linux-kernel - Re: [PATCH v2 2/4] docs: hwmon: (ltc2945): change type of val to ULL in ltc2945_val_to

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Message-ID: <20201118151951.GA174391@roeck-us.net>
Date:   Wed, 18 Nov 2020 07:19:51 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Alexandru Ardelean <ardeleanalex@...il.com>
Cc:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        linux-hwmon@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>, jdelvare@...e.com,
        "Thoren, Mark" <mark.thoren@...log.com>
Subject: Re: [PATCH v2 2/4] docs: hwmon: (ltc2945): change type of val to ULL
 in ltc2945_val_to_reg()

On Wed, Nov 18, 2020 at 04:40:24PM +0200, Alexandru Ardelean wrote:
> On Wed, Nov 11, 2020 at 4:54 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >
> > On 11/11/20 1:12 AM, Alexandru Ardelean wrote:
> > > In order to account for any potential overflows that could occur.
> > >
> > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> >
> > Thinking about it, this can only really happen if the user provides
> > excessive values for limit attributes. Those are currently clamped
> > later, after the conversion. I think it would be better to modify
> > the code to apply a clamp _before_ the conversion as well instead
> > of trying to solve the overflow problem with unsigned long long.
> 
> Coming back to this, I think that using the shunt resistor value
> (which is in micro-ohms), and multiplying with multiples of 1000, the
> chances of overflow grow quite a lot.
> I could be wrong, but that is how it looks when I try to do some math
> with the shunt resistor in place.
> 
> Looking at the clamping code, it looks like the initial version is
> quite simple & straightforward.
> I mean when doing the math and getting values out of range, clamping
> afterwards to 0xffffff for power values is quite handy.
> And clamping everything else to 0xfff for voltage also looks pretty simple.
> We can do some clamping before, but it looks like it's extra math that
> is already done in the conversion code anyway.
> 
> Hopefully, I'm not missing something here :)
>

Using ull operations when not necessary can also be quite expensive,
and I'd rather avoid it. I'd rather see an extra clamp than ull.

Guenter

> >
> > Either case, can you send me a register dump for this chip ?
> > I'd like to write a module test script to actually check if there
> > are any over/underflows or other problems.
> >
> > Thanks,
> > Guenter
> >
> > > ---
> > >  drivers/hwmon/ltc2945.c | 12 ++++++------
> > >  1 file changed, 6 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/hwmon/ltc2945.c b/drivers/hwmon/ltc2945.c
> > > index 1cea710df668..6d4569a25471 100644
> > > --- a/drivers/hwmon/ltc2945.c
> > > +++ b/drivers/hwmon/ltc2945.c
> > > @@ -155,7 +155,7 @@ static long long ltc2945_reg_to_val(struct device *dev, u8 reg)
> > >  }
> > >
> > >  static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > > -                           unsigned long val)
> > > +                           unsigned long long val)
> > >  {
> > >       struct ltc2945_state *st = dev_get_drvdata(dev);
> > >       struct regmap *regmap = st->regmap;
> > > @@ -181,14 +181,14 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > >                       return ret;
> > >               if (control & CONTROL_MULT_SELECT) {
> > >                       /* 25 mV * 25 uV = 0.625 uV resolution. */
> > > -                     val = DIV_ROUND_CLOSEST(val, 625);
> > > +                     val = DIV_ROUND_CLOSEST_ULL(val, 625);
> > >               } else {
> > >                       /*
> > >                        * 0.5 mV * 25 uV = 0.0125 uV resolution.
> > >                        * Divide first to avoid overflow;
> > >                        * accept loss of accuracy.
> > >                        */
> > > -                     val = DIV_ROUND_CLOSEST(val, 25) * 2;
> > > +                     val = DIV_ROUND_CLOSEST_ULL(val, 25) * 2;
> > >               }
> > >               break;
> > >       case LTC2945_VIN_H:
> > > @@ -197,7 +197,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > >       case LTC2945_MAX_VIN_THRES_H:
> > >       case LTC2945_MIN_VIN_THRES_H:
> > >               /* 25 mV resolution. */
> > > -             val /= 25;
> > > +             val = div_u64(val, 25);
> > >               break;
> > >       case LTC2945_ADIN_H:
> > >       case LTC2945_MAX_ADIN_H:
> > > @@ -219,7 +219,7 @@ static int ltc2945_val_to_reg(struct device *dev, u8 reg,
> > >                * dividing the reported current by the sense resistor value
> > >                * in mOhm.
> > >                */
> > > -             val = DIV_ROUND_CLOSEST(val, 25);
> > > +             val = DIV_ROUND_CLOSEST_ULL(val, 25);
> > >               break;
> > >       default:
> > >               return -EINVAL;
> > > @@ -247,7 +247,7 @@ static ssize_t ltc2945_value_store(struct device *dev,
> > >       struct ltc2945_state *st = dev_get_drvdata(dev);
> > >       struct regmap *regmap = st->regmap;
> > >       u8 reg = attr->index;
> > > -     unsigned long val;
> > > +     unsigned long long val;
> > >       u8 regbuf[3];
> > >       int num_regs;
> > >       int regval;
> > >
> >