linux-kernel - [PATCH v2] iio: tmag5273: fix temperature offset

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Message-Id: <20231023-topic-tmag5273x1_temp_offset-v2-1-52ef3f3bd80a@wolfvision.net>
Date:   Fri, 27 Oct 2023 18:21:00 +0200
From:   Javier Carrasco <javier.carrasco@...fvision.net>
To:     Gerald Loacker <gerald.loacker@...fvision.net>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>
Cc:     linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Javier Carrasco <javier.carrasco@...fvision.net>
Subject: [PATCH v2] iio: tmag5273: fix temperature offset

The current offset has the scale already applied to it. The ABI
documentation defines the offset parameter as "offset to be added
to <type>[Y]_raw prior to scaling by <type>[Y]_scale in order to
obtain value in the <type> units as specified in <type>[Y]_raw
documentation"

The right value is obtained at 0 degrees Celsius by the formula provided
in the datasheet:

T = Tsens_t0 + (Tadc_t - Tadc_t0) / Tadc_res

where:
T = 0 degrees Celsius
Tsens_t0 (reference temperature) = 25 degrees Celsius
Tadc_t0 (16-bit format for Tsens_t0) = 17508
Tadc_res = 60.1 LSB/degree Celsius

The resulting offset is 16005.5, which has been truncated to 16005 to
provide an integer value with a precision loss smaller than the 1-LSB
measurement precision.

Fix the offset to apply its value prior to scaling.

Signed-off-by: Javier Carrasco <javier.carrasco@...fvision.net>
---
Changes in v2:
- Fixed typo in the offset value (16005 instead of 16605) (Jonathan Cameron)

- Link to v1: https://lore.kernel.org/r/20231023-topic-tmag5273x1_temp_offset-v1-1-983dca43292c@wolfvision.net
---
 drivers/iio/magnetometer/tmag5273.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/magnetometer/tmag5273.c b/drivers/iio/magnetometer/tmag5273.c
index c5e5c4ad681e..e8c4ca142d21 100644
--- a/drivers/iio/magnetometer/tmag5273.c
+++ b/drivers/iio/magnetometer/tmag5273.c
@@ -356,7 +356,7 @@ static int tmag5273_read_raw(struct iio_dev *indio_dev,
 	case IIO_CHAN_INFO_OFFSET:
 		switch (chan->type) {
 		case IIO_TEMP:
-			*val = -266314;
+			*val = -16005;
 			return IIO_VAL_INT;
 		default:
 			return -EINVAL;

---
base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
change-id: 20231023-topic-tmag5273x1_temp_offset-17774cbce961

Best regards,
-- 
Javier Carrasco <javier.carrasco@...fvision.net>