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iio: cros_ec: Fix the maths for gyro scale calculation

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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: c14dca07a3 ("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>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
This commit is contained in:
Gwendal Grignou 2019-03-13 12:40:02 +01:00 committed by Jonathan Cameron
parent 2e4b88f739
commit 3d02d7082e
1 changed files with 4 additions and 3 deletions
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7
drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c
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@ -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:
/*