Re: [PATCH v5 2/2] iio: adc: add MAX40080 current-sense amplifier driver

[Siratul Islam <siratul.islam@linux.dev>]

On Thu, 2026-07-23 at 09:50 +0300, Stefan Popa wrote:
> The MAX40080 is a bidirectional current-sense amplifier with an
> integrated 12-bit ADC and an I2C/SMBus interface. It measures the
> voltage across an external shunt resistor and the input bus voltage,
> storing the results in an internal FIFO.
> 
...
>  
> +MAXIM MAX40080 CURRENT SENSE AMPLIFIER DRIVER
> +M:	Ciprian Hegbeli <ciprian.hegbeli@analog.com>
> +M:	Stefan Popa <stefan.popa@analog.com>
> +L:	linux-iio@vger.kernel.org
> +S:	Supported
> +W:	https://ez.analog.com/linux-software-drivers
> +F:	Documentation/devicetree/bindings/iio/adc/maxim,max40080.yaml
MAINTAINERS file entry for the binding should be added with the binding patch.
> +F:	drivers/iio/adc/max40080.c
> +
> 
...
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * MAX40080 Digital Current-Sense Amplifier driver
> + *
> + * Copyright 2026 Analog Devices, Inc.
I'd also add the datasheet link here.
/* Datasheet: <https://www.analog.com/media/en/technical-documentation/data-sheets/MAX40080.pdf> */
> + */
> +
...
> +
> +/* CFG.mode field */
> +#define MAX40080_STDBY_MODE		0x00
> +#define MAX40080_SINGLE_MODE		0x02	/* one conversion per Quick Command */
Make "o" in "One" uppercase to maintain consistency with your other comments.
> +
> +/* FIFO_CFG.store_iv field */
> +#define MAX40080_STORE_I_V		0x02
Why I_V and not IV?
> +
> +#define MAX40080_ADC_RES		4096
> +#define MAX40080_INTER_VREF_MV		1250
Andy would usually suggest "mV"
> +#define MAX40080_V_BUFF_GAIN		30
> +#define MAX40080_CSA_50MV_GAIN		25
> +#define MAX40080_CSA_10MV_GAIN		125
Ditto.
> +
> +/*
...
> +
> +static int max40080_update_bits(struct max40080_state *st, u8 reg,
> +				u16 mask, u16 val)
> +{
> +	int ret;
> +	int tmp;
> +
> +	guard(mutex)(&st->lock);
> +
> +	tmp = i2c_smbus_read_word_data(st->client, reg);
> +	if (tmp < 0)
> +		return tmp;
> +
> +	tmp &= ~mask;
> +	tmp |= val & mask;
Could be done in one step? "tmp = (tmp & ~mask) | (val & mask)"
> +
> +	ret = i2c_smbus_write_word_data(st->client, reg, tmp);
> +	if (ret < 0)
> +		return ret;
> +
> +	return 0;
> +}
> +
> +static int max40080_read_iv(struct max40080_state *st, u32 *iv)
> +{
> +	u32 tmp = 0;
> +	int ret, io_ret;
> +
> +	guard(mutex)(&st->lock);
> +
> +	ret = max40080_trigger_measurement(st);
> +	if (ret < 0)
> +		return ret;
> +
> +	/*
> +	 * Wait for the conversion to complete by polling the FIFO valid bit
> +	 * (or bail out on an I2C error). Polling the device's own status makes
> +	 * this independent of the actual conversion time, which varies with the
> +	 * oversampling ratio and the bus speed. The timeout is only a safety
> +	 * ceiling: the worst case is the maximum 128x averaging on both the
> +	 * current and voltage channels at the slowest 15 ksps base rate plus the
> +	 * inter-channel switching time, i.e. roughly 20 ms; 50 ms leaves ample
> +	 * margin.
> +	 */
> +	ret = read_poll_timeout(max40080_read_iv_once, io_ret,
> +				io_ret || (tmp & MAX40080_IV_VALID_MSK),
> +				500, 50000, false, st, &tmp);
> +	*iv = tmp;
Is "iv" intentionally being written even in error path? Ideally it would be 
assigned after the check.
> +	if (ret)
> +		return ret;
> +
> +	return io_ret;
> +}
> +
> +/*
> + * max40080_oversampling_avail[] is ordered so that its index is the FILTER
> + * field value (index 0 = no averaging, index 1 = 8x, ...). Return that index
> + * for an exact match, or -EINVAL for a value that is not on the list.
> + */
> +static int max40080_oversampling_to_filter(int val)
> +{
> +	int i;
> +
Just use for(unsigned int i = 0; i < ARRAY_SIZE(max40080_oversampling_avail); i++);
No need to have a separate declaration of 'i' for "for"
> +	for (i = 0; i < ARRAY_SIZE(max40080_oversampling_avail); i++)
> +		if (max40080_oversampling_avail[i] == val)
> +			return i;
I'd use scope here as it's technically multiline. "for(...) { }"
> +
> +	return -EINVAL;
> +}
> +
> +static int max40080_set_oversampling_ratio(struct max40080_state *st, int val)
> +{
> +	int ret, filter = max40080_oversampling_to_filter(val);
Separate initialization from declaration.

