[PATCH v2 0/3] hwmon: (lm75) add MAX31875 support

[PATCH v2 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.

As suggested by Guenter Roeck, this version reworks the implementation
to extend the existing lm75 driver instead of max31827, since the
MAX31875 is another LM75 variant (similar to max31725/max31726) rather
than a max31827-family part.

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                          | 25 +++++++++++++++++++
 4 files changed, 31 insertions(+), 3 deletions(-)


base-commit: 9955c92abe72564a49c293b1c15cd3b4f02ea6a0
-- 
2.34.1

[PATCH v2 1/3] dt-bindings: hwmon: lm75: add MAX31875

[John Erasmus Mari Geronimo]

Add MAX31875 compatible string to lm75 dt-bindings.

Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.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 b48bf3fd721fe..d56f3ec299105 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 v2 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 ca46754e028b3..54a6a75e8a71b 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 v2 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).

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  | 25 +++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
index e4c4f2b09732d..c68b6c3c2f866 100644
--- a/drivers/hwmon/Kconfig
+++ b/drivers/hwmon/Kconfig
@@ -1525,7 +1525,7 @@ 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 104149a03bad0..886f83c8b0420 100644
--- a/drivers/hwmon/lm75.c
+++ b/drivers/hwmon/lm75.c
@@ -39,6 +39,7 @@ enum lm75_type {		/* keep sorted in alphabetical order */
 	max6625,
 	max6626,
 	max31725,
+	max31875,
 	mcp980x,
 	p3t1750,
 	p3t1755,
@@ -223,6 +224,15 @@ static const struct lm75_params device_params[] = {
 		.default_resolution = 16,
 		.default_sample_time = MSEC_PER_SEC / 20,
 	},
+	[max31875] = {
+		.config_reg_16bits = true,
+		.default_resolution = 10,
+		.resolutions = (u8 []) {8, 9, 10, 12 },
+		.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,
@@ -421,6 +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 >> 7) & 0x1;
+				break;
 			default:
 				return -EINVAL;
 			}
@@ -495,6 +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)
@@ -848,6 +868,7 @@ 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 },
@@ -905,6 +926,10 @@ 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 v2 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. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924084121.82-1-johnerasmusmari.geronimo@analog.com?part=2

Re: [PATCH v2 1/3] dt-bindings: hwmon: lm75: add MAX31875

[sashiko-bot]

> Add MAX31875 compatible string to lm75 dt-bindings.
> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924084121.82-1-johnerasmusmari.geronimo@analog.com?part=1

Re: [PATCH v2 3/3] hwmon: (lm75) add MAX31875 support

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] hwmon: lm75: incorrect bitmasks due to 16-bit byte-swap for MAX31875
- [High] hwmon: lm75: missing resolution state and hardware update for MAX31875
--

commit db9c8895125f3a1241021e25bc19bc84126cb9c6
Author: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.com>

hwmon: (lm75) add MAX31875 support

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).

> diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
> index 104149a03bad0..886f83c8b0420 100644
> --- a/drivers/hwmon/lm75.c
> +++ b/drivers/hwmon/lm75.c

[ ... ]

> @@ -223,6 +224,15 @@ static const struct lm75_params device_params[] = {
>  		.default_resolution = 16,
>  		.default_sample_time = MSEC_PER_SEC / 20,
>  	},
> +	[max31875] = {
> +		.config_reg_16bits = true,
> +		.default_resolution = 10,
> +		.resolutions = (u8 []) {8, 9, 10, 12 },

[Severity: High]
Does the custom switch block for max31875 in lm75_update_interval() bypass
updating data->resolution, rendering this .resolutions mapping dead code?

Without updating data->resolution or the hardware resolution bits, the software
would truncate or misinterpret temperature data (since it remains stuck at 10
bits), and the hardware would be prevented from achieving higher sample rates.

> +		.default_sample_time = 4000,
> +		.num_sample_times = 4,
> +		.sample_times = (unsigned int []){ 125, 250, 1000, 4000 },
> +		.alarm = true,
> +	},

[ ... ]

> @@ -421,6 +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 >> 7) & 0x1;

[Severity: High]
Does this correctly extract the alarm bit? The SMBus word read returns
byte-swapped data (MSB in the lower byte, LSB in the upper byte).

