linux-kernel - Re: [PATCH v2 8/8] iio: adc: stm32-dfsdm: add scaling support to dfsdm

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Date: Wed, 26 Jun 2024 10:59:05 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Olivier Moysan <olivier.moysan@...s.st.com>,
 fabrice.gasnier@...s.st.com,  Jonathan Cameron <jic23@...nel.org>,
 Lars-Peter Clausen <lars@...afoo.de>, Maxime Coquelin
 <mcoquelin.stm32@...il.com>, Alexandre Torgue <alexandre.torgue@...s.st.com>
Cc: Nuno Sa <nuno.sa@...log.com>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 8/8] iio: adc: stm32-dfsdm: add scaling support to
 dfsdm

Hi Olivier,

One thing that I just noticed...

On Tue, 2024-06-25 at 17:07 +0200, Olivier Moysan wrote:
> Add scaling support to STM32 DFSDM.
> 
> When used in an analog context, a DFSDM filter typically converts the data
> from a sigma delta modulator. The IIO device associated to the DFSDM
> filter provides these data as raw data.
> The IIO device can provide scaling information (voltage and offset) to
> allow conversion of raw data into physical values.
> 
> With the new binding based on IIO backend framework, the sigma delta
> modulators are defined as backends providing scaling information.
> 
> The scaling is not supported with legacy binding.
> 
> Signed-off-by: Olivier Moysan <olivier.moysan@...s.st.com>
> Acked-by: Nuno Sa <nuno.sa@...log.com>
> ---
> 

...

> +
> +	case IIO_CHAN_INFO_SCALE:
> +		/*
> +		 * Scale is expressed in mV.
> +		 * When fast mode is disabled, actual resolution may be lower
> +		 * than 2^n, where n = realbits - 1.
> +		 * This leads to underestimating the input voltage.
> +		 * To compensate this deviation, the voltage reference can be
> +		 * corrected with a factor = realbits resolution / actual max
> +		 */
> +		if (adc->backend[idx]) {
> +			iio_backend_read_raw(adc->backend[idx], chan, val,
> val2, mask);

Eve if it does not matter for your usecase, you should still do error handling
as iio_backend_read_raw() can return an error.

> +			*val = div_u64((u64)*val * (u64)BIT(DFSDM_DATA_RES -
> 1), max);
> +			*val2 = chan->scan_type.realbits;
> +			if (chan->differential)
> +				*val *= 2;
> +		}
> +		return IIO_VAL_FRACTIONAL_LOG2;
> +
> +	case IIO_CHAN_INFO_OFFSET:
> +		/*
> +		 * DFSDM output data are in the range [-2^n, 2^n],
> +		 * with n = realbits - 1.
> +		 * - Differential modulator:
> +		 * Offset correspond to SD modulator offset.
> +		 * - Single ended modulator:
> +		 * Input is in [0V, Vref] range,
> +		 * where 0V corresponds to -2^n, and Vref to 2^n.
> +		 * Add 2^n to offset. (i.e. middle of input range)
> +		 * offset = offset(sd) * vref / res(sd) * max / vref.
> +		 */
> +		if (adc->backend[idx]) {
> +			iio_backend_read_raw(adc->backend[idx], chan, val,
> val2, mask);

Same...

- Nuno Sá