Re: [PATCH v2 5/6] staging:iio:ad2s90: Add IIO_CHAN_INFO_SCALE to channel spec and read_raw

[Matheus Tavares Bernardino <matheus.bernardino@usp.br>]

On Sun, Oct 28, 2018 at 1:50 PM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Fri, 26 Oct 2018 23:00:04 -0300
> Matheus Tavares <matheus.bernardino@usp.br> wrote:
>
> > This patch adds the IIO_CHAN_INFO_SCALE mask to ad2s90_chan and
> > implements the relative read behavior at ad2s90_read_raw.
> >
> > Signed-off-by: Victor Colombo <victorcolombo@gmail.com>
> > Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
>
> Hi,
>
> A suggestion inline.  This is a common case that we have infrastructure
> to simplify.  + I think your scale factor is very slightly wrong.
>
> Jonathan
>
> > ---
> >  drivers/staging/iio/resolver/ad2s90.c | 32 ++++++++++++++++++---------
> >  1 file changed, 22 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/staging/iio/resolver/ad2s90.c b/drivers/staging/iio/resolver/ad2s90.c
> > index b4a6a89c11b0..52b656875ed1 100644
> > --- a/drivers/staging/iio/resolver/ad2s90.c
> > +++ b/drivers/staging/iio/resolver/ad2s90.c
> > @@ -34,19 +34,31 @@ static int ad2s90_read_raw(struct iio_dev *indio_dev,
> >       int ret;
> >       struct ad2s90_state *st = iio_priv(indio_dev);
> >
> > -     mutex_lock(&st->lock);
> > +     switch (m) {
> > +     case IIO_CHAN_INFO_SCALE:
> > +             /* 2 * Pi / (2^12 - 1) ~= 0.001534355 */
> > +             *val = 0;
> > +             *val2 = 1534355;
> Definitely 2^12 - 1?  That's a bit unusual if true as it would imply
> that 2^12 - 1 and 0 were the same value.
>
> Imagine a smaller version with on 2^2 bits so 0, 1, 2, 3
> Values of each are
>
> 0, M_PI/2, M_PI, 3*M_PI/2
>
> So the multiplier is 2*M_PI/(2^2) not 2*M_PI/(2^2 - 1)
> 1/2 vs 2/3 * M_PI

Oh, that makes a lot of sense! We used 2^12 - 1 here based on driver
drivers/iio/resolver/ad2s1200.c, whose resolution is also 12 bits, as
the ad2s90.c. Do you think this section is, perhaps, wrong on
ad2s1200.c too, or maybe there is some difference between these two
drivers that I didn't catch regarding the resolution?

Matheus.

> Now this is a very common case so we have the return type
> IIO_VAL_FRACTIONAL_LOG2 to give a more obvious and potentially
> more accurate representation.
>
> > +             return IIO_VAL_INT_PLUS_NANO;
> > +     case IIO_CHAN_INFO_RAW:
> > +             mutex_lock(&st->lock);
> > +
> > +             ret = spi_read(st->sdev, st->rx, 2);
> > +             if (ret < 0) {
> > +                     mutex_unlock(&st->lock);
> > +                     return ret;
> > +             }
> > +
> > +             *val = (((u16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
> >
> > -     ret = spi_read(st->sdev, st->rx, 2);
> > -     if (ret < 0) {
> >               mutex_unlock(&st->lock);
> > -             return ret;
> > -     }
> >
> > -     *val = (((u16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
> > -
> > -     mutex_unlock(&st->lock);
> > +             return IIO_VAL_INT;
> > +     default:
> > +             break;
> > +     }
> >
> > -     return IIO_VAL_INT;
> > +     return -EINVAL;
> >  }
> >
> >  static const struct iio_info ad2s90_info = {
> > @@ -57,7 +69,7 @@ static const struct iio_chan_spec ad2s90_chan = {
> >       .type = IIO_ANGL,
> >       .indexed = 1,
> >       .channel = 0,
> > -     .info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
> > +     .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
> >  };
> >
> >  static int ad2s90_probe(struct spi_device *spi)
>