[PATCH 0/2] Support active-high alert polarity for LM75

[PATCH 0/2] Support active-high alert polarity for LM75

[Markus Stockhausen]

The LM75 configuration register allows to switch the alert polarity.
In default mode the alert output pin is active-low. There are hardware
designs that use this alert output for an hardware assisted automatic
fan speed control. E.g. the D-Link DGS-1250. Implementation is.

- temperature below Tmax threshold -> alert pin low -> fan slow speed
- temperature above Tmax threshold -> alert pin high -> fan high speed

Provide a devicetree configuration option and a driver enhancement to
support these hardware designs.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

[PATCH 1/2] dt-bindings: hwmon: lm75: Add lm75,alert-polarity-active-high property

[Markus Stockhausen]

The LM75 alert pin is asserted based on the value of alert polarity bit of
the configuration register. The device/driver default is 0 which means alert
pin is configured to be active-low. A value of 1 maps to inverted (active-high).

Add an optional boolean property "lm75,alert-polarity-active-high" to
override the alert pin polarity. When absent, the default active-low
polarity is kept.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 Documentation/devicetree/bindings/hwmon/lm75.yaml | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/devicetree/bindings/hwmon/lm75.yaml b/Documentation/devicetree/bindings/hwmon/lm75.yaml
index 0b9fda81e3ec..173751a726f3 100644
--- a/Documentation/devicetree/bindings/hwmon/lm75.yaml
+++ b/Documentation/devicetree/bindings/hwmon/lm75.yaml
@@ -54,6 +54,13 @@ properties:
   interrupts:
     maxItems: 1
 
+  lm75,alert-polarity-active-high:
+    description: Alert pin is asserted based on the value of alert polarity
+      bit of configuration register. Default value is normal (0 which maps to
+      active-low). The other value is inverted (1 which maps to active-high).
+      Specify this property to set the alert polarity to active-high.
+    $ref: /schemas/types.yaml#/definitions/flag
+
 required:
   - compatible
   - reg
-- 
2.54.0

Re: [PATCH 1/2] dt-bindings: hwmon: lm75: Add lm75,alert-polarity-active-high property

[Conor Dooley]

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On Fri, May 01, 2026 at 02:05:17PM +0200, Markus Stockhausen wrote:
> The LM75 alert pin is asserted based on the value of alert polarity bit of
> the configuration register. The device/driver default is 0 which means alert
> pin is configured to be active-low. A value of 1 maps to inverted (active-high).
> 
> Add an optional boolean property "lm75,alert-polarity-active-high" to
> override the alert pin polarity. When absent, the default active-low
> polarity is kept.
> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
> ---
>  Documentation/devicetree/bindings/hwmon/lm75.yaml | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/hwmon/lm75.yaml b/Documentation/devicetree/bindings/hwmon/lm75.yaml
> index 0b9fda81e3ec..173751a726f3 100644
> --- a/Documentation/devicetree/bindings/hwmon/lm75.yaml
> +++ b/Documentation/devicetree/bindings/hwmon/lm75.yaml
> @@ -54,6 +54,13 @@ properties:
>    interrupts:
>      maxItems: 1
>  
> +  lm75,alert-polarity-active-high:

This should be "national" or "adi" as a prefix, lm75 isn't a vendor!

Otherwise, seems reasonable enough to me.

pw-bot: changes-requested

Cheers,
Conor.

> +    description: Alert pin is asserted based on the value of alert polarity
> +      bit of configuration register. Default value is normal (0 which maps to
> +      active-low). The other value is inverted (1 which maps to active-high).
> +      Specify this property to set the alert polarity to active-high.
> +    $ref: /schemas/types.yaml#/definitions/flag
> +
>  required:
>    - compatible
>    - reg
> -- 
> 2.54.0
> 

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[PATCH 2/2] hwmon: (lm75) Support active-high alert polarity

[Markus Stockhausen]

All chips supported by this driver support configurable active-high
alert polarity. This is already documented in the devicetree description.
Add support for it to the driver.

The default polarity of the devices is 0 (active-low). So there is
no need to change the clear mask. For consistency reasons adapt the
hard-coded configuration value of the AS6200. It is the only device
that defaults to active-high polarity.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/hwmon/lm75.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
index f1a1e5b888f6..4b075a7b1cd7 100644
--- a/drivers/hwmon/lm75.c
+++ b/drivers/hwmon/lm75.c
@@ -123,7 +123,9 @@ struct lm75_data {
 
 static const u8 lm75_sample_set_masks[] = { 0 << 5, 1 << 5, 2 << 5, 3 << 5 };
 
-#define LM75_SAMPLE_CLEAR_MASK	(3 << 5)
+#define LM75_ALERT_POLARITY_HIGH_8_BIT	(BIT(2))
+#define LM75_ALERT_POLARITY_HIGH_16_BIT	(BIT(2) << 8)
+#define LM75_SAMPLE_CLEAR_MASK		(3 << 5)
 
