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

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Message-ID: <20180423064417.2e2ng3su23w4i7hx@dell>
Date:   Mon, 23 Apr 2018 07:44:17 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH v2] mfd: tps65911-comparator: Fix an off by one bug

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@...cle.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
Linaro.org │ Open source software for ARM SoCs
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