patch "iio: cros_ec: Fix the maths for gyro scale calculation" added to staging-linus

This is a note to let you know that I've just added the patch titled

iio: cros_ec: Fix the maths for gyro scale calculation

to my staging git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git
in the staging-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 3d02d7082e5823598090530c3988a35f69689943 Mon Sep 17 00:00:00 2001
From: Gwendal Grignou <[email protected]>
Date: Wed, 13 Mar 2019 12:40:02 +0100
Subject: iio: cros_ec: Fix the maths for gyro scale calculation

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

---
 drivers/iio/common/cros_ec_sensors/cros_ec_sensors.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

index 89cb0066a6e0..8d76afb87d87 100644
@@ -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:
 			/*

2.21.0