[PATCH v3 0/2] Support active-high alert polarity for LM75

[PATCH v3 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 v3
- Add Acked-by from Conor Dooley for devicetree commit
- Use device_property_read_bool()
- Match documentation and always overwrite clr_mask during setup
  to enforce default polarity active-low

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

[PATCH v3 1/2] dt-bindings: hwmon: lm75: Add ti,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 "ti,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>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
---
 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..b48bf3fd721f 100644
--- a/Documentation/devicetree/bindings/hwmon/lm75.yaml
+++ b/Documentation/devicetree/bindings/hwmon/lm75.yaml
@@ -54,6 +54,13 @@ properties:
   interrupts:
     maxItems: 1
 
+  ti,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

[PATCH v3 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.

Follow documentation and defensively enforce active-low if property is
not set. This avoids possible inconsistencies for future devices with
wrong parametrization. No API breakage as all current devices have
their parameters set to active-low.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
---
 drivers/hwmon/lm75.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/drivers/hwmon/lm75.c b/drivers/hwmon/lm75.c
index 8b74cc314196..391e6dfcc6b2 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
@@ -725,6 +727,7 @@ static void lm75_remove(void *data)
 static int lm75_generic_probe(struct device *dev, const char *name,
 			      enum lm75_type kind, int irq, struct regmap *regmap)
 {
+	u16 clr_mask, pol_mask, set_mask;
 	struct device *hwmon_dev;
 	struct lm75_data *data;
 	int status, err;
@@ -762,8 +765,18 @@ 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);
+	/* Enforce polarity active-low (default) or active-high (devicetree) */
+	if (!data->params->config_reg_16bits)
+		pol_mask = LM75_ALERT_POLARITY_HIGH_8_BIT;
+	else
+		pol_mask = LM75_ALERT_POLARITY_HIGH_16_BIT;
+
+	clr_mask = data->params->clr_mask | pol_mask;
+	set_mask = data->params->set_mask & ~pol_mask;
+	if (device_property_read_bool(dev, "ti,alert-polarity-active-high"))
+		set_mask |= pol_mask;
+
+	err = lm75_write_config(data, set_mask, clr_mask);
 	if (err)
 		return err;
 
-- 
2.54.0

Re: [PATCH v3 1/2] dt-bindings: hwmon: lm75: Add ti,alert-polarity-active-high property

[Guenter Roeck]

On Mon, May 04, 2026 at 05:10:19PM +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 "ti,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>
> Acked-by: Conor Dooley <conor.dooley@microchip.com>

Applied, after shortening description line length to 75 columns.

Thanks,
Guenter

Re: [PATCH v3 2/2] hwmon: (lm75) Support active-high alert polarity

[Guenter Roeck]

On Mon, May 04, 2026 at 05:10:20PM +0200, Markus Stockhausen wrote:
> 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.
> 
> Follow documentation and defensively enforce active-low if property is
> not set. This avoids possible inconsistencies for future devices with
> wrong parametrization. No API breakage as all current devices have
> their parameters set to active-low.
> 
> Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>

Applied.

Thanks,
Guenter