[PATCH v4 0/3] hwmon: (lm75) add MAX31875 support

[PATCH v4 0/3] hwmon: (lm75) add MAX31875 support

[John Erasmus Mari Geronimo]

Add support for the Analog Devices MAX31875 I2C temperature sensor by
extending the existing lm75 driver.

The MAX31875 provides a 16-bit configuration register, selectable
resolution of 8, 9, 10, or 12 bits, conversion rates of 125 ms, 250 ms,
1 s, or 4 s, and an alarm output.

Changes since v3:
- driver: skip the alert polarity configuration for the MAX31875. It has
  no polarity bit, and the bit used for 16-bit chips is part of its
  conversion rate field.
- driver: set the default resolution and conversion rate at probe so the
  driver state matches the chip.
- The conversion rate field is D2:D1 of the configuration register (see
  the datasheet's Configuration Register Definition). It is read here as
  0x0600 because the driver reads this register without byte swapping.

Tested on a Raspberry Pi 4B with a MAX31875: defaults are restored at
probe after the chip is set to 12-bit/125 ms, update_interval selects
each of the four conversion rates without changing the resolution, and
temp1_alarm follows temp1_max. This was run on the Raspberry Pi 6.12
tree, which reads the configuration register byte-swapped, so the
equivalent bit positions were used there.

Changes since v2:
- dt-bindings: expand the commit message to explain why the MAX31875
  needs its own compatible string instead of reusing an existing
  lm75-family one, and pick up Conor Dooley's Acked-by given
  conditional on that fix.
- driver: drop the .resolutions table, which incorrectly tied
  resolution to the sample-time index even though the two are
  independently configurable on this chip (Guenter Roeck).
- driver: use the BIT() idiom for the alarm bit read to match the
  rest of the file (Guenter Roeck).

Changes since v1:
- Rework the implementation to use the existing lm75 driver instead of
  the previously selected max31827 base driver.
- Add the MAX31875 Device Tree binding, documentation, and driver
  configuration for resolution, conversion rate, and alarm reporting.

John Erasmus Mari Geronimo (3):
  dt-bindings: hwmon: lm75: add MAX31875
  hwmon: (lm75) add MAX31875 documentation
  hwmon: (lm75) add MAX31875 support

 .../devicetree/bindings/hwmon/lm75.yaml       |  1 +
 Documentation/hwmon/lm75.rst                  |  6 ++--
 drivers/hwmon/Kconfig                         |  2 +-
 drivers/hwmon/lm75.c                          | 30 ++++++++++++++++++-
 4 files changed, 35 insertions(+), 4 deletions(-)

-- 
2.34.1

[PATCH v4 1/3] dt-bindings: hwmon: lm75: add MAX31875

[John Erasmus Mari Geronimo]

Add MAX31875 compatible string to lm75 dt-bindings.

The MAX31875 cannot reuse an existing lm75-family compatible such as
adi,adt75: it has a 16-bit configuration register (instead of 8-bit),
four independently selectable conversion rates (125 ms, 250 ms, 1 s,
4 s) and four independently selectable resolutions (8, 9, 10, 12
bits), and reports its alarm through a different configuration
register bit. These differences require dedicated handling in the
lm75 driver, so a distinct compatible string is needed to select it.

Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
---
 Documentation/devicetree/bindings/hwmon/lm75.yaml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Documentation/devicetree/bindings/hwmon/lm75.yaml b/Documentation/devicetree/bindings/hwmon/lm75.yaml
index b48bf3fd7..d56f3ec29 100644
--- a/Documentation/devicetree/bindings/hwmon/lm75.yaml
+++ b/Documentation/devicetree/bindings/hwmon/lm75.yaml
@@ -14,6 +14,7 @@ properties:
   compatible:
     enum:
       - adi,adt75
+      - adi,max31875
       - ams,as6200
       - atmel,at30ts74
       - dallas,ds1775
-- 
2.34.1

[PATCH v4 2/3] hwmon: (lm75) add MAX31875 documentation

[John Erasmus Mari Geronimo]

Document the Maxim MAX31875 I2C temperature sensor as a supported
chip in the lm75 driver documentation.

Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>
---
 Documentation/hwmon/lm75.rst | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/Documentation/hwmon/lm75.rst b/Documentation/hwmon/lm75.rst
index ca46754e0..54a6a75e8 100644
--- a/Documentation/hwmon/lm75.rst
+++ b/Documentation/hwmon/lm75.rst
@@ -66,9 +66,9 @@ Supported chips:
 
 	       http://www.microchip.com/
 
-  * Analog Devices ADT75
+  * Analog Devices ADT75, MAX31875
 
-    Prefix: 'adt75'
+    Prefix: 'adt75', 'max31875'
 
     Addresses scanned: none
 
@@ -76,6 +76,8 @@ Supported chips:
 
 	       https://www.analog.com/adt75
 
+	       https://www.analog.com/max31875
+
   * ST Microelectronics STDS75
 
     Prefix: 'stds75'
-- 
2.34.1

[PATCH v4 3/3] hwmon: (lm75) add MAX31875 support

[John Erasmus Mari Geronimo]

Add support for the Maxim MAX31875 I2C temperature sensor by
extending the lm75 driver. The MAX31875 features a 16-bit
configuration register, configurable resolution (8, 9, 10, or
12 bits), and selectable conversion rates (125ms, 250ms, 1s, 4s).

The MAX31875 has no alert polarity bit, and the bit the driver would
use for 16-bit chips is part of its conversion rate field, so skip
the polarity configuration for it. Also set the default resolution
and conversion rate at probe so the driver state matches the chip.

Suggested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>
---
 drivers/hwmon/Kconfig |  2 +-
 drivers/hwmon/lm75.c  | 30 +++++++++++++++++++++++++++++-
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index 18c682e1a..3123f1282 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -1597,7 +1597,7 @@ config SENSORS_LM75
 	  If you say yes here you get support for one common type of
 	  temperature sensor chip, with models including:
 
-		- Analog Devices ADT75
+		- Analog Devices ADT75, MAX31875
 		- Atmel (now Microchip) AT30TS74
 		- Dallas Semiconductor DS75, DS1775 and DS7505
 		- Global Mixed-mode Technology (GMT) G751
diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
index 2d2d752ae..349648fd0 100644
--- a/drivers/hwmon/lm75.c
+++ b/drivers/hwmon/lm75.c
@@ -38,6 +38,7 @@ enum lm75_type {		/* keep sorted in alphabetical order */
 	max6625,
 	max6626,
 	max31725,
+	max31875,
 	mcp980x,
 	p3t1750,
 	p3t1755,
@@ -222,6 +223,16 @@ static const struct lm75_params device_params[] = {
 		.default_resolution = 16,
 		.default_sample_time = MSEC_PER_SEC / 20,
 	},
+	[max31875] = {
+		.config_reg_16bits = true,
+		.set_mask = 2 << 13,	/* 10-bit resolution */
+		.clr_mask = 3 << 13 | 3 << 9,	/* resolution, 4 s conversion rate */
+		.default_resolution = 10,
+		.default_sample_time = 4000,
+		.num_sample_times = 4,
+		.sample_times = (unsigned int []){ 125, 250, 1000, 4000 },
+		.alarm = true,
+	},
 	[tcn75] = {
 		.default_resolution = 9,
 		.default_sample_time = MSEC_PER_SEC / 18,
@@ -420,6 +431,9 @@ static int lm75_read(struct device *dev, enum hwmon_sensor_types type,
 			case tmp112:
 				*val = !!(regval & BIT(13)) == !!(regval & BIT(2));
 				break;
+			case max31875:
+				*val = !!(regval & BIT(7));
+				break;
 			default:
 				return -EINVAL;
 			}
@@ -494,6 +508,13 @@ static int lm75_update_interval(struct device *dev, long val)
 			return err;
 		data->sample_time = data->params->sample_times[index];
 		break;
+	case max31875:
+		err = regmap_update_bits(data->regmap, LM75_REG_CONF,
+					 0x0600, (3 - index) << 9);
+		if (err < 0)
+			return err;
+		data->sample_time = data->params->sample_times[index];
+		break;
 	case pct2075:
 		err = regmap_write(data->regmap, PCT2075_REG_IDLE, index + 1);
 		if (err)
@@ -770,7 +791,9 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 	data->orig_conf = status;
 
