Re: [PATCH v2] mfd: tps65911-comparator: Fix an off by one bug

Lee Jones
2018-04-23T06:44:17Z

On Fri, 20 Apr 2018, Dan Carpenter wrote:

> The COMP1 and COMP2 elements are in 0 and 1 respectively so this code is
> accessing the wrong elements and one space beyond the end of the array.
> We should be using "id - 1" instead.
>
> The "id" variable is never COMP (0) so that code can be removed.
>
> Fixes: 6851ad3ab346 ("TPS65911: Comparator: Add comparator driver")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
> v2: we can fix the bug and save memory.

Looks like it's as we feared:

http://www.ti.com/lit/ds/symlink/tps65911.pdf (page 52)

There are only 2 comparators 1 and 2.

> diff --git a/drivers/mfd/tps65911-comparator.c b/drivers/mfd/tps65911-comparator.c
> index c0789f81a1c5..887409c3938d 100644
> --- a/drivers/mfd/tps65911-comparator.c
> +++ b/drivers/mfd/tps65911-comparator.c
> @@ -22,7 +22,6 @@
> #include <linux/gpio.h>
> #include <linux/mfd/tps65910.h>
>
> -#define COMP 0
> #define COMP1 1
> #define COMP2 2

Sorry, but I think these defines should describe the indexes into the
supplied struct directly, especially as in this case it is their only
reason for being.

I completely agree with you that defining something that is named '1'
as '0' is not ideal, but IMHO it's better than jumping through hoops
using arithmetic in array indexes.

> @@ -58,14 +57,11 @@ static struct comparator tps_comparators[] = {
>
> static int comp_threshold_set(struct tps65910 *tps65910, int id, int voltage)
> {
> - struct comparator tps_comp = tps_comparators[id];
> + struct comparator tps_comp = tps_comparators[id - 1];
> int curr_voltage = 0;
> int ret;
> u8 index = 0, val;
>
> - if (id = COMP)
> - return 0;
> -
> while (curr_voltage < tps_comp.uV_max) {
> curr_voltage = tps_comp.vsel_table[index];
> if (curr_voltage >= voltage)
> @@ -85,13 +81,10 @@ static int comp_threshold_set(struct tps65910 *tps65910, int id, int voltage)
>
> static int comp_threshold_get(struct tps65910 *tps65910, int id)
> {
> - struct comparator tps_comp = tps_comparators[id];
> + struct comparator tps_comp = tps_comparators[id - 1];
> int ret;
> u8 val;
>
> - if (id = COMP)
> - return 0;
> -
> ret = tps65910->read(tps65910, tps_comp.reg, 1, &val);
> if (ret < 0)
> return ret;

--
Lee Jones [李琼斯]
Linaro Services Technical Lead
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