linux-kernel - [PATCH v2] iio: cros_ec: Fix the maths for gyro scale calculation

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Message-Id: <20190307151041.10113-1-enric.balletbo@collabora.com>
Date:   Thu,  7 Mar 2019 16:10:41 +0100
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     Jonathan Cameron <jic23@...nel.org>, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Gwendal Grignou <gwendal@...omium.org>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Guenter Roeck <groeck@...omium.org>,
        Benson Leung <bleung@...omium.org>,
        Lars-Peter Clausen <lars@...afoo.de>, kernel@...labora.com,
        Hartmut Knaack <knaack.h@....de>
Subject: [PATCH v2] iio: cros_ec: Fix the maths for gyro scale calculation

From: Gwendal Grignou <gwendal@...omium.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@...omium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
---

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..401b22e08d48 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 * 3141592653,
+					180 << (CROS_EC_SENSOR_BITS - 1));
+			ret = IIO_VAL_INT_PLUS_NANO;
 			break;
 		case MOTIONSENSE_TYPE_MAG:
 			/*
-- 
2.20.1