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.

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.

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

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

drivers/iio/adc/Kconfig

drivers/iio/adc/max22531.c

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.

max22531 and max14001 are similar in the interrupts they can provide.
I think max22531's interrupts can be documented more or less like the following.

interrupts: maxItems: 5 items: - description: | Interrupt for signaling when conversion results exceed the upper threshold for ADC readings or fall below the lower threshold. The interrupt source must be attached to one of COUT1 to COUT4 pins. - description: | Alert output that asserts low during a number of different error conditions. The interrupt source must be attached to INT pin.

machschmitt · 2025-08-16T04:04:47Z

Hi @KernelShinobi ,

So, this is my suggestion for MAX22531 register read and write functions.

#define MAX22531_REG_ADDR_MASK		GENMASK(7, 2)
#define MAX22531_REG_WRITE_MASK		BIT(1)

...

static int max22531_reg_read(struct max22531 *adc, unsigned int reg,
			     unsigned int *readval)
{
	u8 cmd;

	cmd = FIELD_PREP(MAX22531_REG_ADDR_MASK, reg);
	*readval = spi_w8r16be(adc->spi_dev, cmd);
	if (*readval < 0)
		return *readval;

	return 0;
}

static int max22531_reg_write(struct max22531 *adc, unsigned int reg,
			      unsigned int writeval)
{
	u8 tx_buf[3];

	tx_buf[0] = FIELD_PREP(MAX22531_REG_ADDR_MASK, reg) | MAX22531_REG_WRITE_MASK;
	put_unaligned_be16(writeval, &tx_buf[1]);

	return spi_write_then_read(adc->spi_dev, tx_buf, ARRAY_SIZE(tx_buf), NULL, 0);
}

I've tested those with MAX22531_EVKIT_A, reading and writing from/to many different registers and the results met the expectations.
Replace all occurrences of regmap_read() with max22531_reg_read() and any occurrence of regmap_write() with max22531_reg_write(). I am still going to do a detailed review of the code in this PR.

machschmitt

Hi @KernelShinobi,

The initial version of max22531 driver looks good overall.
It just needs to be updated according to the suggestions and I think it will be good to go to the mailing lists.

machschmitt · 2025-08-17T03:50:51Z

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

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


max22531 and max14001 are similar in the interrupts they can provide.
I think max22531's interrupts can be documented more or less like the following.

interrupts: maxItems: 5 items: - description: | Interrupt for signaling when conversion results exceed the upper threshold for ADC readings or fall below the lower threshold. The interrupt source must be attached to one of COUT1 to COUT4 pins. - description: | Alert output that asserts low during a number of different error conditions. The interrupt source must be attached to INT pin.

drivers/iio/adc/max22531.c

Add device tree overlay for MAX22531 device. Signed-off-by: Abhinav Jain <[email protected]>

machschmitt

Hi @KernelShinobi ,

The patches look good.
Besides a few comments (most of which are about minor adjustments) there are two commit/patch related things that need to be changed.
(1) The dt-binding patch need to come before the driver code patch. The current order is opposite, meaning the driver code patch is being introduced before the dt-binding patch. Fixing the patch order will also help you fix the MAINTAINER file entry.
(2) The report about how the code was tested must not be in the commit log. It has to be stated in the cover letter and/or under the --- of the patch file.

Before I forget, you'll also need to port the patches to the testing branch of the IIO tree.
Grab the IIO development tree.

git clone git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git
cd iio
git fetch origin testing:testing
git checkout testing

then use git format-patch and git am <patch> to generate patches from your rpi tree and apply them to the IIO tree. You'll probably need to edit the patches to make them apply.

MAINTAINERS

drivers/iio/adc/max22531.c

KernelShinobi · 2025-08-23T14:05:00Z

Hi @KernelShinobi ,

The patches look good. Besides a few comments (most of which are about minor adjustments) there are two commit/patch related things that need to be changed. (1) The dt-binding patch need to come before the driver code patch. The current order is opposite, meaning the driver code patch is being introduced before the dt-binding patch. Fixing the patch order will also help you fix the MAINTAINER file entry. (2) The report about how the code was tested must not be in the commit log. It has to be stated in the cover letter and/or under the --- of the patch file.

Before I forget, you'll also need to port the patches to the testing branch of the IIO tree. Grab the IIO development tree.
git clone git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git
cd iio
git fetch origin testing:testing
git checkout testing
then use git format-patch and git am <patch> to generate patches from your rpi tree and apply them to the IIO tree. You'll probably need to edit the patches to make them apply.

I have reversed the order and fixed conflicts. Will port to iio-testing and submit updates to this PR.

