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SubjectRe: [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@toradex.com>
> > > Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
> > > Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.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@toradex.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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