int ret, filter;

filter = max40080_oversampling_to_filter(val);
> +
> +	if (filter < 0)
> +		return filter;
> +
> +	ret = max40080_update_bits(st, MAX40080_REG_CFG, MAX40080_FILTER_MSK,
> +				   FIELD_PREP(MAX40080_FILTER_MSK, filter));
> +	if (ret)
> +		return ret;
> +
> +	st->oversampling_ratio = val;
> +
> +	return 0;
> +}
> +
> +static int max40080_read_raw(struct iio_dev *indio_dev,
> +			     struct iio_chan_spec const *chan,
> +			     int *val,
> +			     int *val2,
> +			     long mask)
Put "int *val, int *val2, long mask" on the same line. i.e,
static int max40080_read_raw(struct iio_dev *indio_dev,
			     struct iio_chan_spec const *chan,
			     int *val, int *val2, long mask)

> +{
> +	struct max40080_state *st = iio_priv(indio_dev);
> +	int ret;
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_RAW:
> +		if (chan->type == IIO_CURRENT) {
> +			ret = max40080_get_current(st, val);
> +			if (ret)
> +				return ret;
> +		} else if (chan->type == IIO_VOLTAGE) {
> +			ret = max40080_get_voltage(st, val);
> +			if (ret)
> +				return ret;
> +		}
> +
Either have this space before return on all of the paths in this "switch" 
or none of them for consistency.
> +		return IIO_VAL_INT;
> +	case IIO_CHAN_INFO_SCALE:
> +		if (chan->type == IIO_CURRENT) {
> +			/*
> +			 * The selectable current-sense range is exposed through
> +			 * scale: each RANGE setting has its own precomputed
> +			 * mA-per-code value. Userspace picks the range by writing
> +			 * the matching scale.
> +			 */
> +			*val = st->current_scale[st->range][0];
> +			*val2 = st->current_scale[st->range][1];
> +			return IIO_VAL_INT_PLUS_NANO;
> +		}
> +		/* voltage[mV] = raw * Vref[mV] * buffer_gain / ADC_RES */
> +		*val = MAX40080_INTER_VREF_MV * MAX40080_V_BUFF_GAIN;
> +		*val2 = MAX40080_ADC_RES;
> +		return IIO_VAL_FRACTIONAL;
> +	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
> +		ret = max40080_get_oversampling_ratio(st, val);
> +		if (ret)
> +			return ret;
> +		return IIO_VAL_INT;
> +	default:
> +		return -EINVAL;
> +	}
> +}
> +
> +static int max40080_write_raw(struct iio_dev *indio_dev,
> +			      struct iio_chan_spec const *chan,
> +			      int val, int val2, long mask)
> +{
> +	struct max40080_state *st = iio_priv(indio_dev);
> +	unsigned int i;
> +	int ret;
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_SCALE:
> +		/* Only the current channel has a selectable range/scale. */
> +		if (chan->type != IIO_CURRENT)
> +			return -EINVAL;
> +
> +		for (i = 0; i < MAX40080_NUM_RANGES; i++)
> +			if (val == st->current_scale[i][0] &&
> +			    val2 == st->current_scale[i][1])
> +				return max40080_set_range(st, i);
I'd add brackets to this "for" too like above.
> +
> +		return -EINVAL;
> +	case IIO_CHAN_INFO_OVERSAMPLING_RATIO:
> +		ret = max40080_set_oversampling_ratio(st, val);
> +		if (ret)
> +			return ret;
> +		break;
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +static int max40080_reg_access(struct iio_dev *indio_dev,
> +			       unsigned int reg,
> +			       unsigned int write_val,
> +			       unsigned int *read_val)
> +{
> +	struct max40080_state *st = iio_priv(indio_dev);
> +
> +	if (read_val) {
> +		int val = i2c_smbus_read_word_data(st->client, reg);
Declare this "val" at the top.
> +
> +		if (val < 0)
> +			return val;
> +		*read_val = val;
Add blank lines above and below
"*read_val = val;"
> +		return 0;
> +	}
> +