Add device tree documentation for MAX22530-MAX22532 family of ADCs. The MAX22530–MAX22532 are galvanically isolated, 4-channel, multiplexed, 12-bit, analog-to-digital converters (ADC) in the MAXSafe™ family product line. An integrated, isolated, DC-DC converter powers all fieldside circuitry, and this allows field-side diagnostics even when no input signal is present. Signed-off-by: Abhinav Jain <[email protected]>

machschmitt

Hey @KernelShinobi , the commits look better now.
A few last bits from me.

machschmitt · 2025-08-24T03:24:50Z

drivers/iio/adc/max22531.c

+
+	adc = iio_priv(indio_dev);
+	adc->spi_dev = spi;
+	adc->chip_info = chip_info;


Found this while testing but had forgotten to mention it

- adc->chip_info = chip_info; + adc->chip_info = spi_get_device_match_data(spi); + if (!adc->chip_info) + return dev_err_probe(&spi->dev, -EINVAL, "no chip info\n");

Otherwise we get a NullPointerException when probing the driver.

machschmitt · 2025-08-24T03:26:06Z

drivers/iio/adc/max22531.c

+
+static int max22531_probe(struct spi_device *spi)
+{
+	const struct max22531_chip_info *chip_info;


then this is not needed

machschmitt · 2025-08-24T03:32:03Z

drivers/iio/adc/max22531.c

+#define MAX22531_REG_ADC_CHAN(x)	((x) + 1)
+#define MAX22531_REG_FADC_CHAN(x)	((x) + 1)
+
+#define MAX22531_VREF_MV		(18 * MILLI)


oof, I guess I was not very awake when I suggested using MILLI here. That was a misleading suggestion.
(18 * MILLI) results in 18000, which I think is not the correct voltage reference in millivolts.
IIRC, the internal reference is 1.8 V, so revert the define back to 1800.

machschmitt · 2025-08-24T03:33:09Z

drivers/iio/adc/max22531.c

+
+#include <linux/units.h>
+#include <linux/module.h>
+#include <asm/unaligned.h>


After porting the pacthes to the IIO tree, update this to #include <linux/unaligned.h>.

-#include <asm/unaligned.h> +#include <linux/unaligned.h>

machschmitt · 2025-08-24T03:52:11Z

I have reversed the order and fixed conflicts. Will port to iio-testing and submit updates to this PR.

No need to update this PR with a version of the driver from IIO tree. I think that would actually mess up the PR with potentially a lot of changes unrelated to max22531 driver. Update the driver according to comments, port the patches to IIO tree, generate patches from IIO, send the patches to yourself and optionally to me to check they are okay, then, everything being alright, send the patches to the mailing lists as explained by email and exemplified in our last meeting.

Add device support for MAX22530-MAX22531. Implement scale and read functionality for raw/filtered ADC readings. Signed-off-by: Abhinav Jain <[email protected]>

KernelShinobi · 2025-08-25T20:55:54Z

Hey @KernelShinobi , the commits look better now. A few last bits from me.

I have submitted the final changes. For the header, I have kept <asm/unaligned.h> here and <linux/unaligned.h> in the IIO branch. Sending the patches generated from IIO tree to upstream.

KernelShinobi · 2025-08-25T21:36:09Z

I have reversed the order and fixed conflicts. Will port to iio-testing and submit updates to this PR.

No need to update this PR with a version of the driver from IIO tree. I think that would actually mess up the PR with potentially a lot of changes unrelated to max22531 driver. Update the driver according to comments, port the patches to IIO tree, generate patches from IIO, send the patches to yourself and optionally to me to check they are okay, then, everything being alright, send the patches to the mailing lists as explained by email and exemplified in our last meeting.

Patch has been submitted based on the IIO tree:
https://lore.kernel.org/all/[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

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jonathanns reviewed Jul 17, 2025

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

machschmitt requested changes Aug 9, 2025

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machschmitt reviewed Aug 17, 2025

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arch: dts: overlays: Add MAX22531 Overlay

6ef096c

Add device tree overlay for MAX22531 device. Signed-off-by: Abhinav Jain <[email protected]>

KernelShinobi force-pushed the rpi-6.6.y_GSOC_2025_MAX22531 branch from 5bd91fc to 180f2a9 Compare August 19, 2025 23:25

machschmitt approved these changes Aug 20, 2025

View reviewed changes

KernelShinobi force-pushed the rpi-6.6.y_GSOC_2025_MAX22531 branch from 180f2a9 to d8ef1c4 Compare August 23, 2025 14:45

machschmitt approved these changes Aug 24, 2025

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iio: adc: Add initial support for MAX22531 ADC

ccc2372

Add device support for MAX22530-MAX22531. Implement scale and read functionality for raw/filtered ADC readings. Signed-off-by: Abhinav Jain <[email protected]>

KernelShinobi force-pushed the rpi-6.6.y_GSOC_2025_MAX22531 branch from d8ef1c4 to ccc2372 Compare August 25, 2025 20:53

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

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KernelShinobi commented Jul 15, 2025

PR Description

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KernelShinobi commented Aug 23, 2025

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machschmitt commented Jul 27, 2025 •

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