Message-ID: <i3mksgfcza26zbixghsfk2avvxheb32ig6wsqz4zuuqezbok4z@l2rwxoabjkgs>
Date: Thu, 17 Oct 2024 09:27:56 +0200
From: Angelo Dureghello <adureghello@...libre.com>
To: Nuno Sá <noname.nuno@...il.com>
Cc: Nuno Sá <nuno.sa@...log.com>, 
	Lars-Peter Clausen <lars@...afoo.de>, Michael Hennerich <Michael.Hennerich@...log.com>, 
	Jonathan Cameron <jic23@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Olivier Moysan <olivier.moysan@...s.st.com>, linux-iio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, dlechner@...libre.com, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v6 7/8] iio: dac: ad3552r: add high-speed platform driver

Hi Nuno,

On 15.10.2024 09:15, Nuno Sá wrote:
> On Mon, 2024-10-14 at 12:08 +0200, Angelo Dureghello wrote:
> > From: Angelo Dureghello <adureghello@...libre.com>
> > 
> > Add High Speed ad3552r platform driver.
> > 
> > The ad3552r DAC is controlled by a custom (fpga-based) DAC IP
> > through the current AXI backend, or similar alternative IIO backend.
> > 
> > Compared to the existing driver (ad3552r.c), that is a simple SPI
> > driver, this driver is coupled with a DAC IIO backend that finally
> > controls the ad3552r by a fpga-based "QSPI+DDR" interface, to reach
> > maximum transfer rate of 33MUPS using dma stream capabilities.
> > 
> > All commands involving QSPI bus read/write are delegated to the backend
> > through the provided APIs for bus read/write.
> > 
> > Signed-off-by: Angelo Dureghello <adureghello@...libre.com>
> > ---
> 
> Hi Angelo,
> 
> Some more questions from me on top of David's review...
> 
> >  drivers/iio/dac/Kconfig      |  14 ++
> >  drivers/iio/dac/Makefile     |   1 +
> >  drivers/iio/dac/ad3552r-hs.c | 526 +++++++++++++++++++++++++++++++++++++++++++
> >  drivers/iio/dac/ad3552r-hs.h |  18 ++
> >  drivers/iio/dac/ad3552r.h    |   7 +
> >  5 files changed, 566 insertions(+)
> > 
> > diff --git a/drivers/iio/dac/Kconfig b/drivers/iio/dac/Kconfig
> > index fa091995d002..fc11698e88f2 100644
> > --- a/drivers/iio/dac/Kconfig
> > +++ b/drivers/iio/dac/Kconfig
> > @@ -6,6 +6,20 @@
> >  
> >  menu "Digital to analog converters"
> >  
> > +config AD3552R_HS
> > +	tristate "Analog Devices AD3552R DAC High Speed driver"
> > +	select ADI_AXI_DAC
> > +	help
> > +	  Say yes here to build support for Analog Devices AD3552R
> > +	  Digital to Analog Converter High Speed driver.
> > +
> > +          The driver requires the assistance of an IP core to operate,
> > +          since data is streamed into target device via DMA, sent over a
> > +	  QSPI + DDR (Double Data Rate) bus.
> > +
> > +	  To compile this driver as a module, choose M here: the
> > +	  module will be called ad3552r-hs.
> > +
> >  config AD3552R
> >  	tristate "Analog Devices AD3552R DAC driver"
> >  	depends on SPI_MASTER
> > diff --git a/drivers/iio/dac/Makefile b/drivers/iio/dac/Makefile
> > index c92de0366238..d92e08ca93ca 100644
> > --- a/drivers/iio/dac/Makefile
> > +++ b/drivers/iio/dac/Makefile
> > @@ -4,6 +4,7 @@
> >  #
> >  
> >  # When adding new entries keep the list in alphabetical order
> > +obj-$(CONFIG_AD3552R_HS) += ad3552r-hs.o ad3552r-common.o
> >  obj-$(CONFIG_AD3552R) += ad3552r.o ad3552r-common.o
> >  obj-$(CONFIG_AD5360) += ad5360.o
> >  obj-$(CONFIG_AD5380) += ad5380.o
> > diff --git a/drivers/iio/dac/ad3552r-hs.c b/drivers/iio/dac/ad3552r-hs.c
> > new file mode 100644
> > index 000000000000..cb29a600e141
> > --- /dev/null
> > +++ b/drivers/iio/dac/ad3552r-hs.c
> > @@ -0,0 +1,526 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Analog Devices AD3552R
> > + * Digital to Analog converter driver, High Speed version
> > + *
> > + * Copyright 2024 Analog Devices Inc.