 /* The structure below stores the configuration values of the supported devices.
  * In case of being supported multiple configurations, the default one must
@@ -137,7 +139,7 @@ static const struct lm75_params device_params[] = {
 	},
 	[as6200] = {
 		.config_reg_16bits = true,
-		.set_mask = 0x94C0,	/* 8 sample/s, 4 CF, positive polarity */
+		.set_mask = 0x90C0,	/* 8 sample/s, 4 CF */
 		.default_resolution = 12,
 		.default_sample_time = 125,
 		.num_sample_times = 4,
@@ -728,6 +730,7 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 	struct device *hwmon_dev;
 	struct lm75_data *data;
 	int status, err;
+	u16 set_mask;
 
 	data = devm_kzalloc(dev, sizeof(struct lm75_data), GFP_KERNEL);
 	if (!data)
@@ -762,8 +765,15 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 		return err;
 	data->orig_conf = status;
 
-	err = lm75_write_config(data, data->params->set_mask,
-				data->params->clr_mask);
+	set_mask = data->params->set_mask;
+	if (of_property_read_bool(dev->of_node, "lm75,alert-polarity-active-high")) {
+		if (!data->params->config_reg_16bits)
+			set_mask |= LM75_ALERT_POLARITY_HIGH_8_BIT;
+		else
+			set_mask |= LM75_ALERT_POLARITY_HIGH_16_BIT;
+	}
+
+	err = lm75_write_config(data, set_mask, data->params->clr_mask);
 	if (err)
 		return err;
 
-- 
2.54.0

Re: [PATCH 2/2] hwmon: (lm75) Support active-high alert polarity

[Conor Dooley]

[-- Attachment #1: Type: text/plain, Size: 2774 bytes --]

On Fri, May 01, 2026 at 02:05:18PM +0200, Markus Stockhausen wrote:
> All chips supported by this driver support configurable active-high
> alert polarity. This is already documented in the devicetree description.
> Add support for it to the driver.
> 
> The default polarity of the devices is 0 (active-low). So there is
> no need to change the clear mask. For consistency reasons adapt the
> hard-coded configuration value of the AS6200. It is the only device
> that defaults to active-high polarity.

Uh, I dunno if you can do this, changing defaults is an ABI break
typically. What makes it okay to do that in this case?

> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
> ---
>  drivers/hwmon/lm75.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
> index f1a1e5b888f6..4b075a7b1cd7 100644
> --- a/drivers/hwmon/lm75.c
> +++ b/drivers/hwmon/lm75.c
> @@ -123,7 +123,9 @@ struct lm75_data {
>  
>  static const u8 lm75_sample_set_masks[] = { 0 << 5, 1 << 5, 2 << 5, 3 << 5 };
>  
> -#define LM75_SAMPLE_CLEAR_MASK	(3 << 5)
> +#define LM75_ALERT_POLARITY_HIGH_8_BIT	(BIT(2))
> +#define LM75_ALERT_POLARITY_HIGH_16_BIT	(BIT(2) << 8)
> +#define LM75_SAMPLE_CLEAR_MASK		(3 << 5)
>  
>  /* The structure below stores the configuration values of the supported devices.
>   * In case of being supported multiple configurations, the default one must
> @@ -137,7 +139,7 @@ static const struct lm75_params device_params[] = {
>  	},
>  	[as6200] = {
>  		.config_reg_16bits = true,
> -		.set_mask = 0x94C0,	/* 8 sample/s, 4 CF, positive polarity */
> +		.set_mask = 0x90C0,	/* 8 sample/s, 4 CF */
>  		.default_resolution = 12,
>  		.default_sample_time = 125,
>  		.num_sample_times = 4,
> @@ -728,6 +730,7 @@ static int lm75_generic_probe(struct device *dev, const char *name,
>  	struct device *hwmon_dev;
>  	struct lm75_data *data;
>  	int status, err;
> +	u16 set_mask;
>  
>  	data = devm_kzalloc(dev, sizeof(struct lm75_data), GFP_KERNEL);
>  	if (!data)
> @@ -762,8 +765,15 @@ static int lm75_generic_probe(struct device *dev, const char *name,
>  		return err;
>  	data->orig_conf = status;
>  
> -	err = lm75_write_config(data, data->params->set_mask,
> -				data->params->clr_mask);
> +	set_mask = data->params->set_mask;
> +	if (of_property_read_bool(dev->of_node, "lm75,alert-polarity-active-high")) {
> +		if (!data->params->config_reg_16bits)
> +			set_mask |= LM75_ALERT_POLARITY_HIGH_8_BIT;
> +		else
> +			set_mask |= LM75_ALERT_POLARITY_HIGH_16_BIT;
> +	}
> +
> +	err = lm75_write_config(data, set_mask, data->params->clr_mask);
>  	if (err)
>  		return err;
>  
> -- 
> 2.54.0
> 

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Re: [PATCH 2/2] hwmon: (lm75) Support active-high alert polarity

[Guenter Roeck]

