Re: [PATCH] thermal: rcar_gen3_thermal: Fix Wshift-negative-value

From: Daniel Lezcano <daniel.lezcano@linaro.org>
To: Nathan Huckleberry <nhuck@google.com>,
	rui.zhang@intel.com, edubezval@gmail.com
Cc: linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com,
	Yoshihiro Kaneko <ykaneko0929@gmail.com>
Subject: Re: [PATCH] thermal: rcar_gen3_thermal: Fix Wshift-negative-value
Date: Fri, 14 Jun 2019 12:52:17 +0200	[thread overview]
Message-ID: <fd8b8a48-dfb7-1478-2d8d-0953acee39d3@linaro.org> (raw)
In-Reply-To: <20190613211228.34092-1-nhuck@google.com>

Hi Nathan,

On 13/06/2019 23:12, Nathan Huckleberry wrote:
> Clang produces the following warning
> 
> vers/thermal/rcar_gen3_thermal.c:147:33: warning: shifting a negative
> signed value is undefined [-Wshift-negative-value] / (ptat[0] - ptat[2])) +
> FIXPT_INT(TJ_3); ^~~~~~~~~~~~~~~ drivers/thermal/rcar_gen3_thermal.c:126:29
> note: expanded from macro 'FIXPT_INT' #define FIXPT_INT(_x) ((_x) <<
> FIXPT_SHIFT) ~~~~ ^ drivers/thermal/rcar_gen3_thermal.c:150:18: warning:
> shifting a negative signed value is undefined [-Wshift-negative-value]
> tsc->tj_t - FIXPT_INT(TJ_3)); ~~~~~~~~~~~~^~~~~~~~~~~~~~~~
> 
> Upon further investigating it looks like there is no real reason for
> TJ_3 to be -41. Usages subtract -41, code would be cleaner to just add.

All the code seems broken regarding the negative value shifting as the
macros pass an integer:

eg.
        tsc->coef.a2 = FIXPT_DIV(FIXPT_INT(thcode[1] - thcode[0]),
                                 tsc->tj_t - FIXPT_INT(ths_tj_1));

thcode[1] is always < than thcode[0],

thcode[1] - thcode[0] < 0

FIXPT_INT(thcode[1] - thcode[0]) is undefined

Is it done on purpose FIXPT_DIV(FIXPT_INT(thcode[1] - thcode[0]) ?

Try developing the macro with the coef.a2 computation ...

The code quality of this driver could be better, it deserves a rework
IMHO ...

I suggest to revert:

4eb39f79ef443fa566d36bd43f1f578d5c140305
bdc4480a669d476814061b4da6bb006f7048c8e5
6a310f8f97bb8bc2e2bb9db6f49a1b8678c8d144

Rework the coefficient computation and re-introduce what was reverted in
a nicer way.

> Fixes: 4eb39f79ef44 ("thermal: rcar_gen3_thermal: Update value of Tj_1")
> Cc: clang-built-linux@googlegroups.com
> Link: https://github.com/ClangBuiltLinux/linux/issues/531
> Signed-off-by: Nathan Huckleberry <nhuck@google.com>
> ---
>  drivers/thermal/rcar_gen3_thermal.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/thermal/rcar_gen3_thermal.c b/drivers/thermal/rcar_gen3_thermal.c
> index a56463308694..f4b4558c08e9 100644
> --- a/drivers/thermal/rcar_gen3_thermal.c
> +++ b/drivers/thermal/rcar_gen3_thermal.c
> @@ -131,7 +131,7 @@ static inline void rcar_gen3_thermal_write(struct rcar_gen3_thermal_tsc *tsc,
>  #define RCAR3_THERMAL_GRAN 500 /* mili Celsius */
>  
>  /* no idea where these constants come from */
> -#define TJ_3 -41
> +#define TJ_3 41
>  
>  static void rcar_gen3_thermal_calc_coefs(struct rcar_gen3_thermal_tsc *tsc,
>  					 int *ptat, const int *thcode,
> @@ -144,11 +144,11 @@ static void rcar_gen3_thermal_calc_coefs(struct rcar_gen3_thermal_tsc *tsc,
>  	 * the dividend (4095 * 4095 << 14 > INT_MAX) so keep it unscaled
>  	 */
>  	tsc->tj_t = (FIXPT_INT((ptat[1] - ptat[2]) * 157)
> -		     / (ptat[0] - ptat[2])) + FIXPT_INT(TJ_3);
> +		     / (ptat[0] - ptat[2])) - FIXPT_INT(TJ_3);
>  
>  	tsc->coef.a1 = FIXPT_DIV(FIXPT_INT(thcode[1] - thcode[2]),
> -				 tsc->tj_t - FIXPT_INT(TJ_3));
> -	tsc->coef.b1 = FIXPT_INT(thcode[2]) - tsc->coef.a1 * TJ_3;
> +				 tsc->tj_t + FIXPT_INT(TJ_3));
> +	tsc->coef.b1 = FIXPT_INT(thcode[2]) + tsc->coef.a1 * TJ_3;
>  
>  	tsc->coef.a2 = FIXPT_DIV(FIXPT_INT(thcode[1] - thcode[0]),
>  				 tsc->tj_t - FIXPT_INT(ths_tj_1));
> 

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