> > + */
> > +
> > +#include <linux/bitfield.h>
> > +#include <linux/delay.h>
> > +#include <linux/gpio/consumer.h>
> > +#include <linux/iio/backend.h>
> > +#include <linux/iio/buffer.h>
> > +#include <linux/mod_devicetable.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/property.h>
> > +#include <linux/units.h>
> > +
> > +#include "ad3552r.h"
> > +#include "ad3552r-hs.h"
> > +
> > +struct ad3552r_hs_state {
> > +	const struct ad3552r_model_data *model_data;
> > +	struct gpio_desc *reset_gpio;
> > +	struct device *dev;
> > +	struct iio_backend *back;
> > +	bool single_channel;
> > +	struct ad3552r_hs_platform_data *data;
> > +	bool ddr_mode;
> > +};
> > +
> > +static int ad3552r_qspi_update_reg_bits(struct ad3552r_hs_state *st,
> > +					u32 reg, u32 mask, u32 val,
> > +					size_t xfer_size)
> > +{
> > +	u32 rval;
> > +	int err;
> 
> Be consistent. You have a mixture of err and ret. Personally, slight preference for
> 'ret'.
> 
> > +
> > +	err = st->data->bus_reg_read(st->back, reg, &rval, xfer_size);
> > +	if (err)
> > +		return err;
> > +
> > +	rval &= ~mask;
> > +	rval |= val;
> > +
> 
> nit: can be done in one liner...
> 
> > +	return st->data->bus_reg_write(st->back, reg, rval, xfer_size);
> > +}
> > +
> > +static int ad3552r_hs_read_raw(struct iio_dev *indio_dev,
> > +			       struct iio_chan_spec const *chan,
> > +			       int *val, int *val2, long mask)
> > +{
> > +	struct ad3552r_hs_state *st = iio_priv(indio_dev);
> > +	int ret;
> > +
> > +	switch (mask) {
> > +	case IIO_CHAN_INFO_SAMP_FREQ: {
> > +		int sclk;
> > +
> > +		ret = iio_backend_read_raw(st->back, chan, &sclk, 0,
> > +					   IIO_CHAN_INFO_FREQUENCY);
> > +		if (ret != IIO_VAL_INT)
> > +			return -EINVAL;
> > +
> > +		/* Using 4 lanes (QSPI) */
> > +		*val = DIV_ROUND_CLOSEST(sclk * 4 * (1 + st->ddr_mode),
> > +					 chan->scan_type.storagebits);
> 
> If we assume ddr always on, don't forget to put that in a comment. In fact, please
> say that the sampling frequency is only about stream mode (buffering) on.
> 
> > +
> > +		return IIO_VAL_INT;
> > +	}
> > +	case IIO_CHAN_INFO_RAW:
> > +		ret = st->data->bus_reg_read(st->back,
> > +				AD3552R_REG_ADDR_CH_DAC_16B(chan->channel),
> > +				val, 2);
> > +		if (ret)
> > +			return ret;
> > +
> > +		return IIO_VAL_INT;
> 
> Hmm, I think there's an important question here. How useful it is to have "just" raw
> writes? I don't think there's anything preventing us from supporting SCALE and OFFSET
> as the SPI driver? Those are important pieces for useland to be able to compute the
> peak voltage level, right? Or am I missing something?
