Re: [PATCH 1/1] iio: core: Improve precision of __iio_format_value for FRACTIONAL values

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Tue, 29 Jan 2019 17:11:25 +0800
Phil Reid <preid@electromag.com.au> wrote:

> G'day Alex,
> 
> On 29/01/2019 4:32 pm, Alexandru Ardelean wrote:
> > On Tue, Jan 29, 2019 at 8:28 AM Phil Reid <preid@electromag.com.au> wrote:  
> >>
> >> Currently FRACTIONAL values are outputed with 9 digits after the decimal
> >> place. This is not always sufficient to resolve the raw value with 1 bit.
> >> Output FRACTIONAL values to 15 decimal places of precision, regardless
> >> of the number of leading zeros.
> >>
> >> Currently for a 2.5V ref with 24 bits of precision the code outputs only
> >> to 9 decimal places.
> >>
> >> Cur: 0.00014901100000000000 * 16777216 = 2499.989733
> >> New: 0.00014901161193847600 * 16777216 = 2500.000000
> >> Signed-off-by: Phil Reid <preid@electromag.com.au>
> >> ---
> >>
> >> Notes:
> >>      Alternatively I could add additonal FRACTIONAL types that select the new
> >>      behaviour to prevent any possible regressions.
> >>
> >>   drivers/iio/industrialio-core.c | 55 ++++++++++++++++++++++++++++++++++-------
> >>   1 file changed, 46 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> >> index a062cfd..bd9da64 100644
> >> --- a/drivers/iio/industrialio-core.c
> >> +++ b/drivers/iio/industrialio-core.c
> >> @@ -571,11 +571,53 @@ int of_iio_read_mount_matrix(const struct device *dev,
> >>   #endif
> >>   EXPORT_SYMBOL(of_iio_read_mount_matrix);
> >>
> >> +static ssize_t __iio_format_div_prec(char *buf, unsigned int len, s64 x, s32 y)
> >> +{
> >> +       unsigned int prec = 0;
> >> +       unsigned int idx = 0;
> >> +       s64 d;
> >> +
> >> +       if (!len)
> >> +               return 0;
> >> +
> >> +       if (!y)
> >> +               return snprintf(buf, len, "inf");
> >> +
> >> +       if (!x)
> >> +               return snprintf(buf, len, "0");
> >> +
> >> +       if (((x > 0) && (y < 0)) || ((x < 0) && (y > 0))) {
> >> +               buf[idx++] = '-';
> >> +               x = x > 0 ? x : -x;
> >> +               y = y > 0 ? y : -y;
> >> +       }
> >> +
> >> +       d = div64_s64(x, y);
> >> +       idx += snprintf(buf+idx, len-idx, "%d", (int)d);
> >> +       x = x - (y * d);
> >> +       if ((x != 0) && (idx < len-1)) {
> >> +               buf[idx++] = '.';
> >> +               x = x * 10;
> >> +               d = div64_s64(x, y);
> >> +
> >> +               while ((idx < len-1) && (prec < 15)) {
> >> +                       if (d || prec)
> >> +                               prec++;
> >> +                       buf[idx++] = '0' + (char)d;
> >> +                       x = x - (y * d);
> >> +                       if (!x)
> >> +                               break;
> >> +                       x = x * 10;
> >> +                       d = div64_s64(x, y);
> >> +               }
> >> +               buf[idx] = 0;
> >> +       }
> >> +       return idx;
> >> +}
> >> +
> >>   static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
> >>                                    int size, const int *vals)
> >>   {
> >> -       unsigned long long tmp;
> >> -       int tmp0, tmp1;
> >>          bool scale_db = false;
> >>
> >>          switch (type) {
> >> @@ -598,14 +640,9 @@ static ssize_t __iio_format_value(char *buf, size_t len, unsigned int type,
> >>                  else
> >>                          return snprintf(buf, len, "%d.%09u", vals[0], vals[1]);
> >>          case IIO_VAL_FRACTIONAL:
> >> -               tmp = div_s64((s64)vals[0] * 1000000000LL, vals[1]);
> >> -               tmp1 = vals[1];
> >> -               tmp0 = (int)div_s64_rem(tmp, 1000000000, &tmp1);
> >> -               return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
> >> +               return __iio_format_div_prec(buf, len, vals[0], vals[1]);  
> > 
> > Maybe I'm a bit naive, but I'm also a bit curious.
> > If you just bump the numbers here, would it work the same ?
> > 
> > i.e.   10^9 -> 10^15 and "snprintf(buf, len, "%d.%15u", tmp0, abs(tmp1));"  
> I did look at that solution.
> 
> But I was running into overflow issues (even with 64 bit numbers).
> 
> eg: with a 2500 reference and 32 bits.
> 
> 2500 * 10^15 = 2e+18 = 61 bits
> And the result of
> 2500 / 2^32 =  0.000000582076609
> Only provides 9 significant digits with 15 decimal places.
> 
> I was looking to provide 15 significant digits to match a standard double
> precision floating point value.

I'll ask the awkward engineering question.  Is this precision actually valid?
I.e. typical voltage references are +- 0.0x %

The fact we have a 32 bit ADC means you'll detect small changes, but I'm
dubious about whether the absolute value will ever be 'that good'.

If we are going to go out of way to support greater precision we need
a strong justification of why.
To take advantage of these high precision devices you need to take into
account non linear effects, temperature etc.  These will swamp (I think)
any effect of a lack of precision the scale value.

> 
> Proposed solution seemed to work with a wider range, but I admit it's not elegant.
> Certainly interested in other peoples ideas.
> 
> My alternative thought was to introduce a new type that returns the scale
> as a rational. eg: return string like
> scale_rational = 2500/4294967296
> 
> But that'd require existing user space to become aware of the new format.
> 
> 
> > 
> > But in any case, what would be interesting now, is to extend the IIO
> > core logic to provide [somehow] a precision number, default being 9.
> > This could probably be specified on per-channel basis [somehow],
> > similar to other channel params.
> > 
> > So, for example in the default case, if you have "uint32_t precision =
> > 9", you would have the same behavior, with something like
> > 
> >        tmp = div_s64((s64)vals[0] * pow_of_10(precision), vals[1]);
> >        tmp1 = vals[1];
> >        tmp0 = (int)div_s64_rem(tmp, pow_of_10(precision), &tmp1);
> >        return snprintf(buf, len, "%d.%" precision "u", tmp0, abs(tmp1));
> > 
> > Obviously, the above code is just pseudo-code, where pow_of_10()
> > multiplies 10 a "precision" number of times, and the snprintf() would
> > need a temporary buffer to create a format string, which then would be
> > used.  
> 
> Good idea to have some kind of overide, I'll have a look
> 
> > 
> > Thanks
> > Alex
> >   
> >>          case IIO_VAL_FRACTIONAL_LOG2:
> >> -               tmp = shift_right((s64)vals[0] * 1000000000LL, vals[1]);
> >> -               tmp0 = (int)div_s64_rem(tmp, 1000000000LL, &tmp1);
> >> -               return snprintf(buf, len, "%d.%09u", tmp0, abs(tmp1));
> >> +               return __iio_format_div_prec(buf, len, vals[0], 1 << vals[1]);
> >>          case IIO_VAL_INT_MULTIPLE:
> >>          {
> >>                  int i;
> >> --
> >> 1.8.3.1
> >>  
> > 
> >   
> 
>