Because of the swap, hardware bit D7 (alarm) maps to val bit 15. Shifting
regval by 7 incorrectly extracts D15 of the hardware register instead of D7,
returning garbage state when reading temp1_alarm via sysfs.

> +				break;
>  			default:
>  				return -EINVAL;
>  			}

[ ... ]

> @@ -495,6 +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);

[Severity: High]
Does the conversion rate mask 0x0600 apply directly to the byte-swapped regval?

With the swapped value, hardware bits D10/D9 map to bits 2/1 (mask 0x0006).
Using 0x0600 mistakenly clobbers the alarm polarity bit (D2) and fails to set
the conversion rate.

Also, as mentioned above, this block bypasses the default resolution update
logic (data->resolution = data->params->resolutions[index]) and never updates
the hardware resolution bits, silently breaking the configured resolution
mapping.

> +		if (err < 0)
> +			return err;
> +		data->sample_time = data->params->sample_times[index];
> +		break;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924084121.82-1-johnerasmusmari.geronimo@analog.com?part=3

Re: [PATCH v2 3/3] hwmon: (lm75) add MAX31875 support

[Guenter Roeck]

On Thu, Sep 24, 2026 at 04:41:21PM +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).
> 
> 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  | 25 +++++++++++++++++++++++++
>  2 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hwmon/Kconfig b/drivers/hwmon/Kconfig
> index e4c4f2b09732d..c68b6c3c2f866 100644
> --- a/drivers/hwmon/Kconfig
> +++ b/drivers/hwmon/Kconfig
> @@ -1525,7 +1525,7 @@ 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 104149a03bad0..886f83c8b0420 100644
> --- a/drivers/hwmon/lm75.c
> +++ b/drivers/hwmon/lm75.c
> @@ -39,6 +39,7 @@ enum lm75_type {		/* keep sorted in alphabetical order */
>  	max6625,
>  	max6626,
>  	max31725,
> +	max31875,
>  	mcp980x,
>  	p3t1750,
>  	p3t1755,
> @@ -223,6 +224,15 @@ static const struct lm75_params device_params[] = {
>  		.default_resolution = 16,
>  		.default_sample_time = MSEC_PER_SEC / 20,
>  	},
> +	[max31875] = {
> +		.config_reg_16bits = true,
> +		.default_resolution = 10,
> +		.resolutions = (u8 []) {8, 9, 10, 12 },

This isn't how resolutions vs. sample time is supposed to be handled.
For most of the lm75 compatible chips, the sample time is associated
with the resolution: Changing the sample time changes the resolution.
This is not the case here: the chip has separate configuration register
bits for sample time and resolution, and the resolution does not change
automatically if the sample time changes.  The above associates
8/9/10/12 bit resolution with the 125/250/1000/4000 sample times,
which is wrong. Instead, .resolutions should not be set at all,
reflecting the default 10 bit resolution.

> +		.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,
> @@ -421,6 +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 >> 7) & 0x1;

Please use
				*val = !!(regval & BIT(7));
to match the rest of the code. 

Thanks,
Guenter

> +				break;
>  			default:
>  				return -EINVAL;
>  			}
> @@ -495,6 +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)
> @@ -848,6 +868,7 @@ 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 },
> @@ -905,6 +926,10 @@ 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 v2 1/3] dt-bindings: hwmon: lm75: add MAX31875

[Conor Dooley]

[-- Attachment #1: Type: text/plain, Size: 993 bytes --]

On Thu, Sep 24, 2026 at 04:41:19PM +0800, John Erasmus Mari Geronimo wrote:
> Add MAX31875 compatible string to lm75 dt-bindings.

Commit message should explain why this is not compatible with exisiting
devices.
pw-bot: changes-requested
With one,
Acked-by: Conor Dooley <conor.dooley@microchip.com>

> 
> Signed-off-by: John Erasmus Mari Geronimo <johnerasmusmari.geronimo@analog.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 b48bf3fd721fe..d56f3ec299105 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
> 

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