> 
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +}
> > +
> > +static int ad3552r_hs_write_raw(struct iio_dev *indio_dev,
> > +				struct iio_chan_spec const *chan,
> > +				int val, int val2, long mask)
> > +{
> > +	struct ad3552r_hs_state *st = iio_priv(indio_dev);
> > +
> > +	switch (mask) {
> > +	case IIO_CHAN_INFO_RAW:
> > +		iio_device_claim_direct_scoped(return -EBUSY, indio_dev) {
> > +			return st->data->bus_reg_write(st->back,
> > +				    AD3552R_REG_ADDR_CH_DAC_16B(chan->channel),
> > +				    val, 2);
> > +		}
> > +		unreachable();
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +}
> > +
> > +static int ad3552r_hs_buffer_postenable(struct iio_dev *indio_dev)
> > +{
> > +	struct ad3552r_hs_state *st = iio_priv(indio_dev);
> > +	struct iio_backend_data_fmt fmt = {
> > +		.type = IIO_BACKEND_DATA_UNSIGNED
> > +	};
> > +	int loop_len, val, err;
> > +
> > +	/* Inform DAC chip to switch into DDR mode */
> > +	err = ad3552r_qspi_update_reg_bits(st,
> > +					   AD3552R_REG_ADDR_INTERFACE_CONFIG_D,
> > +					   AD3552R_MASK_SPI_CONFIG_DDR,
> > +					   AD3552R_MASK_SPI_CONFIG_DDR, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	/* Inform DAC IP to go for DDR mode from now on */
> > +	err = iio_backend_ddr_enable(st->back);
> > +	if (err) {
> > +		dev_warn(st->dev, "could not set DDR mode, not streaming");
> 
> To me, this is an error so I would treat it as such. dev_err()
> 
> > +		goto exit_err;
> > +	}
> > +
> > +	st->ddr_mode = true;
> > +
> > +	switch (*indio_dev->active_scan_mask) {
> > +	case AD3552R_CH0_ACTIVE:
> > +		st->single_channel = true;
> > +		loop_len = 2;
> > +		val = AD3552R_REG_ADDR_CH_DAC_16B(0);
> > +		break;
> > +	case AD3552R_CH1_ACTIVE:
> > +		st->single_channel = true;
> > +		loop_len = 2;
> > +		val = AD3552R_REG_ADDR_CH_DAC_16B(1);
> > +		break;
> > +	case AD3552R_CH0_CH1_ACTIVE:
> > +		st->single_channel = false;
> > +		loop_len = 4;
> > +		val = AD3552R_REG_ADDR_CH_DAC_16B(1);
> > +		break;
> > +	default:
> > +		err = -EINVAL;
> > +		goto exit_err_ddr;
> > +	}
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_STREAM_MODE,
> > +				      loop_len, 1);
> > +	if (err)
> > +		goto exit_err_ddr;
> > +
> > +	err = iio_backend_data_transfer_addr(st->back, val);
> > +	if (err)
> > +		goto exit_err_ddr;
> > +
> > +	err = iio_backend_data_format_set(st->back, 0, &fmt);
> > +	if (err)
> > +		goto exit_err_ddr;
> > +
> > +	err = iio_backend_data_stream_enable(st->back);
> > +	if (err)
> > +		goto exit_err_ddr;
> > +
> > +	return 0;
> > +
> > +exit_err_ddr:
> > +	iio_backend_ddr_disable(st->back);
> > +
> > +exit_err:
> > +	ad3552r_qspi_update_reg_bits(st,
> > +				     AD3552R_REG_ADDR_INTERFACE_CONFIG_D,
> > +				     AD3552R_MASK_SPI_CONFIG_DDR,
> > +				     0, 1);
> > +
> > +	iio_backend_ddr_disable(st->back);
> > +
> > +	st->ddr_mode = false;
> 
> 'ddr_mode' is pretty much used for the sampling freq, right? If we go the way of just
> reporting the buffering sampling freq, I guess you can drop this variable.
