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

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Message-ID: <aFGVAWi7CZAy0E8k@JSANTO12-L01.ad.analog.com>
Date: Tue, 17 Jun 2025 13:17:05 -0300
From: Jonathan Santos <jonath4nns@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Jonathan Santos <Jonathan.Santos@...log.com>, 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, dlechner@...libre.com
Subject: Re: [PATCH v11 11/11] iio: adc: ad7768-1: add low pass -3dB cutoff
 attribute

On 06/13, Andy Shevchenko wrote:
> 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.
> 

Every reference I have specifies it as 0.433, so I believe that is it.

> > +};
> 
> ...
> 
> > +	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? ...?
> 

Yes, MILLI.

Thanks,
Jonathan S.

> -- 
> With Best Regards,
> Andy Shevchenko
> 
>