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

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 28 Aug 2025 11:42:28 -0500
David Lechner <dlechner@baylibre.com> 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, introducing the avg parameter is not needed at this time
> since we currently only support a single filter mode which always has an
> AVG value of 1. But it helps to show where the factor comes from in the
> datasheet and will be used in the future when supporting additional
> filter types.
> 
> 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.
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
> ---
> This is one of those unfortunate cases where we are fixing a bug but we
> risk breaking userspace that may be depending on the buggy behavior.
> 
> I intentionally didn't include a Fixes: tag for this reason.
> 
> Given some of the other shortcomings of this driver, like using an
> integer IIO_CHAN_INFO_SAMP_FREQ value when it really needs to allow a
> fractional values, it makes me hopeful that no one is caring too much
> about the exact value of the sampling frequency. So I'm more willing
> than I would normally be to take a risk on making this change.
> 
> I also have a plan to resolve things if we do find we broke someone and
> need to revert the change. We have a recent devicetree fix [1] for these
> chips that would allow us to detect "new" users using the correct DT
> bindings and "old" users using the buggy bindings. So we could use this
> as a way to also enable the old buggy sampling frequency behavior only
> for "old" users while allowing "new" users to get the correct behavior.

I'm not convinced this is a good plan as it may avoid regressions on a
particular board, but they are going to get unexpected changes if say
they order a new board of same type that has a newer DT.

Anyway hopefully we won't need it!

> 
> [1] https://lore.kernel.org/linux-iio/20250825-iio-adc-ad7124-proper-clock-support-v2-0-4dcff9db6b35@baylibre.com/

Just one comment on the comment.

I'd like some more eyes on this though as whilst it seems reasonable
the whole different modes bit changing timings is not particularly obvious.

> ---
>  drivers/iio/adc/ad7124.c | 26 ++++++++++++++++++--------
>  1 file changed, 18 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7124.c b/drivers/iio/adc/ad7124.c
> index 3fc24f5fffc8f200c8656cb97f9e7f80546f688b..d75ef4d5de233c2a548c732b36440d0d82c86f34 100644
> --- a/drivers/iio/adc/ad7124.c
> +++ b/drivers/iio/adc/ad7124.c
> @@ -84,10 +84,11 @@
>  #define AD7124_CONFIG_PGA		GENMASK(2, 0)
>  
>  /* AD7124_FILTER_X */
> -#define AD7124_FILTER_FS		GENMASK(10, 0)
>  #define AD7124_FILTER_FILTER		GENMASK(23, 21)
>  #define AD7124_FILTER_FILTER_SINC4		0
>  #define AD7124_FILTER_FILTER_SINC3		2
> +#define AD7124_FILTER_SINGLE_CYCLE	BIT(16)
> +#define AD7124_FILTER_FS		GENMASK(10, 0)
>  
>  #define AD7124_MAX_CONFIGS	8
>  #define AD7124_MAX_CHANNELS	16
> @@ -250,9 +251,10 @@ static int ad7124_set_mode(struct ad_sigma_delta *sd,
>  	return ad_sd_write_reg(&st->sd, AD7124_ADC_CONTROL, 2, st->adc_control);
>  }
>  
> -static void ad7124_set_channel_odr(struct ad7124_state *st, unsigned int channel, unsigned int odr)
> +static void ad7124_set_channel_odr(struct ad7124_state *st, unsigned int channel,
> +				   unsigned int odr, unsigned int avg)
>  {
> -	unsigned int fclk, odr_sel_bits;
> +	unsigned int fclk, factor, odr_sel_bits;
>  
>  	fclk = clk_get_rate(st->mclk);
>  	/*
> @@ -261,8 +263,12 @@ static void ad7124_set_channel_odr(struct ad7124_state *st, unsigned int channel
>  	 * fCLK is the master clock frequency
>  	 * FS[10:0] are the bits in the filter register
>  	 * FS[10:0] can have a value from 1 to 2047
> +	 * When multiple channels in the sequencer or the SINGLE_CYCLE bit is
This sentence doesn't read. Maybe something with a few more words like.

	 * When multiple channels are enabled in the sequencer, the
	 * SINGLE_CYCLE bit is set or when certain filter modes are enabled,...

> +	 * or when certain filter modes are enabled, there is an extra factor
> +	 * of (4 + AVG - 1) to allow for settling time.
>  	 */
> -	odr_sel_bits = DIV_ROUND_CLOSEST(fclk, odr * 32);
> +	factor = 32 * (4 + avg - 1);
> +	odr_sel_bits = DIV_ROUND_CLOSEST(fclk, odr * factor);