linux-kernel - Re: [PATCH v11 11/11] iio: adc: ad7768-1: add low pass -3dB cutoff attribute

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Message-ID: <aEwd4cS7j0Vvypg8@smile.fi.intel.com>
Date: Fri, 13 Jun 2025 15:47:29 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Jonathan Santos <Jonathan.Santos@...log.com>
Cc: linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
	andy@...nel.org, nuno.sa@...log.com, Michael.Hennerich@...log.com,
	marcelo.schmitt@...log.com, jic23@...nel.org, robh@...nel.org,
	krzk+dt@...nel.org, conor+dt@...nel.org, marcelo.schmitt1@...il.com,
	linus.walleij@...aro.org, brgl@...ev.pl, lgirdwood@...il.com,
	broonie@...nel.org, jonath4nns@...il.com, dlechner@...libre.com
Subject: Re: [PATCH v11 11/11] iio: adc: ad7768-1: add low pass -3dB cutoff
 attribute

On Wed, Jun 11, 2025 at 08:52:03AM -0300, 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.

> +/* -3dB cutoff frequency multipliers (relative to ODR) for each filter type. */
> +static const int ad7768_filter_3db_odr_multiplier[] = {
> +	[AD7768_FILTER_SINC5] = 204,		/* 0.204 */
> +	[AD7768_FILTER_SINC3] = 262,		/* 0.2617 */
> +	[AD7768_FILTER_SINC3_REJ60] = 262,	/* 0.2617 */
> +	[AD7768_FILTER_WIDEBAND] = 433,		/* 0.433 */

Just to be sure, is it 0.433 or 0.4333(3) actually? Sometimes datasheets have
rounding that even may lead to problems (see TSC issues for some of the Intel
CPUs in the past). That's behind my question.

> +};

...

> +	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);

MILLI? KILO/ MICRO/MILLI? ...?

-- 
With Best Regards,
Andy Shevchenko