On 5/1/26 10:05, Conor Dooley wrote:
> On Fri, May 01, 2026 at 02:05:18PM +0200, Markus Stockhausen wrote:
>> All chips supported by this driver support configurable active-high
>> alert polarity. This is already documented in the devicetree description.
>> Add support for it to the driver.
>>
>> The default polarity of the devices is 0 (active-low). So there is
>> no need to change the clear mask. For consistency reasons adapt the
>> hard-coded configuration value of the AS6200. It is the only device
>> that defaults to active-high polarity.
> 
> Uh, I dunno if you can do this, changing defaults is an ABI break
> typically. What makes it okay to do that in this case?
> 

Turns out the driver doesn't actually set the polarity bit due to a driver
bug, and no one noticed. Also, _if_ active high polarity is set, the _alarm
attribute shows the wrong value due to a second bug in the driver.
Given all that, there is no real ABI breakage.

Guenter

Re: [PATCH 2/2] hwmon: (lm75) Support active-high alert polarity

[Conor Dooley]

[-- Attachment #1: Type: text/plain, Size: 1087 bytes --]

On Fri, May 01, 2026 at 12:32:47PM -0700, Guenter Roeck wrote:
> On 5/1/26 10:05, Conor Dooley wrote:
> > On Fri, May 01, 2026 at 02:05:18PM +0200, Markus Stockhausen wrote:
> > > All chips supported by this driver support configurable active-high
> > > alert polarity. This is already documented in the devicetree description.
> > > Add support for it to the driver.
> > > 
> > > The default polarity of the devices is 0 (active-low). So there is
> > > no need to change the clear mask. For consistency reasons adapt the
> > > hard-coded configuration value of the AS6200. It is the only device
> > > that defaults to active-high polarity.
> > 
> > Uh, I dunno if you can do this, changing defaults is an ABI break
> > typically. What makes it okay to do that in this case?
> > 
> 
> Turns out the driver doesn't actually set the polarity bit due to a driver
> bug, and no one noticed. Also, _if_ active high polarity is set, the _alarm
> attribute shows the wrong value due to a second bug in the driver.
> Given all that, there is no real ABI breakage.

Cool :)

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[PATCH v2 0/2] Support active-high alert polarity for LM75

[Markus Stockhausen]

The LM75 configuration register allows to switch the alert polarity.
In default mode the alert output pin is active-low. There are hardware
designs that use this alert output for an hardware assisted automatic
fan speed control. E.g. the D-Link DGS-1250 implements

- temperature below Tmax threshold -> alert pin low -> fan slow speed
- temperature above Tmax threshold -> alert pin high -> fan high speed

Provide a devicetree configuration option and a driver enhancement to
support these hardware designs.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

---

Changes in v2
- Carve out AS6200 polarity fix into separate series
- Rename devicetree prefix from lm75 to ti

[PATCH 2/2] hwmon: (lm75) Support active-high alert polarity

[Markus Stockhausen]

LM75 devices supported by this driver support configurable active-high
alert polarity. This is already documented in the devicetree description.
Add support for it to the driver.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/hwmon/lm75.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
index 8b74cc314196..9542f04e215e 100644
--- a/drivers/hwmon/lm75.c
+++ b/drivers/hwmon/lm75.c
@@ -123,7 +123,9 @@ struct lm75_data {
 
 static const u8 lm75_sample_set_masks[] = { 0 << 5, 1 << 5, 2 << 5, 3 << 5 };
 
-#define LM75_SAMPLE_CLEAR_MASK	(3 << 5)
+#define LM75_ALERT_POLARITY_HIGH_8_BIT	(BIT(2))
+#define LM75_ALERT_POLARITY_HIGH_16_BIT	(BIT(2) << 8)
+#define LM75_SAMPLE_CLEAR_MASK		(3 << 5)
 
 /* The structure below stores the configuration values of the supported devices.
  * In case of being supported multiple configurations, the default one must
@@ -728,6 +730,7 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 	struct device *hwmon_dev;
 	struct lm75_data *data;
 	int status, err;
+	u16 set_mask;
 
 	data = devm_kzalloc(dev, sizeof(struct lm75_data), GFP_KERNEL);
 	if (!data)
@@ -762,8 +765,15 @@ static int lm75_generic_probe(struct device *dev, const char *name,
 		return err;
 	data->orig_conf = status;
 
-	err = lm75_write_config(data, data->params->set_mask,
-				data->params->clr_mask);
+	set_mask = data->params->set_mask;
+	if (of_property_read_bool(dev->of_node, "ti,alert-polarity-active-high")) {
+		if (!data->params->config_reg_16bits)
+			set_mask |= LM75_ALERT_POLARITY_HIGH_8_BIT;
+		else
+			set_mask |= LM75_ALERT_POLARITY_HIGH_16_BIT;
+	}
+
+	err = lm75_write_config(data, set_mask, data->params->clr_mask);
 	if (err)
 		return err;
 
-- 
2.54.0