[PATCH 3.12 42/82] iio: ak8975: Fix calculation formula for convert micro tesla to gauss unit

Greg Kroah-Hartman
2014-02-20T23:52:29Z

From: Beomho Seo <beomho.seo@samsung.com>

commit bef44abccb2677e8d16e50b75316d4fd1061be81 upstream.

This effects the reported scale of the raw values, and thus userspace
applications that use this value.

One micro tesla equal 0.01 gauss. So I have fixed calculation formula And add RAW_TO_GAUSS macro.
ASA is in the range of 0 to 255. If multiply 0.003, calculation result(in_magn_[*]_scale) is
always 0. So multiply 3000 and return and IIO_VAL_INT_PLUS_MICRO.
As a result, read_raw call back function return accurate scale value.

Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/iio/magnetometer/ak8975.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

--- a/drivers/iio/magnetometer/ak8975.c
+++ b/drivers/iio/magnetometer/ak8975.c
@@ -85,6 +85,7 @@
#define AK8975_MAX_CONVERSION_TIMEOUT 500
#define AK8975_CONVERSION_DONE_POLL_TIME 10
#define AK8975_DATA_READY_TIMEOUT ((100*HZ)/1000)
+#define RAW_TO_GAUSS(asa) ((((asa) + 128) * 3000) / 256)

/*
* Per-instance context data for the device.
@@ -265,15 +266,15 @@ static int ak8975_setup(struct i2c_clien
*
* Since 1uT = 100 gauss, our final scale factor becomes:
*
- * Hadj = H * ((ASA + 128) / 256) * 3/10 * 100
- * Hadj = H * ((ASA + 128) * 30 / 256
+ * Hadj = H * ((ASA + 128) / 256) * 3/10 * 1/100
+ * Hadj = H * ((ASA + 128) * 0.003) / 256
*
* Since ASA doesn't change, we cache the resultant scale factor into the
* device context in ak8975_setup().
*/
- data->raw_to_gauss[0] = ((data->asa[0] + 128) * 30) >> 8;
- data->raw_to_gauss[1] = ((data->asa[1] + 128) * 30) >> 8;
- data->raw_to_gauss[2] = ((data->asa[2] + 128) * 30) >> 8;
+ data->raw_to_gauss[0] = RAW_TO_GAUSS(data->asa[0]);
+ data->raw_to_gauss[1] = RAW_TO_GAUSS(data->asa[1]);
+ data->raw_to_gauss[2] = RAW_TO_GAUSS(data->asa[2]);

return 0;
}
@@ -428,8 +429,9 @@ static int ak8975_read_raw(struct iio_de
case IIO_CHAN_INFO_RAW:
return ak8975_read_axis(indio_dev, chan->address, val);
case IIO_CHAN_INFO_SCALE:
- *val = data->raw_to_gauss[chan->address];
- return IIO_VAL_INT;
+ *val = 0;
+ *val2 = data->raw_to_gauss[chan->address];
+ return IIO_VAL_INT_PLUS_MICRO;
}
return -EINVAL;
}