Re: [PATCH v3] iio: cros_ec: Fix the maths for gyro scale calculation

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 13 Mar 2019 12:40:02 +0100
Enric Balletbo i Serra <enric.balletbo@collabora.com> wrote:

> From: Gwendal Grignou <gwendal@chromium.org>
> 
> Calculation did not use IIO_DEGREE_TO_RAD and implemented a variant to
> avoid precision loss as we aim a nano value. The offset added to avoid
> rounding error, though, doesn't give us a close result to the expected
> value. E.g.
> 
> For 1000dps, the result should be:
> 
>     (1000 * pi ) / 180 >> 15 ~= 0.000532632218
> 
> But with current calculation we get
> 
>     $ cat scale
>     0.000547890
> 
> Fix the calculation by just doing the maths involved for a nano value
> 
>    val * pi * 10e12 / (180 * 2^15)
> 
> so we get a closer result.
> 
>     $ cat scale
>     0.000532632
> 
> Fixes: c14dca07a31d ("iio: cros_ec_sensors: add ChromeOS EC Contiguous Sensors driver")
> Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Applied to the fixes-togreg branch of iio.git and marked for stable.

Thanks,

Jonathan

> ---
> 
> Changes in v3:
> - Fix 'warning: this decimal constant is unsigned only in ISO C90' spotted by kbuild test robot.
> 
> Changes in v2:
> - Do the maths instead of play with dividers as pointed by Jonathan.
> 
>  drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c
> index 89cb0066a6e0..8d76afb87d87 100644
> --- a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c
> +++ b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c
> @@ -103,9 +103,10 @@ static int cros_ec_sensors_read(struct iio_dev *indio_dev,
>  			 * Do not use IIO_DEGREE_TO_RAD to avoid precision
>  			 * loss. Round to the nearest integer.
>  			 */
> -			*val = div_s64(val64 * 314159 + 9000000ULL, 1000);
> -			*val2 = 18000 << (CROS_EC_SENSOR_BITS - 1);
> -			ret = IIO_VAL_FRACTIONAL;
> +			*val = 0;
> +			*val2 = div_s64(val64 * 3141592653ULL,
> +					180 << (CROS_EC_SENSOR_BITS - 1));
> +			ret = IIO_VAL_INT_PLUS_NANO;
>  			break;
>  		case MOTIONSENSE_TYPE_MAG:
>  			/*