Re: [PATCH v2 2/2] iio: temperature: add driver support for ti tmp117

[Lars-Peter Clausen]

On 4/3/21 4:58 PM, Puranjay Mohan wrote:
> On Fri, Apr 2, 2021 at 1:43 PM Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> > On 4/1/21 11:16 AM, Puranjay Mohan wrote:
> > > TMP117 is a Digital temperature sensor with integrated NV memory.
> > >
> > > Add support for tmp117 driver in iio subsystem.
> > > Datasheet:-https://www.ti.com/lit/gpn/tmp117
> > >
> > > Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> > Nice and clean driver. Just some comments about the CALIBBIAS.
> >
> > > [...]
> > > +#define TMP117_RESOLUTION_10UC               78125
> > Isn't the unit here 100 uC?
> it is 7.8125 milli degrees_C so 78125 x 10^-4 milli degrees_C
> which is 78125 x 10^-4 x 10^3 micro degrees_C
> so it becomes 78125 x 10^-1 micro degrees_C = 78125 10_microdegrees_C.
> Did it in detail so I remember it in the future. I guess you thought
> it as 0.78125 millidegrees_C?
Ah, I get it, it is a tenth micro degree, not tens of micro degrees, 
sorry got confused.
> [...]
>
> > I think that would be quite unexpected behavior. The unit should be the
> > same. Here in the read function you can just return the register value.
> Okay, if you feel that would be right then I will do it.
Yea, I think reading and writing in different units would be a bit 
confusing.
> > Just make sure to properly sign extend like for the RAW property.
> >
> > > +             return IIO_VAL_INT_PLUS_MICRO;
> > > [...]
> > > +}
> > > +
> > > +static int tmp117_write_raw(struct iio_dev *indio_dev,
> > > +             struct iio_chan_spec const *channel, int val,
> > > +             int val2, long mask)
> > > +{
> > > +     struct tmp117_data *data = iio_priv(indio_dev);
> > > +     s16 off;
> > > +
> > > +     switch (mask) {
> > > +     case IIO_CHAN_INFO_CALIBBIAS:
> > > +             off = (s16)val;
> > This should have some input validation to avoid writing bogus values to
> > the register when the value is out of range. You can either reject out
> > of range values or clamp them into the valid range (using the clamp()
> > macro).
> the maximum value which this register takes is 0xffff, so it should
> get clamped automatically when casting to s16?
> I might be wrong here.
Casting will truncate the upper bits. So something like 0x12345 gets 
turned into 0x2345.
> > > +             return i2c_smbus_write_word_swapped(data->client,
> > > +                                             TMP117_REG_TEMP_OFFSET, off);
> > > +
> > > +     default:
> > > +             return -EINVAL;
> > > +     }
> > > +}
> > > +
> > [...]