Date:   Mon, 7 Feb 2022 09:05:11 -0500
From:   Liam Beguin <liambeguin@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Peter Rosin <peda@...ntia.se>, andy.shevchenko@...il.com,
        lars@...afoo.de, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        robh+dt@...nel.org
Subject: Re: [PATCH v13 06/11] iio: afe: rescale: make use of units.h

On Sat, Feb 05, 2022 at 05:54:04PM +0000, Jonathan Cameron wrote:
> On Tue, 1 Feb 2022 14:28:28 -0500
> Liam Beguin <liambeguin@...il.com> wrote:
> 
> > Hi Peter,
> > 
> > On Mon, Jan 31, 2022 at 03:50:22PM +0100, Peter Rosin wrote:
> > > Hi!
> > > 
> > > I noticed that I have not reviewed this patch. Sorry for my low
> > > bandwidth.
> > > 
> > > On 2022-01-30 17:10, Liam Beguin wrote:  
> > > > Make use of well-defined SI metric prefixes to improve code readability.
> > > > 
> > > > Signed-off-by: Liam Beguin <liambeguin@...il.com>
> > > > ---
> > > >  drivers/iio/afe/iio-rescale.c | 14 +++++++-------
> > > >  1 file changed, 7 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > > > index 67273de46843..27c6664915ff 100644
> > > > --- a/drivers/iio/afe/iio-rescale.c
> > > > +++ b/drivers/iio/afe/iio-rescale.c
> > > > @@ -51,11 +51,11 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> > > >  		}
> > > >  		fallthrough;
> > > >  	case IIO_VAL_FRACTIONAL_LOG2:
> > > > -		tmp = (s64)*val * 1000000000LL;
> > > > +		tmp = (s64)*val * GIGA;
> > > >  		tmp = div_s64(tmp, rescale->denominator);
> > > >  		tmp *= rescale->numerator;
> > > >  
> > > > -		tmp = div_s64_rem(tmp, 1000000000LL, &rem);
> > > > +		tmp = div_s64_rem(tmp, GIGA, &rem);  
> > > 
> > > It is NOT easy for me to say which of GIGA/NANO is most fitting.
> > > There are a couple of considerations:  
> > 
> > I agree with you that the choice behind GIGA/NANO can be a bit
> > confusing.
> > 
> > In my opinion, these defines makes the code easier to read if you
> > consider them as multipliers with no physical meaning, basically a
> > pretty name for a power of 10.
> > 
> > By this logic, we wouldn't ever use FEMTO to DECI.
> 
> Not sure if it would help but maybe it's worth a local define
> of something like
> 
> #define MULT9 1000000000LL
> to loose the association with any particular SI basis and
> just indicate it's a bit number being used to retain precision
> in some maths?  Would need a comment to stop people sending
> patches to replace it with GIGA though ;)
> 
> My ultimate preference here is for whatever works for Peter and
> Liam as the people who are mostly likely to have to deal
> with any changes to this driver in the future.

Hi Jonathan,

My preference here is to keep GIGA, if it makes everyone more
comfortable, I can add a comment explaing the intention of the
multiplication?

