Re: [PATCH v2] backlight: pwm_bl: Fix uninitialized variable

On Mon, Jul 23, 2018 at 08:23:43AM +0100, Lee Jones wrote:

On Thu, 19 Jul 2018, Daniel Thompson wrote:

quoted

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 <redacted>
Signed-off-by: Daniel Thompson <redacted>
Tested-by: Marcel Ziswiler <redacted>
---

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 ... ;)

quoted

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.

quoted

 		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?

No, it's bogus. Looks like when I broke the if statement into two
clauses I ended up flipping the && to an ||.

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) {

quoted

+			/*
+			 * 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
Follow Linaro: Facebook | Twitter | Blog