iio: adc: Add initial driver support for MAX22531 #2854

KernelShinobi · 2025-07-15T21:12:26Z

MAX22531 is a galvanically isolated, field-side self-powered, 4-channel, multiplexed, 12-bit ADC.

PR Description

Basic IIO ADC max22531 driver features:

Definitions for register addresses and useful constants (if any).
Definitions for the IIO channels.
A device state struct.
A read_raw function implementation (may be just a mock for now).
An iio_info struct populated with a reference to the read_raw function.
A probe function to initialize and setup the device.
SPI and device tree MODULE_DEVICE_TABLEs

PR Type

New feature (a change that adds new functionality)

PR Checklist

I have conducted a self-review of my own code changes
I could not test the DT overlay and/or the driver probing. However, I have compiled it on my machine and used static tools to ensure the code is okay.

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>

machschmitt

Hi @KernelShinobi , a few comments about this initial driver version.
Note that, aside from the comments above, there are still some improvements that need to be made to make this driver usable (e.g. read ADC sample data, provide scale to convert output codes to mV). Those features shall be implemented next. Though, overall, this looks good to me as an initial driver version.

machschmitt · 2025-07-17T05:02:55Z

arch/arm/boot/dts/overlays/rpi-max22531-overlay.dts

+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2835-spi", "brcm,bcm2711";


You may need to add brcm,bcm2712 to this list to actually indicate this overlay is compatible with rpi5.

- compatible = "brcm,bcm2835-spi", "brcm,bcm2711"; + compatible = "brcm,bcm2712", "brcm,bcm2835-spi", "brcm,bcm2711";

machschmitt · 2025-07-17T05:10:06Z

arch/arm/boot/dts/overlays/rpi-max22531-overlay.dts

+	max22531@0 {
+		compatible = "maxim,max22531";
+		reg = <0>;	/* Using CS0 on spi0 */
+		spi-max-frequency = <10000000>;


10 MHz is relatively fast for a rpi SPI controller. If you observe transfers bringing strange results or failing when you receive the hw, then consider decreasing the SPI clock frequency.

machschmitt · 2025-07-17T05:13:51Z

drivers/iio/adc/Kconfig

+        tristate "Analog Devices MAX22531 ADC Driver"
+        depends on SPI
+        help
+          Say yes here to build support for this driver.


Not a thing crucial for the moment, but I'd add the usual config help text boilerplate here. Upstream folks may request it, and there's nothing preventing us from adding a few more lines of text here.

machschmitt · 2025-07-17T05:15:53Z

drivers/iio/adc/max22531.c

+		.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),		\
+		.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),	\
+		.scan_index = ch,					\
+		.output = 0,						\


not needed

- .output = 0, \

machschmitt · 2025-07-17T05:17:57Z

drivers/iio/adc/max22531.c

+	MAX22531_CHANNEL(1),
+	MAX22531_CHANNEL(2),
+	MAX22531_CHANNEL(3),
+	IIO_CHAN_SOFT_TIMESTAMP(2),


Let's leave the timestamp channel for after buffer support is implemented. Timestamp channels are not very useful on their own.

machschmitt · 2025-07-17T05:53:16Z

drivers/iio/adc/max22531.c

+	adc = iio_priv(indio_dev);
+	adc->spi_dev = spi;
+
+	indio_dev->name = "max22531";


this can be better set according to the specific device (max22530, max22531, or max22532) if a chip_info struct is implemented. See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/iio/adc/ad7124.c?h=v6.16-rc6#n196 for an example.

machschmitt · 2025-07-17T05:53:44Z

drivers/iio/adc/max22531.c

+}
+
+static const struct spi_device_id max22531_id[] = {
+	{ "max22531" },


provide entries for max22530, and max22532 too

machschmitt · 2025-07-17T05:54:48Z

drivers/iio/adc/max22531.c

+MODULE_DEVICE_TABLE(spi, max22531_id);
+
+static const struct of_device_id max22531_spi_of_id[] = {
+	{ .compatible = "adi,max22531" },


same here, add OF entries for matching with max22530 and max22532.

machschmitt · 2025-07-17T06:12:37Z

drivers/iio/adc/max22531.c

+	adc->vref = devm_regulator_get(&spi->dev, "vref");
+	if (IS_ERR(adc->vref))
+		dev_err(&spi->dev, "Failed to retrieve vref\n");
+
+	ret = regulator_enable(adc->vref);
+	if (ret)
+		return ret;


vref is an internal voltage reference that is always on/enabled. If I'm not missing anything, we can just use vref. E.g.

#define MAX22531_VREF_MV 1800

thus we don't need to keep a vref regulator field in struct max22351.
For regulators, the driver should request VDDL and VDDPL regulators, as those must be supplied to power the MAX22531 chip.

machschmitt · 2025-07-17T06:32:26Z

drivers/iio/adc/max22531.c

+};
+
+static const struct regmap_config regmap_config = {
+	.reg_bits = 16,


The SPI read/write commands (datasheet Table 1 and Table 2) have 8-bit header so we would have .reg_bits = 8 (or reg_bits = 6 maybe).
Though, the header also contains the W/R and burst bits.
I think the W/R can be handled with
.write_flag_mask = BIT(1)
the burst bit skipped with
.pad_bits = 1
See https://elixir.bootlin.com/linux/v6.15.6/source/include/linux/regmap.h#L254

jonathanns · 2025-07-17T12:42:49Z

drivers/iio/adc/max22531.c

+
+	adc->regmap = devm_regmap_init_spi(spi, &regmap_config);
+	if (IS_ERR(adc->regmap))
+		dev_err(&spi->dev, "regmap init failure\n");


Usually when something critical fails in the probe it is better to stop and return, like:
return dev_err_probe(&spi->dev, PTR_ERR(adc->regmap), "regmap init failure\n")

Same for other similar cases here.

psychild added 4 commits July 10, 2025 15:58

arch: dts: overlays: Add MAX22531 Overlay

09db106

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>

iio: adc: Update DT Overlay for MAX22531

15b53e9

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>

iio: adc: Add initial basic support for MAX22531 ADC

fad64a4

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>

iio: adc: Add regmap support and device tree support

3aec57d

Signed-off-by: Abhinav Jain <jain.abhinav177@gmail.com>

KernelShinobi requested review from nunojsa, mhennerich, dbogdan, btogorean, chegbeli, buha, amiclaus, rbolboac, danmois, CiprianRegus and machschmitt as code owners July 15, 2025 21:12

machschmitt reviewed Jul 17, 2025

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jonathanns reviewed Jul 17, 2025

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iio: adc: Add initial driver support for MAX22531 #2854

iio: adc: Add initial driver support for MAX22531 #2854

KernelShinobi commented Jul 15, 2025

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KernelShinobi commented Jul 15, 2025

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