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 <[email protected]>

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.

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

drivers/iio/adc/Kconfig

drivers/iio/adc/max22531.c

machschmitt · 2025-07-27T21:01:45Z

Hello, so I've set up MAX2531_EVKIT_A and tested the initial MAX22531 driver version.

I've added "brcm,bcm2712" to device tree overlay compatible list and added another dev_info print at the end of driver probe.

Here's the result

@KernelShinobi , please, update the PR code according to the provided suggestions.

Despite the message printed to kernel logs, the MAX22531_REG_PROD_ID read is clearly not successful, as we should get 0x0001 from that register. As we talked about in a previous meeting, drop the regmap implementation and make direct use of SPI transfers to comply with the protocol described on datasheet page 19 correctly.

I'll test it again after you update the driver code and register access functions accordingly.

Improve initial DT Overlay for MAX22530-MAX22532. Add MAX22530-MAX22532 device tree documentation. Signed-off-by: Abhinav Jain <[email protected]>

Add device entries for MAX22530 and MAX22532. Retrieve logic and DC-DC power supplies post fixes in DT overlay. Add chip_info structure for the device. Fix regmap_config for SPI read/write commands as per the datasheet. Implement scale in max22531_read_raw function. Add boilerplate text to Kconfig for the driver. Signed-off-by: Abhinav Jain <[email protected]>

machschmitt

Hey @KernelShinobi , the MAX22531 driver is looking pretty good overall.
Aside from a few suggestions of mine, I think there's not much else to do to get to a functional working driver.
I'll add another comment with the code for register access when I get it properly working.
Oh, one more thing, please, also run checkpatch to fix any leftover codestyle issue (if any). E.g.

./scripts/checkpatch.pl --terse --codespell --color=always --strict --file drivers/iio/adc/max22531.c

Cheers,
Marcelo

machschmitt · 2025-08-09T00:55:44Z

Documentation/devicetree/bindings/iio/adc/adi,max22531.yaml

+
+  Datasheet can be found here
+    https://www.analog.com/media/en/technical-documentation/data-sheets/max22530-max22532.pdf
+


Reference the spi-peripheral-props here

$ref: /schemas/spi/spi-peripheral-props.yaml#

machschmitt · 2025-08-09T01:04:31Z

Documentation/devicetree/bindings/iio/adc/adi,max22531.yaml

+  '#address-cells':
+    const: 1
+
+  '#size-cells':
+    const: 0


These are currently not needed.
#address-cells and #size-cells may be needed if ADC channels eventually need to be described in dt. For example

properties: ... '#address-cells': const: 1 '#size-cells': const: 0 patternProperties: "^channel@[0-9a-f]$": $ref: /schemas/iio/adc/adc.yaml# ... examples: - | spi { #address-cells = <1>; #size-cells = <0>; max22531: adc@0 { compatible = "adi,max22531"; ... #address-cells = <1>; #size-cells = <0>; channel@0 { reg = <0>; }; ...

Though, I currently don't see any need to describe MAX22531 channels in dt so would drop #address-cells and #size-cells .

machschmitt · 2025-08-09T01:07:00Z

Documentation/devicetree/bindings/iio/adc/adi,max22531.yaml

+  allOf:
+    - $ref: /schemas/spi/spi-peripheral-props.yaml#


This can be declared right after the main dt-binding description, as mentioned above.

machschmitt · 2025-08-09T01:13:59Z

Documentation/devicetree/bindings/iio/adc/adi,max22531.yaml

+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        max22531: adc@0 {


No need to have the phandle in the example

- max22531: adc@0 { + max2253adc@0 {

machschmitt · 2025-08-09T01:18:27Z

drivers/iio/adc/Kconfig

+        depends on SPI
+        help
+          Say yes here to build support for field-side self-powered 12-bit
+	  isolated Maxim ADCs. (max22530, max22531, max22532).


nitpicking: is the indentation correct at this line?

machschmitt · 2025-08-09T01:27:02Z

drivers/iio/adc/max22531.c

+#define MAX22531_REG_ADC1		0x01
+#define MAX22531_REG_ADC2		0x02
+#define MAX22531_REG_ADC3		0x03
+#define MAX22531_REG_ADC4		0x04


