
> 
> I'm OK either way and do not need any explicit authorship to be
> expressed for me.

In this instance I suggest keeping Reported-by and Tested-by.

> > > Tested-by: Marcel Ziswiler <marcel.ziswiler@...adex.com>
> > > ---
> > >  drivers/video/backlight/pwm_bl.c | 17 ++++++++---------
> > >  1 file changed, 8 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/video/backlight/pwm_bl.c
> > > b/drivers/video/backlight/pwm_bl.c
> > > index 9ee4c1b735b2..e3c22b79fbcd 100644
> > > --- a/drivers/video/backlight/pwm_bl.c
> > > +++ b/drivers/video/backlight/pwm_bl.c
> > > @@ -299,15 +299,14 @@ static int pwm_backlight_parse_dt(struct
> > > device *dev,
> > >  		 * interpolation between each of the values of
> > > brightness levels
> > >  		 * and creates a new pre-computed table.
> > >  		 */
> > > -		of_property_read_u32(node, "num-interpolated-
> > > steps",
> > > -				     &num_steps);
> > > -
> > > -		/*
> > > -		 * Make sure that there is at least two entries in
> > > the
> > > -		 * brightness-levels table, otherwise we can't
> > > interpolate
> > > -		 * between two points.
> > > -		 */
> > > -		if (num_steps) {
> > > +		if ((of_property_read_u32(node, "num-interpolated-
> > > steps",
> > > +					  &num_steps) == 0) &&
> > > num_steps) {
> > 
> > This is pretty ugly, and isn't it suffering from over-bracketing?  My
> > suggestion would be to break out the invocation of
> > of_property_read_u32() from the if and test only the result.
> > 
> > 		of_property_read_u32(node, "num-interpolated-steps", 
> > &num_steps);
> 
> you mean:
> 
> 		ret = of_property_read_u32(node, "num-interpolated-
> steps", &num_steps);
> 
> > 		if (!ret && num_steps) {
> > 
> > I haven't checked the underling code, but is it even feasible for
> > of_property_read_u32() to not succeed AND for num_steps to be set?
> > 
> > If not, the check for !ret if superfluous and you can drop it.
> 
> No, then we are back to the initial issue of num_steps potentially not
> being initialised. We really want both of_property_read_u32() to
> succeed AND num_steps to actually be set.

I also think num_steps should be pre-initialised.

Then it will only be set if of_property_read_u32() succeeds.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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