> 
> > +
> > +	return err;
> > +}
> > +
> > +static int ad3552r_hs_buffer_predisable(struct iio_dev *indio_dev)
> > +{
> > +	struct ad3552r_hs_state *st = iio_priv(indio_dev);
> > +	int err;
> > +
> > +	err = iio_backend_data_stream_disable(st->back);
> > +	if (err)
> > +		return err;
> > +
> > +	/* Inform DAC to set in SDR mode */
> > +	err = ad3552r_qspi_update_reg_bits(st,
> > +					   AD3552R_REG_ADDR_INTERFACE_CONFIG_D,
> > +					   AD3552R_MASK_SPI_CONFIG_DDR,
> > +					   0, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = iio_backend_ddr_disable(st->back);
> > +	if (err)
> > +		return err;
> > +
> > +	st->ddr_mode = false;
> > +
> > +	return 0;
> > +}
> > +
> > +static int ad3552r_hs_set_output_range(struct ad3552r_hs_state *st,
> > +				       unsigned int mode)
> > +{
> > +	return ad3552r_qspi_update_reg_bits(st,
> > +				AD3552R_REG_ADDR_CH0_CH1_OUTPUT_RANGE,
> > +				AD3552R_MASK_CH_OUTPUT_RANGE,
> > +				FIELD_PREP(AD3552R_MASK_CH0_RANGE, mode) |
> > +				FIELD_PREP(AD3552R_MASK_CH1_RANGE, mode),
> > +				1);
> 
> I think you only call this function once, right? I would do this inline FWIW...
> 
> > +}
> > +
> > +static int ad3552r_hs_reset(struct ad3552r_hs_state *st)
> > +{
> > +	int err;
> > +
> > +	st->reset_gpio = devm_gpiod_get_optional(st->dev,
> > +						 "reset", GPIOD_OUT_LOW);
> > +	if (IS_ERR(st->reset_gpio))
> > +		return PTR_ERR(st->reset_gpio);
> > +
> 
> I suspect you actually want GPIOD_OUT_HIGH? Assuming the reset is active low, you
> need to properly set it as such in DT. Then gpiolib will take care of things for you.
> Note that, GPIOD_OUT_HIGH means "give me the pin in the asserted state". So if it's
> active low, then it will be effectively be low.
> 
> > +	if (st->reset_gpio) {
> > +		fsleep(10);
> > +		gpiod_set_value_cansleep(st->reset_gpio, 1);
> 
> Here you want to bring it out of reset and so de-assert the pin:
> 
> gpiod_set_value_cansleep(st->reset_gpio, 0);
> 
> Again, as long as you set the pin as active low in DT, gpiolib will negate the value
> for you internally.
> 

fixed all the rest. And added scale and offset (now readable from sysfs)
for the 2 channels, tested that using different ranges on the 2 channels works,
including custom ranges.

On this point i decided to use the active-high logic (active-low as negated reset
was more correct) becouse ad3552r.c is doing the same.
I can use the correct active-low logic here anyway.

> > +	} else {
> > +		err = ad3552r_qspi_update_reg_bits(st,
> > +					AD3552R_REG_ADDR_INTERFACE_CONFIG_A,
> > +					AD3552R_MASK_SOFTWARE_RESET,
> > +					AD3552R_MASK_SOFTWARE_RESET, 1);
> > +		if (err)
> > +			return err;
> > +	}
> > +	msleep(100);
> 
> nit: fsleep()
> 
> > +
> > +	return 0;
> > +}
> > +
> > +static int ad3552r_hs_scratch_pad_test(struct ad3552r_hs_state *st)
> > +{
> > +	int err, val;
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_SCRATCH_PAD,
> > +				      AD3552R_SCRATCH_PAD_TEST_VAL1, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = st->data->bus_reg_read(st->back, AD3552R_REG_ADDR_SCRATCH_PAD,
> > +				     &val, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	if (val != AD3552R_SCRATCH_PAD_TEST_VAL1) {
> > +		dev_err(st->dev,
> > +			"SCRATCH_PAD_TEST mismatch. Expected 0x%x, Read 0x%x\n",
> > +			AD3552R_SCRATCH_PAD_TEST_VAL1, val);
> > +		return -EIO;
