Re: [PATCH 1/3] iio: max31856: add option for setting mains filter rejection frequency

[Andrea Merello <andrea.merello@gmail.com>]

Il giorno dom 6 ott 2019 alle ore 09:54 Jonathan Cameron
<jic23@kernel.org> ha scritto:
>
> On Mon, 23 Sep 2019 14:17:12 +0200
> Andrea Merello <andrea.merello@gmail.com> wrote:
>
> > This sensor has an embedded notch filter for reducing interferences caused
> > by the power mains. This filter can be tuned to reject either 50Hz or 60Hz
> > (and harmonics).
> >
> > Currently the said setting is left alone (the sensor defaults to 60Hz).
> > This patch introduces a IIO attribute that allows the user to set the said
> > filter to the desired frequency.
> >
> > NOTE: this has been intentionally not tied to any DT property to allow
> > the configuration of this setting from userspace, e.g. with a GUI or by
> > reading a configuration file, or maybe reading a GPIO tied to a physical
> > switch or accessing some device that can autodetect the line frequency.
> >
> > Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
> This one is not something that can be expect to be known from the setup
> of the device as it will depend on local mains frequency.
>
> So fine, to have it as a userspace control, but the name is too generic.
> We already have a number of filter attributes and we should try to
> work out how to bring it inline with them.

Sure

> in_X_filter_low_pass_3db_frequency
> in_X_filter_high_pass_3db_frequency
>
> So would,
> in_X_filter_notch_center_frequency work?
> ( I suppose we should use the American spelling ;)

Yes, it seems OK in this case. I will produce a V2 with this modification.

> This kind of ignores the harmonics aspect but at least documents the
> main frequency being blocked.

I think this is perfectly fine: the user wants to set the notch filter
center frequency to either 60Hz or 50Hz to match the line frequency,
then she/he expects the filter to simply "work" somehow; IMO the
harmonic thing does not needed to be explicit.

> There is a slight complexity that we have devices that have dual
> notchfilters (50 and 60Hz) and ones where you can turn it off entirely.
>
> I suppose no value would count as off and we could perhaps use a list
> for both on at the same time (though that's a bit horrible).

IMHO it seems reasonable. Maybe for all-off and both-on conditions we
could also use magic strings like i.e. "all" and "off".

> > ---
> >  drivers/iio/temperature/max31856.c | 49 ++++++++++++++++++++++++++++++
> >  1 file changed, 49 insertions(+)
> >
> > diff --git a/drivers/iio/temperature/max31856.c b/drivers/iio/temperature/max31856.c
> > index 73ed550e3fc9..d12613f7ba3c 100644
> > --- a/drivers/iio/temperature/max31856.c
> > +++ b/drivers/iio/temperature/max31856.c
> > @@ -23,6 +23,7 @@
> >  #define MAX31856_CR0_1SHOT         BIT(6)
> >  #define MAX31856_CR0_OCFAULT       BIT(4)
> >  #define MAX31856_CR0_OCFAULT_MASK  GENMASK(5, 4)
> > +#define MAX31856_CR0_FILTER_50HZ   BIT(0)
> >  #define MAX31856_TC_TYPE_MASK      GENMASK(3, 0)
> >  #define MAX31856_FAULT_OVUV        BIT(1)
> >  #define MAX31856_FAULT_OPEN        BIT(0)
> > @@ -63,6 +64,7 @@ static const struct iio_chan_spec max31856_channels[] = {
> >  struct max31856_data {
> >       struct spi_device *spi;
> >       u32 thermocouple_type;
> > +     bool filter_50hz;
> >  };
> >
> >  static int max31856_read(struct max31856_data *data, u8 reg,
> > @@ -123,6 +125,11 @@ static int max31856_init(struct max31856_data *data)
> >       reg_cr0_val &= ~MAX31856_CR0_1SHOT;
> >       reg_cr0_val |= MAX31856_CR0_AUTOCONVERT;
> >
> > +     if (data->filter_50hz)
> > +             reg_cr0_val |= MAX31856_CR0_FILTER_50HZ;
> > +     else
> > +             reg_cr0_val &= ~MAX31856_CR0_FILTER_50HZ;
> > +
> >       return max31856_write(data, MAX31856_CR0_REG, reg_cr0_val);
> >  }
> >
> > @@ -249,12 +256,53 @@ static ssize_t show_fault_oc(struct device *dev,
> >       return show_fault(dev, MAX31856_FAULT_OPEN, buf);
> >  }
> >
> > +static ssize_t show_filter(struct device *dev,
> > +                        struct device_attribute *attr,
> > +                        char *buf)
> > +{
> > +     struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> > +     struct max31856_data *data = iio_priv(indio_dev);
> > +
> > +     return sprintf(buf, "%d\n", data->filter_50hz ? 50 : 60);
> > +}
> > +
> > +static ssize_t set_filter(struct device *dev,
> > +                       struct device_attribute *attr,
> > +                       const char *buf,
> > +                       size_t len)
> > +{
> > +     struct iio_dev *indio_dev = dev_to_iio_dev(dev);
> > +     struct max31856_data *data = iio_priv(indio_dev);
> > +     unsigned int freq;
> > +     int ret;
> > +
> > +     ret = kstrtouint(buf, 10, &freq);
> > +     if (ret)
> > +             return ret;
> > +
> > +     switch (freq) {
> > +     case 50:
> > +             data->filter_50hz = true;
> > +             break;
> > +     case 60:
> > +             data->filter_50hz = false;
> > +             break;
> > +     default:
> > +             return -EINVAL;
> > +     }
> > +
> > +     max31856_init(data);
> > +     return len;
> > +}
> > +
> >  static IIO_DEVICE_ATTR(fault_ovuv, 0444, show_fault_ovuv, NULL, 0);
> >  static IIO_DEVICE_ATTR(fault_oc, 0444, show_fault_oc, NULL, 0);
> > +static IIO_DEVICE_ATTR(filter, 0644, show_filter, set_filter, 0);
> >
> >  static struct attribute *max31856_attributes[] = {
> >       &iio_dev_attr_fault_ovuv.dev_attr.attr,
> >       &iio_dev_attr_fault_oc.dev_attr.attr,
> > +     &iio_dev_attr_filter.dev_attr.attr,
> >       NULL,
> >  };
> >
> > @@ -280,6 +328,7 @@ static int max31856_probe(struct spi_device *spi)
> >
> >       data = iio_priv(indio_dev);
> >       data->spi = spi;
> > +     data->filter_50hz = false;
> >
> >       spi_set_drvdata(spi, indio_dev);
> >
>