[PATCH v2] iio: tmag5273: fix temperature offset

[PATCH v2] iio: tmag5273: fix temperature offset

[Javier Carrasco]

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@wolfvision.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@wolfvision.net>

Re: [PATCH v2] iio: tmag5273: fix temperature offset

[Jonathan Cameron]

On Tue, 21 Nov 2023 06:48:39 +0100
Javier Carrasco <javier.carrasco@wolfvision.net> wrote:

> 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@wolfvision.net>
Applied to the fixes-togreg branch of iio.git and marked for stable. Thanks,

Jonathan

> ---
> 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,