Re: [RFC 1/4] iio: Introduce a new fractional value type

[Jonathan Cameron <jic23@kernel.org>]

On 08/16/2012 10:18 AM, Lars-Peter Clausen wrote:
> This series is just a request for comments right now and at this point it is
> barley tested.
> 
> Currently IIO uses a decimal fixed point representations for real type numbers.
> This patch introduces a new representation for rational type numbers. The number
> will be expressed by specifying a numerator and denominator. For converting a
> raw value to a processed value multiply it by the numerator and divide it by the
> denominator.
> 
> The reasoning for introducing this new type is that for a lot of devices the
> scale can be represented easily by a fractional number, but it is not possible
> to represent it as fixed point number without rounding.  E.g. for a simple DAC
> the scale is often the reference voltage divided by the number of possible
> values (Usually 2**n_bits - 1). Each driver currently implements the conversion
> of this fraction to a fixed point number on its own.
> 
> Also when it comes to the in-kernel interface this allows to directly use the
> fractional factors to convert a raw value to a processed value. This should on
> one hand require less instructions and on the other hand increase the
> precession.
Hmm.. Is the range of this going to be large enough?  Or do we need to have
variants of the fractional value?

Otherwise this looks like a reasonable idea to me. Few comments inline.
> 
> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
> ---
>  drivers/iio/industrialio-core.c |    9 +++++++++
>  include/linux/iio/types.h       |    1 +
>  2 files changed, 10 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index 2ec266e..69e60c5 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -365,6 +365,7 @@ static ssize_t iio_read_channel_info(struct device *dev,
>  {
>  	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
>  	struct iio_dev_attr *this_attr = to_iio_dev_attr(attr);
> +	unsigned long long tmp;
>  	int val, val2;
>  	bool scale_db = false;
>  	int ret = indio_dev->info->read_raw(indio_dev, this_attr->c,
> @@ -390,6 +391,14 @@ static ssize_t iio_read_channel_info(struct device *dev,
>  			return sprintf(buf, "-%d.%09u\n", val, -val2);
>  		else
>  			return sprintf(buf, "%d.%09u\n", val, val2);
> +	case IIO_VAL_FRACTIONAL:
> +		tmp = div_s64((s64)val * 1000000000LL, val2);
> +		val2 = do_div(tmp, 1000000000LL);
> +		val = tmp;
> +		return sprintf(buf, "%d.%09u\n", val, val2);
> +
> +		/* Or maybe we could do something like this:
> +		return sprintf(buf, "%d/%d\n", val, val2); */
Not sure on this one...  Will wait for anyone elses comments.
So far we have pretty printed eveything the same. Hence this will
add a fair bit of complexity to userspace code (and will cause
issues for any existing code!)

>  	default:
>  		return 0;
>  	}
> diff --git a/include/linux/iio/types.h b/include/linux/iio/types.h
> index 44e3977..8ef5fde 100644
> --- a/include/linux/iio/types.h
> +++ b/include/linux/iio/types.h
> @@ -57,5 +57,6 @@ enum iio_modifier {
>  #define IIO_VAL_INT_PLUS_MICRO 2
>  #define IIO_VAL_INT_PLUS_NANO 3
>  #define IIO_VAL_INT_PLUS_MICRO_DB 4
> +#define IIO_VAL_FRACTIONAL 5
I wonder if it is worth leaving a gap here for
any more of the int plus form?
>  
>  #endif /* _IIO_TYPES_H_ */
>