realtek: hwmon: add LM75 alert pin polarity swap patch

Allow to configure the LM75 alert pin to active-high instead
of its default active-low. This patch is needed for the D-Link
DGS-1250 series where the alert pin steers the fan speed
between low and high.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22589
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

diff --git a/target/linux/realtek/patches-6.12/810-lm75-alert-polarity-swap.patch b/target/linux/realtek/patches-6.12/810-lm75-alert-polarity-swap.patch
new file mode 100644 (file)
index 0000000..694640a
--- /dev/null
+++ b/target/linux/realtek/patches-6.12/810-lm75-alert-polarity-swap.patch
@@ -0,0 +1,59 @@
+From 92ea53e058d40ebda7326016494e3c21dc536c53 Mon Sep 17 00:00:00 2001
+From: Markus Stockhausen <markus.stockhausen@gmx.de>
+Date: Mon, 23 Mar 2026 22:12:01 +0200
+Subject: [PATCH] hwmon: lm75 alert polarity swap
+
+The LM75 can steer the alert polarity. In default mode the alert
+output pin is active-low. This can not be configured with the
+existing LM75 driver. 
+
+There are hardware designs that use this alert output for an automatic 
+fan speed control. E.g. the D-Link DGS-1250. This works as follows
+
+- temperature below Tmax threshold -> alert pin low -> fan slow speed
+- temperature above Tmax threshold -> alert pin high -> fan high speed
+
+As one can see the hardware design requires the alert pin to be 
+configured in mode active-high to work as described. Add a LM75 DTS 
+property "alert-polarity-active-high" that allows to swap the alert 
+pin behaviour during initialization to active-high.
+
+Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
+
+--- a/drivers/hwmon/lm75.c
++++ b/drivers/hwmon/lm75.c
+@@ -121,7 +121,8 @@ 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_SAMPLE_CLEAR_MASK                (3 << 5)
++#define LM75_ALERT_POLARITY_HIGH      BIT(2)
+ 
+ /* The structure below stores the configuration values of the supported devices.
+  * In case of being supported multiple configurations, the default one must
+@@ -631,6 +632,7 @@ static int lm75_probe(struct i2c_client
+       struct device *hwmon_dev;
+       struct lm75_data *data;
+       int status, err;
++      u16 set_mask;
+ 
+       if (!i2c_check_functionality(client->adapter,
+                       I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA))
+@@ -680,9 +682,14 @@ static int lm75_probe(struct i2c_client
+       }
+       data->orig_conf = status;
+       data->current_conf = status;
++      
++      set_mask = data->params->set_mask;
++      if (of_property_read_bool(dev->of_node, "alert-polarity-active-high")) {
++              pr_info("set lm75 alert to active high\n");
++              set_mask |= LM75_ALERT_POLARITY_HIGH;
++      }
+ 
+-      err = lm75_write_config(data, data->params->set_mask,
+-                              data->params->clr_mask);
++      err = lm75_write_config(data, set_mask, data->params->clr_mask);
+       if (err)
+               return err;
+