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

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Date:   Fri, 27 Oct 2023 15:15:07 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Javier Carrasco <javier.carrasco@...fvision.net>
Cc:     Gerald Loacker <gerald.loacker@...fvision.net>,
        Lars-Peter Clausen <lars@...afoo.de>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: tmag5273: fix temperature offset

On Mon, 23 Oct 2023 11:50:02 +0200
Javier Carrasco <javier.carrasco@...fvision.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

So base units for temperature are milli degrees celsius. 
T = 1000 * (25 + (adc - 17508) / 60.1)

T =  1000/60.1 * (25 * 60.1 + adc - 17508)
T = 10000/601 * (-16005.5 + adc)  
So I think the maths is a little off..
> 
> 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 16605.5, which has been truncated to 16005 to
Interesting - the truncated value you have looks good to be but that's
not matching with the resulting offset or the value below...

> 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>
> ---
>  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..d22ca39007b6 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 = -16605;
>  			return IIO_VAL_INT;
>  		default:
>  			return -EINVAL;
> 
> ---
> base-commit: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1
> change-id: 20231023-topic-tmag5273x1_temp_offset-17774cbce961
> 
> Best regards,