linux-kernel - Re: [PATCH v8 12/14] iio: afe: rescale: add temperature transducers

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Date:   Sat, 28 Aug 2021 22:33:25 -0400
From:   Liam Beguin <liambeguin@...il.com>
To:     Peter Rosin <peda@...ntia.se>
Cc:     jic23@...nel.org, lars@...afoo.de, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        robh+dt@...nel.org
Subject: Re: [PATCH v8 12/14] iio: afe: rescale: add temperature transducers

On Thu, Aug 26, 2021 at 10:56:11AM +0200, Peter Rosin wrote:
> On 2021-08-20 21:17, Liam Beguin wrote:
> > From: Liam Beguin <lvb@...hos.com>
> > 
> > A temperature transducer is a device that converts a thermal quantity
> > into any other physical quantity. This patch add support for temperature
> > to voltage (like the LTC2997) and temperature to current (like the
> > AD590) linear transducers.
> > In both cases these are assumed to be connected to a voltage ADC.
> > 
> > Signed-off-by: Liam Beguin <lvb@...hos.com>
> > ---
> >  drivers/iio/afe/iio-rescale.c | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> > 
> > diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> > index 8cdcb6ffb563..12de44058bea 100644
> > --- a/drivers/iio/afe/iio-rescale.c
> > +++ b/drivers/iio/afe/iio-rescale.c
> > @@ -427,11 +427,38 @@ static int rescale_temp_sense_rtd_props(struct device *dev,
> >  	return 0;
> >  }
> >  
> > +static int rescale_temp_transducer_props(struct device *dev,
> > +					 struct rescale *rescale)
> > +{
> > +	s32 offset = 0;
> > +	s32 sense = 1;
> > +	s32 alpha;
> > +	s64 tmp;
> > +	int ret;
> > +
> > +	device_property_read_u32(dev, "sense-offset-millicelsius", &offset);
> > +	device_property_read_u32(dev, "sense-resistor-ohms", &sense);
> > +	ret = device_property_read_u32(dev, "alpha-ppm-per-celsius", &alpha);
> > +	if (ret) {
> > +		dev_err(dev, "failed to read alpha-ppm-per-celsius: %d\n", ret);
> > +		return ret;
> > +	}
> > +
> > +	rescale->numerator = 1000000;
> > +	rescale->denominator = alpha * sense;
> > +
> > +	tmp = (s64)offset * (s64)alpha * (s64)sense;
> > +	rescale->offset = div_s64(tmp, (s32)1000000);
> 
> Error: Too many casts :-)

Oof! You're right, that doesn't look too good...

I guess I haven't looked at this part of the series for a few
revisions, my bad.

> 
> I think it would make sense to lose tmp, and just spell it out in one
> statement?
> 
> 	rescale->offset = div_s64((s64)offset * rescale->denominator,
> 				  rescale->numerator);
> 
> Because you are prepping the offset so that it will survive a later rescaler
> multiplication, and all the "random" multiplications and divisions don't
> make that very clear.
> 

Yeah, I agree! The way I had it spelled out wasn't much more readable.
I'll use your suggestion instead.

Thanks,
Liam

> Cheers,
> Peter
> 
> > +
> > +	return 0;
> > +}
> > +
> >  enum rescale_variant {
> >  	CURRENT_SENSE_AMPLIFIER,
> >  	CURRENT_SENSE_SHUNT,
> >  	VOLTAGE_DIVIDER,
> >  	TEMP_SENSE_RTD,
> > +	TEMP_TRANSDUCER,
> >  };
> >  
> >  static const struct rescale_cfg rescale_cfg[] = {
> > @@ -451,6 +478,10 @@ static const struct rescale_cfg rescale_cfg[] = {
> >  		.type = IIO_TEMP,
> >  		.props = rescale_temp_sense_rtd_props,
> >  	},
> > +	[TEMP_TRANSDUCER] = {
> > +		.type = IIO_TEMP,
> > +		.props = rescale_temp_transducer_props,
> > +	},
> >  };
> >  
> >  static const struct of_device_id rescale_match[] = {
> > @@ -462,6 +493,8 @@ static const struct of_device_id rescale_match[] = {
> >  	  .data = &rescale_cfg[VOLTAGE_DIVIDER], },
> >  	{ .compatible = "temperature-sense-rtd",
> >  	  .data = &rescale_cfg[TEMP_SENSE_RTD], },
> > +	{ .compatible = "temperature-transducer",
> > +	  .data = &rescale_cfg[TEMP_TRANSDUCER], },
> >  	{ /* sentinel */ }
> >  };
> >  MODULE_DEVICE_TABLE(of, rescale_match);
> >