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SubjectRe: [PATCH v2] backlight: pwm_bl: Fix uninitialized variable
On Thu, 19 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.
>
> Additionally fix a small grammar error that was identified and
> tighten up return code checking of DT properties, both of which came
> up during review of this patch.
>
> 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>
> Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
> ---
>
> Notes:
> v2:
> - Simplify SoB chain (with Marcel's permission)
> - Separate complex if statement and make other similar calls use same
> return code checking approach
> - Tidy up comment formatting and fix pre-existing grammar error
>
> drivers/video/backlight/pwm_bl.c | 25 ++++++++++++-------------
> 1 file changed, 12 insertions(+), 13 deletions(-)

I'm hesitant to provide feedback on this, as I feel as though I've
messed you around enough, however ... ;)

> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> index 9ee4c1b735b2..f7799f62fea0 100644
> --- a/drivers/video/backlight/pwm_bl.c
> +++ b/drivers/video/backlight/pwm_bl.c
> @@ -284,30 +284,29 @@ static int pwm_backlight_parse_dt(struct device *dev,
> ret = of_property_read_u32_array(node, "brightness-levels",
> data->levels,
> data->max_brightness);
> - if (ret < 0)
> + if (!ret)
> return ret;
>
> ret = of_property_read_u32(node, "default-brightness-level",
> &value);
> - if (ret < 0)
> + if (!ret)
> return ret;

Just FYI (it didn't even make it to 'nit' status), this should really
be done in a separate patch since it is unrelated to the rest of the
patch.

> data->dft_brightness = value;
>
> /*
> * This property is optional, if is set enables linear
> - * interpolation between each of the values of brightness levels
> - * and creates a new pre-computed table.
> + * 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) {
> + ret = of_property_read_u32(node, "num-interpolated-steps",
> + &num_steps);
> + if (!ret || num_steps) {

Not sure if it's even possible for of_property_read_u32() to fail AND
still populate num_steps, however this check makes it sound like that's
okay. Is that correct?

I can't help but think that this all 'just goes away' if you
pre-initialise num_steps. I wouldn't let the "do not initialise too
far away from the code using variable" affect this. However, if
you're insistent, perhaps consider moving the declaration to just
below:

if (data->max_brightness > 0) {

> + /*
> + * Make sure that there are at least two entries in
> + * the brightness-levels table, otherwise we can't
> + * interpolate between two points.
> + */
> if (data->max_brightness < 2) {
> dev_err(dev, "can't interpolate\n");
> return -EINVAL;

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