Re: [PATCH v2 3/6] iio: ad7949: Support configuration read-back

["Popa, Stefan Serban" <StefanSerban.Popa@analog.com>]

On Sb, 2019-05-11 at 11:31 +0100, Jonathan Cameron wrote:
> [External]
> 
> 
> On Tue, 7 May 2019 14:53:32 -0500
> Adam Michaelis <adam.michaelis@collins.com> wrote:
> 
> > 
> > On Sun, May 5, 2019 at 9:42 AM Jonathan Cameron <jic23@kernel.org>
> > wrote:
> > > 
> > > 
> > > On Thu,  2 May 2019 11:14:29 -0500
> > > Adam Michaelis <adam.michaelis@rockwellcollins.com> wrote:
> > > 
> > > > 
> > > > Adds device tree parameter to set the configuration read-back bit
> > > > in the configuration register to tell the AD7949 to include the
> > > > value of
> > > > the configuration register at the time the current sample was
> > > > acquired
> > > > when reading from the part.
> > > > 
> > > > Further work must be done to make read-back information available
> > > > to
> > > > consumer.
> > > This needs some explanation of why it is useful at all. I'm certainly
> > > unclear
> > > on why it would be useful to configure this at boot time.
> > > 
> > > Code looks fine.
> > > 
> > > Jonathan
> > > 
> > The configuration read-back feature is being maintained from the
> > original version of this driver. Before adding the device tree entry,
> > there was no way to change this setting other than debugfs raw access
> > to the SPI interface, and there is still no access to the returned
> > configuration data should the feature be enabled. I would be willing
> > to remove the feature altogether, but wanted to tread softly on
> > existing features.
> Ah. Makes sense.  My gut feeling is to drop it.
> 
> Anyone at Analog have a view on this?
> 
> Thanks,
> 
> Jonathan
> 

Hi, 

It is not obvious to me why this feature would be useful. I would not add
it.

Regards,
-Stefan

> > 
> > 
> > Adam
> > > 
> > > > 
> > > > 
> > > > Signed-off-by: Adam Michaelis <adam.michaelis@rockwellcollins.com>
> > > > ---
> > > >       V2: Add some defines to reduce use of magic numbers.
> > > > ---
> > > >  drivers/iio/adc/ad7949.c | 12 +++++++++++-
> > > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > > > index afc1361af5fb..7820e1097787 100644
> > > > --- a/drivers/iio/adc/ad7949.c
> > > > +++ b/drivers/iio/adc/ad7949.c
> > > > @@ -69,6 +69,7 @@ struct ad7949_adc_spec {
> > > >   * @iio_dev: reference to iio structure
> > > >   * @spi: reference to spi structure
> > > >   * @ref_sel: selected reference voltage source
> > > > + * @cfg_readback: whether reads will include configuration data
> > > >   * @resolution: resolution of the chip
> > > >   * @cfg: copy of the configuration register
> > > >   * @current_channel: current channel in use
> > > > @@ -80,6 +81,7 @@ struct ad7949_adc_chip {
> > > >       struct iio_dev *indio_dev;
> > > >       struct spi_device *spi;
> > > >       enum ad7949_ref_sel ref_sel;
> > > > +     bool cfg_readback;
> > > >       u8 resolution;
> > > >       u16 cfg;
> > > >       unsigned int current_channel;
> > > > @@ -283,7 +285,11 @@ static int ad7949_spi_init(struct
> > > > ad7949_adc_chip *ad7949_adc)
> > > >                       AD7949_CFG_REF_SEL_MASK;
> > > >       adc_config |= (AD7949_CFG_SEQ_DISABLED <<
> > > > AD7949_CFG_SEQ_SHIFT) &
> > > >                       AD7949_CFG_SEQ_MASK;
> > > > -     adc_config |= AD7949_CFG_READBACK_DIS;
> > > > +
> > > > +     if (ad7949_adc->cfg_readback)
> > > > +             adc_config |= AD7949_CFG_READBACK_EN;
> > > > +     else
> > > > +             adc_config |= AD7949_CFG_READBACK_DIS;
> > > > 
> > > >       ret = ad7949_spi_write_cfg(ad7949_adc,
> > > >                       adc_config,
> > > > @@ -331,6 +337,10 @@ static int ad7949_spi_probe(struct spi_device
> > > > *spi)
> > > >       indio_dev->num_channels = spec->num_channels;
> > > >       ad7949_adc->resolution = spec->resolution;
> > > > 
> > > > +     ad7949_adc->cfg_readback = of_property_read_bool(
> > > > +                     ad7949_adc->indio_dev->dev.of_node,
> > > > +                     "adi,cfg-readback");
> > > > +
> > > >       ret = of_property_read_u32(ad7949_adc->indio_dev-
> > > > >dev.of_node,
> > > >                       "adi,reference-select",
> > > >                       &temp);
> > --