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Message-ID: <7125491f-880c-486e-b417-8fc1e0c20c88@foss.st.com>
Date: Wed, 26 Jun 2024 18:15:15 +0200
From: Olivier MOYSAN <olivier.moysan@...s.st.com>
To: Nuno Sá <noname.nuno@...il.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 Nuno,
On 6/26/24 10:59, Nuno Sá wrote:
> 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.
>
Ack. On the same occasion, I will switch to a dedicated API.
Olivier
>> + *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
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