On Mon, 7 Feb 2022 09:05:11 -0500 Liam Beguin <liambeguin@xxxxxxxxx> wrote: > 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@xxxxxxxxx> 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@xxxxxxxxx> > > > > > --- > > > > > 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? > Works for me. J > 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; > >
