linux-kernel - Re: [PATCH v2] iio: adc: ad7124: fix sample rate for multi-channel use

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Message-ID: <aLnYk6RPePeACmex@smile.fi.intel.com>
Date: Thu, 4 Sep 2025 21:21:07 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: David Lechner <dlechner@...libre.com>
Cc: Michael Hennerich <Michael.Hennerich@...log.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Nuno Sá <nuno.sa@...log.com>,
	Andy Shevchenko <andy@...nel.org>, linux-iio@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: adc: ad7124: fix sample rate for multi-channel
 use

On Thu, Sep 04, 2025 at 11:19:56AM -0500, David Lechner wrote:
> Change how the FS[10:0] field of the FILTER register is calculated to
> get consistent sample rates when only one channel is enabled vs when
> multiple channels are enabled in a buffered read.
> 
> By default, the AD7124 allows larger sampling frequencies when only one
> channel is enabled. It assumes that you will discard the first sample or
> so to allow for settling time and then no additional settling time is
> needed between samples because there is no multiplexing due to only one
> channel being enabled. The conversion formula to convert between the
> sampling frequency and the FS[10:0] field is:
> 
>     fADC = fCLK / (FS[10:0] x 32)
> 
> which is what the driver has been using.
> 
> On the other hand, when multiple channels are enabled, there is
> additional settling time needed when switching between channels so the
> calculation to convert between becomes:
> 
>     fADC = fCLK / (FS[10:0] x 32 x (4 + AVG - 1))
> 
> where AVG depends on the filter type selected and the power mode.
> 
> The FILTER register has a SINGLE_CYCLE bit that can be set to force the
> single channel case to use the same timing as the multi-channel case.
> 
> Before this change, the first formula was always used, so if all of the
> in_voltageY_sampling_frequency attributes were set to 10 Hz, then doing
> a buffered read with 1 channel enabled would result in the requested
> sampling frequency of 10 Hz. But when more than one channel was
> enabled, the actual sampling frequency would be 2.5 Hz per channel,
> which is 1/4 of the requested frequency.
> 
> After this change, the SINGLE_CYCLE flag is now always enabled and the
> multi-channel formula is now always used. This causes the sampling
> frequency to be consistent regardless of the number of channels enabled.
> 
> Technically, the sincx+sinc1 filter modes can't currently be selected
> so there is some temporarily dead code in ad7124_get_avg() until filter
> support is added.
> 
> The AD7124_FILTER_FS define is moved while we are touching this to
> keep the bit fields in descending order to be consistent with the rest
> of the file.

...

>  	tmp = FIELD_PREP(AD7124_FILTER_FILTER, cfg->filter_type) |
> +		AD7124_FILTER_SINGLE_CYCLE |
>  		FIELD_PREP(AD7124_FILTER_FS, cfg->odr_sel_bits);

Seems to me that this is not indented correctly, with that in mind I would

	tmp = FIELD_PREP(AD7124_FILTER_FILTER, cfg->filter_type) |
	      FIELD_PREP(AD7124_FILTER_FS, cfg->odr_sel_bits) |
	      AD7124_FILTER_SINGLE_CYCLE;


-- 
With Best Regards,
Andy Shevchenko