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

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Message-ID: <CAJkfWY6U3RF=2X4geFsUhFADf9x0GO8s28KQmR7TvnVgo_WTig@mail.gmail.com>
Date:   Tue, 13 Aug 2019 13:12:43 -0700
From:   Nathan Huckleberry <nhuck@...gle.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rui.zhang@...el.com, edubezval@...il.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Yoshihiro Kaneko <ykaneko0929@...il.com>
Subject: Re: [PATCH] thermal: rcar_gen3_thermal: Fix Wshift-negative-value

I'm not familiar enough with the code to rewrite these parts of the
driver. Silencing the warnings while maintaining the same
functionality was the goal of this patch.

On Fri, Jun 14, 2019 at 3:52 AM Daniel Lezcano
<daniel.lezcano@...aro.org> 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.
>
>
> > 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));
> >
>
>
> --
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