> 
> This is in probing right? dev_err_probe()
> 
> > +	}
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_SCRATCH_PAD,
> > +				      AD3552R_SCRATCH_PAD_TEST_VAL2, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = st->data->bus_reg_read(st->back, AD3552R_REG_ADDR_SCRATCH_PAD,
> > +				     &val, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	if (val != AD3552R_SCRATCH_PAD_TEST_VAL2) {
> > +		dev_err(st->dev,
> > +			"SCRATCH_PAD_TEST mismatch. Expected 0x%x, Read 0x%x\n",
> > +			AD3552R_SCRATCH_PAD_TEST_VAL2, val);
> > +		return -EIO;
> 
> ditto
> 
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static int ad3552r_hs_setup_custom_gain(struct ad3552r_hs_state *st,
> > +					u16 gain, u16 offset)
> > +{
> > +	int err;
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_CH_OFFSET(0),
> > +				      offset, 1);
> > +	if (err)
> > +		return dev_err_probe(st->dev, err, "Error writing register\n");
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_CH_OFFSET(1),
> > +				      offset, 1);
> > +	if (err)
> > +		return dev_err_probe(st->dev, err, "Error writing register\n");
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_CH_GAIN(0),
> > +				      gain, 1);
> > +	if (err)
> > +		return dev_err_probe(st->dev, err, "Error writing register\n");
> > +
> > +	err = st->data->bus_reg_write(st->back, AD3552R_REG_ADDR_CH_GAIN(1),
> > +				      gain, 1);
> > +	if (err)
> > +		return dev_err_probe(st->dev, err, "Error writing register\n");
> > +
> > +	return 0;
> > +}
> > +
> > +static int ad3552r_hs_setup(struct ad3552r_hs_state *st)
> > +{
> > +	u8 gs_p, gs_n;
> > +	s16 goffs;
> > +	u16 id, rfb;
> > +	u16 gain = 0, offset = 0;
> > +	u32 val, range;
> > +	int err;
> > +
> > +	err = ad3552r_hs_reset(st);
> > +	if (err)
> > +		return err;
> > +
> > +	err = iio_backend_ddr_disable(st->back);
> > +	if (err)
> > +		return err;
> > +
> > +	err = ad3552r_hs_scratch_pad_test(st);
> > +	if (err)
> > +		return err;
> > +
> > +	err = st->data->bus_reg_read(st->back, AD3552R_REG_ADDR_PRODUCT_ID_L,
> > +				     &val, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	id = val;
> > +
> > +	err = st->data->bus_reg_read(st->back, AD3552R_REG_ADDR_PRODUCT_ID_H,
> > +				     &val, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	id |= val << 8;
> > +	if (id != st->model_data->chip_id)
> > +		dev_info(st->dev, "Chip ID error. Expected 0x%x, Read 0x%x\n",
> > +			 AD3552R_ID, id);
> > +
> > +	err = st->data->bus_reg_write(st->back,
> > +				      AD3552R_REG_ADDR_SH_REFERENCE_CONFIG,
> > +				      0, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = st->data->bus_reg_write(st->back,
> > +				      AD3552R_REG_ADDR_TRANSFER_REGISTER,
> > +				      AD3552R_MASK_QUAD_SPI |
> > +				      AD3552R_MASK_STREAM_LENGTH_KEEP_VALUE, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = iio_backend_data_source_set(st->back, 0, IIO_BACKEND_EXTERNAL);
> > +	if (err)
> > +		return err;
> > +
> > +	err = iio_backend_data_source_set(st->back, 1, IIO_BACKEND_EXTERNAL);
> > +	if (err)
> > +		return err;
> > +
> > +	err = ad3552r_get_ref_voltage(st->dev);
> > +	if (err < 0)
> > +		return err;
> > +
> > +	val = err;
> 
> Then, 'err' is not an error. I don't really like of mixing return values (errors)
> with values. Please pass the value as a pointer argument to the function.