Cheers,
Liam

> Jonathan
> 
> 
> > 
> > Cheers,
> > Liam
> > 
> > > A) 1000000000 is just a big value (GIGA fits). Something big is
> > >    needed to not lose too much precision.
> > > B) 1000000000 is what the IIO core uses to print fractional-log
> > >    values with nano precision (NANO fits). This is not really
> > >    relevant in this context.
> > > C) 1000000000 makes the int-plus-nano and fractional-log cases
> > >    align (NANO fits). This last consideration is introduced with
> > >    patch 4/11.
> > > 
> > > There is simply no correct define to use. And whichever define is
> > > chosen makes the other interpretation less obvious. Which is not
> > > desirable, obscures things and make both GIGA and NANO bad
> > > options.
> > > 
> > > So, I stepped back to the description provided by Andy in the
> > > comments of v11:
> > > 
> > > On 2021-12-22 19:59, Andy Shevchenko wrote:
> > > | You should get the proper power after the operation.
> > > | Write a formula (mathematically speaking) and check each of them for this.
> > > | 
> > > | 10^-5/10^-9 == 1*10^4 (Used NANO)
> > > | 10^-5/10^9 == 1/10^-14 (Used GIGA)
> > > | 
> > > | See the difference?
> > > 
> > > No, I don't really see the difference, that just makes me totally
> > > confused. Dividing by 10^-9 or multiplying by 10^9 is as we all
> > > know exactly the same, and the kernel cannot deal directly with
> > > 10^-9 so both will look the same in code (multiplying by 10^9). So,
> > > you must be referring to the "real formula" behind the code. But
> > > in that case, if the "real formula" behind the (then equivalent)
> > > code had instead been
> > > 
> > > 	10^-5*10^9 == 1*10^4 (Used GIGA)
> > > 	10^-5*10^-9 == 1/10^-14 (Used NANO)
> > > 
> > > the outcome is the opposite. NANO turns GIGA and vice versa.
> > > 
> > > Since you can express the same thing differently in math too, it
> > > all crumbles for me. Because of this duality, it will be a matter
> > > of taste if GIGA or NANO fits best in any given instance. Sometimes
> > > (perhaps commonly) it will be decidedly easy to pick one of them,
> > > but in other cases (see above) we will end up with a conflict.
> > > 
> > > What to do then? Or, what am I missing?
> > > 
> > > My taste says NANO in this case, since A) is just some big number
> > > and not really about units and B) is as stated not really relevant.
> > > Which makes C) win the argument for me.
> > >   
> > > >  		*val = tmp;
> > > >  
> > > >  		if (!rem)
> > > > @@ -71,7 +71,7 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
> > > >  
> > > >  		*val2 = rem / (int)tmp;
> > > >  		if (rem2)
> > > > -			*val2 += div_s64((s64)rem2 * 1000000000LL, tmp);
> > > > +			*val2 += div_s64((s64)rem2 * GIGA, tmp);  
> > > 
> > > Here, 1000000000 matches the above use. If we go with NANO above,
> > > we should go with NANO here as well.
> > >   
> > > >  		return IIO_VAL_INT_PLUS_NANO;
> > > >  	case IIO_VAL_INT_PLUS_NANO:
> > > > @@ -332,8 +332,8 @@ static int rescale_current_sense_amplifier_props(struct device *dev,
> > > >  	 * gain_div / (gain_mult * sense), while trying to keep the
> > > >  	 * numerator/denominator from overflowing.
> > > >  	 */
> > > > -	factor = gcd(sense, 1000000);
> > > > -	rescale->numerator = 1000000 / factor;
> > > > +	factor = gcd(sense, MEGA);
> > > > +	rescale->numerator = MEGA / factor;  
> > > 
> > > Here, the 1000000 number comes from the unit of the sense resistor
> > > (micro-ohms), so I would have preferred MICRO. But who can tell
> > > if we -mathematically speaking- have divided the given resistance
> > > integer by 10^6 (MEGA) or multiplied it with 10^-6 (MICRO) to
> > > account for the unit? Or if we divided the other values with
> > > 10^6 (MEGA) (or multiplied by 10^-6, MICRO) to make them fit the
> > > unit of the shunt resistance?
> > > 
> > > All of the above is of course equivalent so both MEGA and MICRO
> > > are correct. But as stated, MICRO makes to most sense as that is
> > > what connects the code to reality and hints at where the value
> > > is coming from. For me anyway.
> > >   
> > > >  	rescale->denominator = sense / factor;
> > > >  
> > > >  	factor = gcd(rescale->numerator, gain_mult);
> > > > @@ -361,8 +361,8 @@ static int rescale_current_sense_shunt_props(struct device *dev,
> > > >  		return ret;
> > > >  	}
> > > >  
> > > > -	factor = gcd(shunt, 1000000);
> > > > -	rescale->numerator = 1000000 / factor;
> > > > +	factor = gcd(shunt, MEGA);
> > > > +	rescale->numerator = MEGA / factor;  
> > > 
> > > Same here, 1000000 comes from the micro-ohms unit of the shunt
> > > resistor, so I would have preferred MICRO.
> > > 
> > > 
> > > 
> > > Sorry for the long mail. I blame the duality of these ambiguous
> > > SI-defines that are a bit confusing to me.
> > > 
> > > Cheers,
> > > Peter
> > >   
> > > >  	rescale->denominator = shunt / factor;
> > > >  
> > > >  	return 0;  
> 

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