linux-kernel - Re: [PATCH v2 16/16] iio: adc: ad7768-1: add low pass -3dB cutoff attribute

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Message-ID: <a429dcb8-04e2-4e6b-a648-feb32757f887@baylibre.com>
Date: Mon, 27 Jan 2025 19:27:43 -0600
From: David Lechner <dlechner@...libre.com>
To: Jonathan Santos <Jonathan.Santos@...log.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: lars@...afoo.de, Michael.Hennerich@...log.com,
 marcelo.schmitt@...log.com, jic23@...nel.org, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, jonath4nns@...il.com,
 marcelo.schmitt1@...il.com
Subject: Re: [PATCH v2 16/16] iio: adc: ad7768-1: add low pass -3dB cutoff
 attribute

On 1/27/25 9:14 AM, Jonathan Santos wrote:
> Ad7768-1 has a different -3db frequency multiplier depending on
> the filter type configured. The cutoff frequency also varies according
> to the current ODR.
> 
> Add a readonly low pass -3dB frequency cutoff attribute to clarify to
> the user which bandwidth is being allowed depending on the filter
> configurations.
> 
> Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> ---
> v2 Changes:
> * New patch in v2.
> ---
>  drivers/iio/adc/ad7768-1.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7768-1.c b/drivers/iio/adc/ad7768-1.c
> index 6d0b430a8d54..daf91ef6f77b 100644
> --- a/drivers/iio/adc/ad7768-1.c
> +++ b/drivers/iio/adc/ad7768-1.c
> @@ -149,6 +149,18 @@ enum ad7768_scan_type {
>  	AD7768_SCAN_TYPE_HIGH_SPEED,
>  };
>  
> +/*
> + * -3dB cutoff frequency multipliers (relative to ODR) for
> + * each filter type. Values are multiplied by 1000.
> + */
> +static const int ad7768_filter_3db_odr_multiplier[] = {
> +	[SINC5] = 204,
> +	[SINC5_DEC_X8] = 204,
> +	[SINC5_DEC_X16] = 204,
> +	[SINC3] = 261,
> +	[WIDEBAND] = 433,
> +};
> +
>  static const int ad7768_mclk_div_rates[4] = {
>  	16, 8, 4, 2,
>  };
> @@ -202,7 +214,8 @@ static const struct iio_chan_spec ad7768_channels[] = {
>  		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),
>  		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),
>  		.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ) |
> -					   BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO),
> +					   BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO) |
> +					   BIT(IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY),
>  		.ext_info = ad7768_ext_info,
>  		.indexed = 1,
>  		.channel = 0,
> @@ -802,7 +815,7 @@ static int ad7768_read_raw(struct iio_dev *indio_dev,
>  {
>  	struct ad7768_state *st = iio_priv(indio_dev);
>  	const struct iio_scan_type *scan_type;
> -	int scale_uv, ret;
> +	int scale_uv, ret, temp;
>  
>  	scan_type = iio_get_current_scan_type(indio_dev, chan);
>  	if (IS_ERR(scan_type))
> @@ -842,6 +855,12 @@ static int ad7768_read_raw(struct iio_dev *indio_dev,
>  	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
>  		*val = st->oversampling_ratio;
>  
> +		return IIO_VAL_INT;
> +
> +	case IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY:
> +		temp = st->samp_freq * ad7768_filter_3db_odr_multiplier[st->filter_type];
> +		*val = DIV_ROUND_CLOSEST(temp, 1000);
> +
>  		return IIO_VAL_INT;
>  	}
>  

If this is read-only, do we actually need it? It looks like the attribute is
going to return 1 of 3 values that exactly matches the filter_type attribute,
so this is not really giving any additional info.