 	/* Enforce polarity active-low (default) or active-high (devicetree) */
-	if (!data->params->config_reg_16bits)
+	if (data->kind == max31875)
+		pol_mask = 0;	/* no alert polarity bit */
+	else if (!data->params->config_reg_16bits)
 		pol_mask = LM75_ALERT_POLARITY_HIGH_8_BIT;
 	else
 		pol_mask = LM75_ALERT_POLARITY_HIGH_16_BIT;
@@ -847,6 +870,7 @@ static const struct i2c_device_id lm75_i2c_ids[] = {
 	{ .name = "max6626", .driver_data = max6626 },
 	{ .name = "max31725", .driver_data = max31725 },
 	{ .name = "max31726", .driver_data = max31725 },
+	{ .name = "max31875", .driver_data = max31875 },
 	{ .name = "mcp980x", .driver_data = mcp980x },
 	{ .name = "p3t1750", .driver_data = p3t1750 },
 	{ .name = "p3t1755", .driver_data = p3t1755 },
@@ -904,6 +928,10 @@ static const struct of_device_id lm75_of_match[] = {
 		.compatible = "adi,adt75",
 		.data = (void *)adt75
 	},
+	{
+		.compatible = "adi,max31875",
+		.data = (void *)max31875
+	},
 	{
 		.compatible = "ams,as6200",
 		.data = (void *)as6200
-- 
2.34.1

Re: [PATCH v4 1/3] dt-bindings: hwmon: lm75: add MAX31875

[sashiko-bot]

> Add MAX31875 compatible string to lm75 dt-bindings.
> 
> The MAX31875 cannot reuse an existing lm75-family compatible such as
> adi,adt75: it has a 16-bit configuration register (instead of 8-bit),
> four independently selectable conversion rates (125 ms, 250 ms, 1 s,
> 4 s) and four independently selectable resolutions (8, 9, 10, 12
> bits), and reports its alarm through a different configuration
> register bit. These differences require dedicated handling in the
> lm75 driver, so a distinct compatible string is needed to select it.
> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261023091550.1494-1-johnerasmusmari.geronimo@analog.com?part=1

Re: [PATCH v4 2/3] hwmon: (lm75) add MAX31875 documentation

[sashiko-bot]

> Document the Maxim MAX31875 I2C temperature sensor as a supported
> chip in the lm75 driver documentation.
> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261023091550.1494-1-johnerasmusmari.geronimo@analog.com?part=2

Re: [PATCH v4 3/3] hwmon: (lm75) add MAX31875 support

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] hwmon: (lm75) wrong shutdown bit used for max31875 in probe and PM callbacks
--

commit 35ce43dc6fc53387d6a1aa3a7019a45b202c43af
Author: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

hwmon: (lm75) add MAX31875 support

This commit adds support for the Analog Devices / Maxim MAX31875 I2C
temperature sensor to the lm75 hwmon driver, configuring its 16-bit
configuration register, resolution, conversion rate, and alarm bit.

> diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
> index 2d2d752aeac9d..349648fd08e15 100644
> --- a/drivers/hwmon/lm75.c
> +++ b/drivers/hwmon/lm75.c
[ ... ]
> @@ -222,6 +223,16 @@ static const struct lm75_params device_params[] = {
>  		.default_resolution = 16,
>  		.default_sample_time = MSEC_PER_SEC / 20,
>  	},
> +	[max31875] = {
> +		.config_reg_16bits = true,
> +		.set_mask = 2 << 13,	/* 10-bit resolution */
> +		.clr_mask = 3 << 13 | 3 << 9,	/* resolution, 4 s conversion rate */
> +		.default_resolution = 10,
> +		.default_sample_time = 4000,
> +		.num_sample_times = 4,
> +		.sample_times = (unsigned int []){ 125, 250, 1000, 4000 },
> +		.alarm = true,
> +	},

[Severity: Medium]
Does the driver manipulate the wrong shutdown bit for max31875 during
probe and system suspend/resume?