This can become more concise if we add a macro for those

-#define MAX22531_REG_ADC1 0x01 -#define MAX22531_REG_ADC2 0x02 -#define MAX22531_REG_ADC3 0x03 -#define MAX22531_REG_ADC4 0x04 +#define MAX22531_REG_ADC_CHAN(x) ((x) + 1)

machschmitt · 2025-08-09T01:32:38Z

drivers/iio/adc/max22531.c

+		switch(chan->channel) {
+		case 0:
+			reg = MAX22531_REG_ADC1;
+			break;
+		case 1:
+			reg = MAX22531_REG_ADC2;
+			break;
+		case 2:
+			reg = MAX22531_REG_ADC3;
+			break;
+		case 3:
+			reg = MAX22531_REG_ADC3;
+			break;
+		default:
+			return -EINVAL;
+	}
+	ret = regmap_read(adc->regmap, reg, val);
+	if (ret) 
+		return ret;
+	return IIO_VAL_INT;


then this can be simplified to something like

ret = regmap_read(adc->regmap, MAX22531_REG_ADC_CHAN(chan->channel), val); if (ret) return ret; return IIO_VAL_INT;

using regmap_read() as a placeholder for the register read function, which might end up not being called through the regmap API.

machschmitt · 2025-08-09T02:02:22Z

drivers/iio/adc/max22531.c

+	case IIO_CHAN_INFO_SCALE:
+		switch(chan->channel) {
+		case 0:
+			reg = MAX22531_REG_FADC1;
+			break;
+		case 1:
+			reg = MAX22531_REG_FADC2;
+			break;
+		case 2:
+			reg = MAX22531_REG_FADC3;
+			break;
+		case 3:
+			reg = MAX22531_REG_FADC4;
+			break;
+		default:
+			return -EINVAL;
+	}
+
+	reg = regmap_read(adc->regmap, reg, val);
+	if (ret)
+		return ret;
+	return IIO_VAL_INT;


hmm, the chip data sheet says "Filtering averages the most recent 4 readings." on page 17.
I would use in_Y_mean_raw interface [1] to provide that similarly to non-averaged reads.

case IIO_CHAN_INFO_AVERAGE_RAW: ret = regmap_read(adc->regmap, MAX22531_REG_FADC_CHAN(chan->channel), val); if (ret) return ret; return IIO_VAL_INT;

Again, using regmap_read() as a placeholder. Also, register macro name up to you.
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git/tree/Documentation/ABI/testing/sysfs-bus-iio?h=testing#n406

machschmitt · 2025-08-09T02:06:53Z

drivers/iio/adc/max22531.c

+	unsigned int ret, prod_id;
+	const struct max22531_chip_info *info;
+	struct max22531 *adc;
+	struct iio_dev *indio_dev;


nitpicking: Prefer reverse fir tree order, a.k.a. reverse Christmas tree or reverse XMAS tree for local variable declarations.
See
Documentation/process/maintainer-tip.rst
Documentation/process/maintainer-netdev.rst
Documentation/process/maintainer-kvm-x86.rst

machschmitt · 2025-08-09T02:26:50Z

Documentation/devicetree/bindings/iio/adc/adi,max22531.yaml

+  vddpl-supply:
+    description:
+      Isolated DC-DC converter power supply.
+


Due to the "DO attempt to make bindings complete even if a driver doesn't support some features." guideline, we will also need to document the interrupt.
There should be plenty of examples of devices that can fire interrupts when readings surpass/undergo a threshold.
One device I recall having something similar is AD7091R-2/-4/-5/-8 (see Documentation/devicetree/bindings/iio/adc/adi,ad7091r5.yaml).
I get the impression max22531 interrupt will be a bit trickier to document since the interrupt may be triggered by conditions other than the threshold.

psychild added 4 commits July 10, 2025 15:58

arch: dts: overlays: Add MAX22531 Overlay

09db106

Signed-off-by: Abhinav Jain <[email protected]>

iio: adc: Update DT Overlay for MAX22531

15b53e9

Signed-off-by: Abhinav Jain <[email protected]>

iio: adc: Add initial basic support for MAX22531 ADC

fad64a4

Signed-off-by: Abhinav Jain <[email protected]>

iio: adc: Add regmap support and device tree support

3aec57d

Signed-off-by: Abhinav Jain <[email protected]>

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

View reviewed changes

jonathanns reviewed Jul 17, 2025

View reviewed changes

drivers/iio/adc/max22531.c Outdated Show resolved Hide resolved

psychild added 2 commits August 7, 2025 14:34

iio: adc: Device tree improvements for MAX22530-MAX22532

8de7c35

Improve initial DT Overlay for MAX22530-MAX22532. Add MAX22530-MAX22532 device tree documentation. Signed-off-by: Abhinav Jain <[email protected]>

machschmitt requested changes Aug 9, 2025

View reviewed changes


		Datasheet can be found here
		https://www.analog.com/media/en/technical-documentation/data-sheets/max22530-max22532.pdf

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

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KernelShinobi commented Jul 15, 2025

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machschmitt commented Jul 27, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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machschmitt commented Jul 27, 2025 •

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