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    Subject[PATCH 4.2 024/120] iio: Add inverse unit conversion macros
    Date
    4.2-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Lars-Peter Clausen <lars@metafoo.de>

    commit c689a923c867eac40ed3826c1d9328edea8b6bc7 upstream.

    Add inverse unit conversion macro to convert from standard IIO units to
    units that might be used by some devices.

    Those are useful in combination with scale factors that are specified as
    IIO_VAL_FRACTIONAL. Typically the denominator for those specifications will
    contain the maximum raw value the sensor will generate and the numerator
    the value it maps to in a specific unit. Sometimes datasheets specify those
    in different units than the standard IIO units (e.g. degree/s instead of
    rad/s) and so we need to do a unit conversion.

    >From a mathematical point of view it does not make a difference whether we
    apply the unit conversion to the numerator or the inverse unit conversion
    to the denominator since (x / y) / z = x / (y * z). But as the denominator
    is typically a larger value and we are rounding both the numerator and
    denominator to integer values using the later method gives us a better
    precision (E.g. the relative error is smaller if we round 8000.3 to 8000
    rather than rounding 8.3 to 8).

    This is where in inverse unit conversion macros will be used.

    Marked for stable as used by some upcoming fixes.

    Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
    Signed-off-by: Jonathan Cameron <jic23@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/linux/iio/iio.h | 17 +++++++++++++++++
    1 file changed, 17 insertions(+)

    --- a/include/linux/iio/iio.h
    +++ b/include/linux/iio/iio.h
    @@ -645,6 +645,15 @@ int iio_str_to_fixpoint(const char *str,
    #define IIO_DEGREE_TO_RAD(deg) (((deg) * 314159ULL + 9000000ULL) / 18000000ULL)

    /**
    + * IIO_RAD_TO_DEGREE() - Convert rad to degree
    + * @rad: A value in rad
    + *
    + * Returns the given value converted from rad to degree
    + */
    +#define IIO_RAD_TO_DEGREE(rad) \
    + (((rad) * 18000000ULL + 314159ULL / 2) / 314159ULL)
    +
    +/**
    * IIO_G_TO_M_S_2() - Convert g to meter / second**2
    * @g: A value in g
    *
    @@ -652,4 +661,12 @@ int iio_str_to_fixpoint(const char *str,
    */
    #define IIO_G_TO_M_S_2(g) ((g) * 980665ULL / 100000ULL)

    +/**
    + * IIO_M_S_2_TO_G() - Convert meter / second**2 to g
    + * @ms2: A value in meter / second**2
    + *
    + * Returns the given value converted from meter / second**2 to g
    + */
    +#define IIO_M_S_2_TO_G(ms2) (((ms2) * 100000ULL + 980665ULL / 2) / 980665ULL)
    +
    #endif /* _INDUSTRIAL_IO_H_ */



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