For devices with config_reg_16bits set to true, lm75_i2c_reg_read() and
lm75_i2c_reg_write() use i2c_smbus_read_word_data() and
i2c_smbus_write_word_data(), placing the first I2C byte (datasheet D15:D8)
into bits 7:0 of the register value and the second I2C byte (datasheet
D7:D0) into bits 15:8.

Under this layout, MAX31875 places One-Shot (D8) at BIT(0) and Shutdown
(D0) at BIT(8), whereas LM75_SHUTDOWN is defined as 0x01 (BIT(0)).

In drivers/hwmon/lm75.c, lm75_generic_probe() calls lm75_write_config():

static inline int lm75_write_config(struct lm75_data *data, u16 set_mask,
				    u16 clr_mask)
{
	return regmap_update_bits(data->regmap, LM75_REG_CONF,
				  clr_mask | set_mask | LM75_SHUTDOWN, set_mask);
}

And the power management callbacks in drivers/hwmon/lm75.c also use
LM75_SHUTDOWN:

static int lm75_suspend(struct device *dev)
{
	struct lm75_data *data = dev_get_drvdata(dev);

	return regmap_update_bits(data->regmap, LM75_REG_CONF, LM75_SHUTDOWN, LM75_SHUTDOWN);
}

static int lm75_resume(struct device *dev)
{
	struct lm75_data *data = dev_get_drvdata(dev);

	return regmap_update_bits(data->regmap, LM75_REG_CONF, LM75_SHUTDOWN, 0);
}

Because device_params[max31875].clr_mask does not include BIT(8) and
lm75_write_config(), lm75_suspend(), and lm75_resume() operate on
LM75_SHUTDOWN (BIT(0)), won't system suspend and resume toggle the
One-Shot bit instead of placing the chip into shutdown mode, and won't
lm75_generic_probe() fail to clear the shutdown bit if the sensor was
already in shutdown mode prior to probe?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261023091550.1494-1-johnerasmusmari.geronimo@analog.com?part=3

Re: [PATCH v4 1/3] dt-bindings: hwmon: lm75: add MAX31875

[Guenter Roeck]

On Fri, Oct 23, 2026 at 05:15:48PM +0800, John Erasmus Mari Geronimo wrote:
> Add MAX31875 compatible string to lm75 dt-bindings.
> 
> The MAX31875 cannot reuse an existing lm75-family compatible such as
> adi,adt75: it has a 16-bit configuration register (instead of 8-bit),
> four independently selectable conversion rates (125 ms, 250 ms, 1 s,
> 4 s) and four independently selectable resolutions (8, 9, 10, 12
> bits), and reports its alarm through a different configuration
> register bit. These differences require dedicated handling in the
> lm75 driver, so a distinct compatible string is needed to select it.
> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>
> Acked-by: Conor Dooley <conor.dooley@microchip.com>

Applied.

Thanks,
Guenter

Re: [PATCH v4 2/3] hwmon: (lm75) add MAX31875 documentation

[Guenter Roeck]

On Fri, Oct 23, 2026 at 05:15:49PM +0800, John Erasmus Mari Geronimo wrote:
> Document the Maxim MAX31875 I2C temperature sensor as a supported
> chip in the lm75 driver documentation.
> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

Applied.

Thanks
Guenter

Re: [PATCH v4 3/3] hwmon: (lm75) add MAX31875 support

[Guenter Roeck]

On Fri, Oct 23, 2026 at 05:15:50PM +0800, John Erasmus Mari Geronimo wrote:
> Add support for the Maxim MAX31875 I2C temperature sensor by
> extending the lm75 driver. The MAX31875 features a 16-bit
> configuration register, configurable resolution (8, 9, 10, or
> 12 bits), and selectable conversion rates (125ms, 250ms, 1s, 4s).
> 
> The MAX31875 has no alert polarity bit, and the bit the driver would
> use for 16-bit chips is part of its conversion rate field, so skip
> the polarity configuration for it. Also set the default resolution
> and conversion rate at probe so the driver state matches the chip.
> 
> Suggested-by: Guenter Roeck <linux@roeck-us.net>
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

Applied.

Thanks,
Guenter