> +	return i2c_smbus_write_word_data(st->client, reg, write_val);
> +}
> +
> +/*
> + * Configure the device from the cached state. The device powers up in standby
> + * with PEC enabled (CFG POR = 0x0060), so PEC is kept enabled throughout.
> + */
> +static int max40080_init(struct max40080_state *st)
> +{
> +	u16 fifo_cfg, cfg;
> +	int ret, filter;
> +
> +	filter = max40080_oversampling_to_filter(st->oversampling_ratio);
> +	if (filter < 0)
> +		return filter;
> +
> +	/*
> +	 * Put the device in standby before (re)configuring the FIFO: the FIFO
> +	 * configuration register can only be written while the device is not
> +	 * converting.
> +	 */
> +	cfg = FIELD_PREP(MAX40080_MODE_MSK, MAX40080_STDBY_MODE) |
> +	      FIELD_PREP(MAX40080_PEC_EN_MSK, 1);
I'd have a blank line before the "ret" assignments.
> +	ret = i2c_smbus_write_word_data(st->client, MAX40080_REG_CFG, cfg);
> +	if (ret)
> +		return ret;
> +
> +	/* Store a matched current+voltage pair per conversion. */
> +	fifo_cfg = FIELD_PREP(MAX40080_STORE_IV_MSK, MAX40080_STORE_I_V);
Here too.
> +	ret = i2c_smbus_write_word_data(st->client, MAX40080_REG_FIFO_CFG,
> +					fifo_cfg);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * Use single-measurement mode: the device stays idle and converts once
> +	 * per SMBus Quick Command (see max40080_trigger_measurement()), so each
> +	 * read returns a fresh sample rather than a queued FIFO entry.
> +	 */
> +	cfg = FIELD_PREP(MAX40080_MODE_MSK, MAX40080_SINGLE_MODE) |
> +	      FIELD_PREP(MAX40080_PEC_EN_MSK, 1) |
> +	      FIELD_PREP(MAX40080_RANGE_MSK, st->range) |
> +	      FIELD_PREP(MAX40080_FILTER_MSK, filter);
> +
Like you have here.
> +	ret = i2c_smbus_write_word_data(st->client, MAX40080_REG_CFG, cfg);
> +	if (ret)
> +		return ret;
> +
> +	return 0;
> +}
> +
> +static int max40080_probe(struct i2c_client *client)
> +{
> +
...
> +
> +	indio_dev = devm_iio_device_alloc(dev, sizeof(*st));
> +	if (!indio_dev)
> +		return -ENOMEM;
> +
> +	i2c_set_clientdata(client, indio_dev);
> +
Is it used?
> +	st = iio_priv(indio_dev);
> +	st->client = client;
> +
> +	ret = devm_mutex_init(dev, &st->lock);
> +	if (ret)
> +		return ret;
> +
> +	ret = device_property_read_u32(dev, propname, &st->shunt_resistor_uohm);
> +	if (ret)
> +		return dev_err_probe(dev, ret, "can't read %s\n", propname);
Add a space here.
> +	if (!st->shunt_resistor_uohm)
> +		return dev_err_probe(dev, -EINVAL, "%s must be non-zero\n",
> +				     propname);
> +
> +	max40080_calc_current_scale(st);
> +
> +	/* Defaults: 50 mV range (index 0), no averaging. */
> +	st->range = 0;
> +	st->oversampling_ratio = 1;
> +
> +	indio_dev->name = "max40080";
> +	indio_dev->info = &max40080_info;
> +	indio_dev->modes = INDIO_DIRECT_MODE;
> +	indio_dev->channels = max40080_channels;
> +	indio_dev->num_channels = ARRAY_SIZE(max40080_channels);
> +
> +	ret = max40080_init(st);
> +	if (ret)
> +		return ret;
> +
> +	return devm_iio_device_register(dev, indio_dev);
> +}
> +
> +static const struct i2c_device_id max40080_i2c_ids[] = {
> +	{ "max40080" },
use named initializer. i.e, 
{ .name = "max40080" },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(i2c, max40080_i2c_ids);
> +
> 
...
> +MODULE_AUTHOR("Ciprian Hegbeli <ciprian.hegbeli@analog.com>");
> +MODULE_AUTHOR("Stefan Popa <stefan.popa@analog.com>");
> +MODULE_DESCRIPTION("Analog Devices MAX40080 current-sense amplifier driver");
> +MODULE_LICENSE("GPL");