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

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Date:   Wed, 28 Aug 2019 16:52:20 +0800
From:   Zhang Rui <rui.zhang@...el.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Nathan Huckleberry <nhuck@...gle.com>, edubezval@...il.com
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Yoshihiro Kaneko <ykaneko0929@...il.com>,
        wsa+renesas@...g-engineering.com
Subject: Re: [PATCH] thermal: rcar_gen3_thermal: Fix Wshift-negative-value

On Fri, 2019-06-14 at 12:52 +0200, Daniel Lezcano wrote:
> 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.

Sounds reasonable to me.

Yoshihiro,
can you please clarify on this? Or else I will revert the above commits
first?

Also CC Wolfram Sang, the driver author.

thanks,
rui
> 
> 
> > Fixes: 4eb39f79ef44 ("thermal: rcar_gen3_thermal: Update value of
> > Tj_1")
> > Cc: clang-built-linux@...glegroups.com
> > Link: https://github.com/ClangBuiltLinux/linux/issues/531
> > Signed-off-by: Nathan Huckleberry <nhuck@...gle.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));
> > 
> 
>