Re: [PATCH v3 2/5] iio: add processed write API

[Romain Gantois <romain.gantois@bootlin.com>]

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Hello Andy,

On Thursday, 6 November 2025 17:07:18 CET Andy Shevchenko wrote:
> On Thu, Nov 06, 2025 at 03:11:47PM +0100, Romain Gantois wrote:
> > Add a function to allow IIO consumers to write a processed value to a
> > channel.
> 
> ...
> 
> > +int iio_divide_by_value(int *result, s64 numerator,
> > +			unsigned int type, int val, int val2)
> > +{
> > +	s64 tmp_num, tmp_den;
> > +
> > +	switch (type) {
> > +	case IIO_VAL_INT:
> > +		tmp_num = numerator;
> > +		tmp_den = val;
> > +		break;
> > +	case IIO_VAL_INT_PLUS_MICRO:
> > +	case IIO_VAL_INT_PLUS_NANO:
> > +		switch (type) {
> > +		case IIO_VAL_INT_PLUS_MICRO:
> > +			tmp_num = MICRO;
> > +			tmp_den = MICRO;
> > +			break;
> > +
> > +		case IIO_VAL_INT_PLUS_NANO:
> > +			tmp_num = NANO;
> > +			tmp_den = NANO;
> > +			break;
> > +		}
> > 
> > +		tmp_num *= numerator;
> > +		tmp_den = (s64)abs(val) * tmp_den + (s64)abs(val2);
> 
> Here is a subtle bug. The problematic piece is abs(). See
> https://lore.kernel.org/r/20251106152051.2361551-1-andriy.shevchenko@linux.i
> ntel.com for the answer.

Oh wow, that's a nasty one indeed.

> > +EXPORT_SYMBOL_GPL(iio_write_channel_processed_scale);
> 
> Can we start using namespaced exports?
> 

Sounds good, but won't it look strange to have only 
iio_write_channel_processed_scale() use a namespaced export?

...
> > +/**
> > + * iio_write_channel_processed_scale() - scale and write processed value
> > to a given channel + * @chan:		The channel being queried.
> > + * @val:		Value to write.
> > + * @scale:		Processed value is divided by this scale factor 
during the
> > conversion. + *
> > + * This function writes a processed value to a channel. A processed value
> > means + * that this value will have the correct unit and not some device
> > internal + * representation. If the device does not support writing a
> > processed value, the + * function will query the channel's scale and
> > offset and write an appropriately + * transformed raw value.
> > 
> > + * Context: May sleep.
> 
> The above kernel-doc doesn't have this!
> 
> > + * Return: an error code or 0.
> 
> Be consistent with the existing code, and even in your own change.
> 
> ("Return" section name, "Context" section presence, etc.)

I'll match the "Return" section with what I used for iio_divide_by_value() 
then, since the format used for iio_read_channel_label() is broken.

> 
> Use Perl (original) kernel-doc for now, the Python has a significant
> regression (the fix is pending to go to Linus' branch).

Thanks for the heads up.

-- 
Romain Gantois, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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