linux-kernel - Re: [PATCH v5 4/7] iio: adc: ad7380: add support for pseudo-differential parts

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Message-ID: <20240324130135.35f4b0eb@jic23-huawei>
Date: Sun, 24 Mar 2024 13:01:35 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Julien Stephan <jstephan@...libre.com>
Cc: Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich
 <Michael.Hennerich@...log.com>, Nuno Sá <nuno.sa@...log.com>,
 David Lechner <dlechner@...libre.com>, Rob Herring <robh+dt@...nel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley
 <conor+dt@...nel.org>, Liam Girdwood <lgirdwood@...il.com>, Mark Brown
 <broonie@...nel.org>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, kernel test robot
 <lkp@...el.com>
Subject: Re: [PATCH v5 4/7] iio: adc: ad7380: add support for
 pseudo-differential parts

On Tue, 19 Mar 2024 11:11:25 +0100
Julien Stephan <jstephan@...libre.com> wrote:

> From: David Lechner <dlechner@...libre.com>
> 
> Add support for AD7383, AD7384 pseudo-differential compatible parts.
> Pseudo differential parts require common mode voltage supplies so add
> the support for them and add the support of IIO_CHAN_INFO_OFFSET to
> retrieve the offset
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> Signed-off-by: Julien Stephan <jstephan@...libre.com>

Hi Julien,

A few aditional comments inline.  The one about
optional regulators may be something others disagree with.
Mark, perhaps you have time to comment.
Is this usage of devm_regulator_get_optional() to check a real regulator
is supplied (as we are going to get the voltage) sensible?  Feels wrong
given the regulator is the exact opposite of optional.

Jonathan

>  struct ad7380_state {
>  	const struct ad7380_chip_info *chip_info;
>  	struct spi_device *spi;
>  	struct regmap *regmap;
>  	unsigned int vref_mv;
> +	unsigned int vcm_mv[2];
>  	/*
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
> @@ -304,6 +333,11 @@ static int ad7380_read_raw(struct iio_dev *indio_dev,
>  		*val2 = chan->scan_type.realbits;
>  
>  		return IIO_VAL_FRACTIONAL_LOG2;
> +	case IIO_CHAN_INFO_OFFSET:
> +		*val = st->vcm_mv[chan->channel] * (1 << chan->scan_type.realbits)
> +			/ st->vref_mv;

So this maths seems to be right to me, but it took me a while to figure it out.
Perhaps a comment would help along the lines of this is transforming

	(raw * scale) + vcm_mv
to
	(raw + vcm_mv / scale) * scale
as IIO ABI says offset is applied before scale.

> +
> +		return IIO_VAL_INT;
>  	}
>  
>  	return -EINVAL;
> @@ -350,7 +384,7 @@ static int ad7380_probe(struct spi_device *spi)
>  	struct iio_dev *indio_dev;
>  	struct ad7380_state *st;
>  	struct regulator *vref;
> -	int ret;
> +	int ret, i;
>  
>  	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
>  	if (!indio_dev)
> @@ -394,6 +428,40 @@ static int ad7380_probe(struct spi_device *spi)
>  		st->vref_mv = AD7380_INTERNAL_REF_MV;
>  	}
>  
> +	if (st->chip_info->num_vcm_supplies > ARRAY_SIZE(st->vcm_mv))
> +		return dev_err_probe(&spi->dev, -EINVAL,
> +				     "invalid number of VCM supplies\n");
> +
> +	/*
> +	 * pseudo-differential chips have common mode supplies for the negative
> +	 * input pin.
> +	 */
> +	for (i = 0; i < st->chip_info->num_vcm_supplies; i++) {
> +		struct regulator *vcm;
> +
> +		vcm = devm_regulator_get_optional(&spi->dev,

Why optional?

> +						  st->chip_info->vcm_supplies[i]);
> +		if (IS_ERR(vcm))

This will fail if it's not there, so I'm guessing you are using this to avoid
getting to the regulator_get_voltage?  If it's not present I'd rely on that
failing rather than the confusing handling here.

When the read of voltage wasn't in probe this would have resulted in a problem
much later than initial setup, now it is, we are just pushing it down a few lines.

Arguably we could have a devm_regulator_get_not_dummy()
that had same implementation to as get_optional() but whilst it's called that
I think it's confusing to use like this.

> +			return dev_err_probe(&spi->dev, PTR_ERR(vcm),
> +					     "Failed to get %s regulator\n",
> +					     st->chip_info->vcm_supplies[i]);
> +
> +		ret = regulator_enable(vcm);
> +		if (ret)
> +			return ret;
> +
> +		ret = devm_add_action_or_reset(&spi->dev,
> +					       ad7380_regulator_disable, vcm);
> +		if (ret)
> +			return ret;
> +
> +		ret = regulator_get_voltage(vcm);

I'd let this fail if we have a dummy regulator.

> +		if (ret < 0)
> +			return ret;
> +
> +		st->vcm_mv[i] = ret / 1000;
> +	}
> +