> 
> > +
> > +	err = ad3552r_qspi_update_reg_bits(st,
> > +				AD3552R_REG_ADDR_SH_REFERENCE_CONFIG,
> > +				AD3552R_MASK_REFERENCE_VOLTAGE_SEL,
> > +				val, 1);
> > +	if (err)
> > +		return err;
> > +
> > +	err = ad3552r_get_drive_strength(st->dev, &val);
> > +	if (!err) {
> > +		err = ad3552r_qspi_update_reg_bits(st,
> > +					AD3552R_REG_ADDR_INTERFACE_CONFIG_D,
> > +					AD3552R_MASK_SDO_DRIVE_STRENGTH,
> > +					val, 1);
> > +		if (err)
> > +			return err;
> > +	}
> > +
> > +	struct fwnode_handle *child __free(fwnode_handle) =
> > +				device_get_named_child_node(st->dev, "channel");
> > +	if (!child)
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * One of "adi,output-range-microvolt" or "custom-output-range-config"
> > +	 * must be available in fdt.
> > +	 */
> > +	err = ad3552r_get_output_range(st->dev, st->model_data, child, &range);
> > +	if (!err)
> > +		return ad3552r_hs_set_output_range(st, range);
> > +	if (err != -ENOENT)
> > +		return err;
> 
> It seems to me you're already getting the span values so it should be possible to
> export them via sysfs as the spi driver, right?
> 
> > +
> > +	err = ad3552r_get_custom_gain(st->dev, child, &gs_p, &gs_n, &rfb,
> > +				      &goffs);
> > +	if (err)
> > +		return err;
> > +
> > +	gain = ad3552r_calc_custom_gain(gs_p, gs_n, goffs);
> > +	offset = abs(goffs);
> > +
> > +	return ad3552r_hs_setup_custom_gain(st, gain, offset);
> > +}
> > +
> > +static const struct iio_buffer_setup_ops ad3552r_hs_buffer_setup_ops = {
> > +	.postenable = ad3552r_hs_buffer_postenable,
> > +	.predisable = ad3552r_hs_buffer_predisable,
> > +};
> > +
> > +#define AD3552R_CHANNEL(ch) { \
> > +	.type = IIO_VOLTAGE, \
> > +	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW), \
> > +	.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ), \
> > +	.output = 1, \
> > +	.indexed = 1, \
> > +	.channel = (ch), \
> > +	.scan_index = (ch), \
> > +	.scan_type = { \
> > +		.sign = 'u', \
> > +		.realbits = 16, \
> > +		.storagebits = 16, \
> > +		.endianness = IIO_BE, \
> > +	} \
> > +}
> > +
> > +static const struct iio_chan_spec ad3552r_hs_channels[] = {
> > +	AD3552R_CHANNEL(0),
> > +	AD3552R_CHANNEL(1),
> > +};
> > +
> > +static const struct iio_info ad3552r_hs_info = {
> > +	.read_raw = &ad3552r_hs_read_raw,
> > +	.write_raw = &ad3552r_hs_write_raw,
> > +};
> > +
> > +static int ad3552r_hs_probe(struct platform_device *pdev)
> > +{
> > +	struct ad3552r_hs_state *st;
> > +	struct iio_dev *indio_dev;
> > +	int ret;
> > +
> > +	indio_dev = devm_iio_device_alloc(&pdev->dev, sizeof(*st));
> > +	if (!indio_dev)
> > +		return -ENOMEM;
> > +
> > +	st = iio_priv(indio_dev);
> > +	st->dev = &pdev->dev;
> > +
> > +	st->data = pdev->dev.platform_data;
> > +	if (!st->data)
> > +		dev_err_probe(st->dev, -ENODEV, "No platform data !");
> 
> return dev_err_probe()
> 
> > +
> > +	st->back = devm_iio_backend_get(&pdev->dev, NULL);
> > +	if (IS_ERR(st->back))
> > +		return PTR_ERR(st->back);
> > +
> > +	ret = devm_iio_backend_enable(&pdev->dev, st->back);
> > +	if (ret)
> > +		return ret;
> > +
> > +	st->model_data = device_get_match_data(&pdev->dev);
> 
> error handling...
> 
> if (!st->model_data)
> 	return -ENODEV; (or -EINVAL) - it seems there's no consensus in what to
> return here.
> 
> - Nuno Sá
> 

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