Re: [PATCH 3/3 v4] iio: magnetometer: ak8974: Provide scaling

[Jonathan Cameron <jic23@kernel.org>]

On Mon, 20 Apr 2020 22:40:22 +0200
Linus Walleij <linus.walleij@linaro.org> wrote:

> The manual for the HSCDTD008A gives us a scaling for the
> three axis as +/- 2.4mT (24 Gauss) per axis.
> 
> The manual for the AMI305 and AMI306 gives us a scaling
> for the three axis as +/- 12 Gauss per axis.
> 
> Tests with the HSCDTD008A sensor, cat the raw values:
> $ cat in_magn_*_raw
> raw
> 45
> 189
> -19
> 
> The scaling factor in in_magn_*_scale is 0.001464843,
> which gives:
> 0.065 Gauss
> 0.277 Gauss
> -0.027 Gauss
> 
> The earths magnetic field is in the range of 0.25 to 0.65
> Gauss on the surface according to Wikipedia, so these
> seem like reasonable values.
> 
> Again we are guessing that the AK8974 has a 12 bit ADC,
> based on the similarity with AMI305 and AMI306.
> 
> Cc: Nick Reitemeyer <nick.reitemeyer@web.de>
> Cc: Stephan Gerhold <stephan@gerhold.net>
> Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Applied, but they'll sit in my testing branch for a while yet
most likely so can rebase (add tags / drop them etc) if others
still wish to comment.

Thanks,

Jonathan

> ---
> ChangeLog v3->v4:
> - Do not multiply the range with 2, instead divide by
>   two in divisor and dividend. (Thus it becomes 2^11
>   and 2^14 respectively.)
> - Use the IIO_VAL_FRACTIONAL_LOG2 trick and pass 2^11
>   or 2^14 as range.
> ChangeLog v2->v3:
> - Scale the 2.4mT/24Gauss to 15 bits for the HSCDTD008A.
> - Scale the 12 Gauss to 12 bits for the AMI305/AMI306
> - Use 12 bits for the other variants.
> - Return directly in the raw read function.
> ChangeLog v1->v2:
> - Split out the measurement refactoring.
> ---
>  drivers/iio/magnetometer/ak8974.c | 45 ++++++++++++++++++++++++++++++-
>  1 file changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/magnetometer/ak8974.c b/drivers/iio/magnetometer/ak8974.c
> index fb6d84b79a7a..2d500be9da40 100644
> --- a/drivers/iio/magnetometer/ak8974.c
> +++ b/drivers/iio/magnetometer/ak8974.c
> @@ -606,6 +606,48 @@ static int ak8974_read_raw(struct iio_dev *indio_dev,
>  		if (ret)
>  			return ret;
>  		return IIO_VAL_INT;
> +	case IIO_CHAN_INFO_SCALE:
> +		switch (ak8974->variant) {
> +		case AK8974_WHOAMI_VALUE_AMI306:
> +		case AK8974_WHOAMI_VALUE_AMI305:
> +			/*
> +			 * The datasheet for AMI305 and AMI306, page 6
> +			 * specifies the range of the sensor to be
> +			 * +/- 12 Gauss.
> +			 */
> +			*val = 12;
> +			/*
> +			 * 12 bits are used, +/- 2^11
> +			 * [ -2048 .. 2047 ] (manual page 20)
> +			 * [ 0xf800 .. 0x07ff ]
> +			 */
> +			*val2 = 11;
> +			return IIO_VAL_FRACTIONAL_LOG2;
> +		case AK8974_WHOAMI_VALUE_HSCDTD008A:
> +			/*
> +			 * The datasheet for HSCDTF008A, page 3 specifies the
> +			 * range of the sensor as +/- 2.4 mT per axis, which
> +			 * corresponds to +/- 2400 uT = +/- 24 Gauss.
> +			 */
> +			*val = 24;
> +			/*
> +			 * 15 bits are used (set up in CTRL4), +/- 2^14
> +			 * [ -16384 .. 16383 ] (manual page 24)
> +			 * [ 0xc000 .. 0x3fff ]
> +			 */
> +			*val2 = 14;
> +			return IIO_VAL_FRACTIONAL_LOG2;
> +		default:
> +			/* GUESSING +/- 12 Gauss */
> +			*val = 12;
> +			/* GUESSING 12 bits ADC +/- 2^11 */
> +			*val2 = 11;
> +			return IIO_VAL_FRACTIONAL_LOG2;
> +		}
> +		break;
> +	default:
> +		/* Unknown request */
> +		break;
>  	}
>  
>  	return -EINVAL;
> @@ -670,7 +712,8 @@ static const struct iio_chan_spec_ext_info ak8974_ext_info[] = {
>  		.type = IIO_MAGN,					\
>  		.modified = 1,						\
>  		.channel2 = IIO_MOD_##axis,				\
> -		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
> +		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |		\
> +			BIT(IIO_CHAN_INFO_SCALE),			\
>  		.ext_info = ak8974_ext_info,				\
>  		.address = index,					\
>  		.scan_index = index,					\