fix(settings): store fan trip temperatures with a decimal so F round-trips

Fan temperatures are stored in C because that is what the kernel wants,
but a whole degree C cannot name every whole degree F: 85F rounds to
29C, which reads back as 84F. 60 of the 136 F values in the settable
range drift a degree that way, so a value entered in F frequently -- but
not always -- came back changed after a page reload.

The saved value now carries one decimal place. A tenth of a degree C is
0.18F, so the worst-case storage error is 0.09F and every whole degree F
survives the round trip; a value entered in C is still a whole number.

Two consumers assumed an integer and had to be fixed with it:

- applyThermalSettings() read the setting with getRawSettingInt(), whose
  hand-rolled digit loop does not stop at a decimal point and turns
  "29.4" into 2884. The kernel accepts the resulting 2884000-millidegree
  trip write, so the fan would simply have stopped turning on after the
  next boot. It now reads the raw string and parses a float.

- SetGPIOFanProperties() built millidegrees for the gpio-fan overlay by
  appending "000" to the setting; it now multiplies.

Displaying in C also sets step="any" on the input, since a value holding
a tenth is invalid against the default step of 1. Existing whole-degree
values stay valid and stable -- they just aren't exact until re-entered.

Fixes #2840

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: dkulp

src/boot/FPPINIT_Config.cpp

@@ -13,6 +13,7 @@
 #include <chrono>
 #include "fpp-json.h"
 #include <cctype>
+#include <cmath>
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
@@ -410,10 +411,18 @@ void applyThermalSettings(bool captureDefaults) {
                 defaultsChanged = true;
             }
         }
-        int temp = getRawSettingInt("FanTrip_" + key, -1);
-        if (temp > 0) {
-            printf("FPP - Setting fan trip point %s to %dC\n", key.c_str(), temp);
-            PutFileContents(tripFile, std::to_string(temp * 1000));
+        // Read as a string and parse a float: the setting holds degrees C to one
+        // decimal place so that whole degrees F survive the conversion both ways
+        // (see PutSetting() in www/api/controllers/settings.php).  getRawSettingInt()
+        // must not be used here - its hand-rolled digit loop does not stop at the
+        // decimal point and would turn "29.4" into 2884.
+        std::string tempStr;
+        if (getRawSetting("FanTrip_" + key, tempStr)) {
+            double temp = strtod(tempStr.c_str(), nullptr);
+            if (temp > 0) {
+                printf("FPP - Setting fan trip point %s to %.1fC\n", key.c_str(), temp);
+                PutFileContents(tripFile, std::to_string((int) std::lround(temp * 1000.0)));
+            }
         }
     });
     if (defaultsChanged) {

www/api/controllers/settings.php

@@ -121,7 +121,15 @@ function PutSetting()
     $setting = params('SettingName');
 
     if (($setting == 'GPIOFanTemperature' || str_starts_with($setting, 'FanTrip_')) && isset($settings['temperatureInF']) && $settings['temperatureInF'] == 1) {
-        $value = round(($value - 32) * 5 / 9);
+        // Store one decimal place, not a whole degree C.  A whole degree C is too
+        // coarse to name every whole degree F: 85F rounds to 29C, which reads back
+        // as 84F, and 60 of the 136 values in range drift like that.  A tenth of a
+        // degree C is 0.18F, so the worst case error is 0.09F and every whole
+        // degree F survives the round trip.  Consumers of the stored value must
+        // therefore parse a float - see applyThermalSettings() in
+        // src/boot/FPPINIT_Config.cpp and SetGPIOFanProperties() in
+        // www/common/settings.php.  A value entered in C stays a whole number.
+        $value = round(($value - 32) * 5 / 9, 1);
     }
 
     // A failed write must not fall through to the apply/restart steps below:

www/common.php

@@ -821,7 +821,7 @@ function PrintFanThermalSettings()
                 'description' => $desc,
                 'tip' => $tip,
                 'type' => 'number',
-                'default' => (int) round($currentTemp),
+                'default' => round($currentTemp, 1),
                 'min' => 20,
                 'max' => 95,
                 'step' => 1,
@@ -1096,13 +1096,25 @@ function PrintSetting($setting, $callback = '', $options = array(), $plugin = ''
                 $step = isset($s['step']) ? $s['step'] : 1;
                 $default = isset($s['default']) ? $s['default'] : "0";
 
-                if (($setting == 'GPIOFanTemperature' || str_starts_with($setting, 'FanTrip_')) && isset($settings['temperatureInF']) && $settings['temperatureInF'] == 1) {
-                    $origVal = isset($settings[$setting]) ? $settings[$setting] : $default;
-                    $settings[$setting] = round(($origVal * 9 / 5) + 32);
-                    $min = round(($min * 9 / 5) + 32);
-                    $max = round(($max * 9 / 5) + 32);
-                    $default = round(($default * 9 / 5) + 32);
-                    $suffix = ' F';
+                // Temperature settings are always stored in C because that is what the
+                // kernel wants, but may be entered and displayed in F.  A whole number
+                // of C cannot represent every whole number of F, so the stored value is
+                // allowed a decimal (see PutSetting()); without it 60 of the 136 usable
+                // F values come back a degree lower after a save/reload round trip.
+                if ($setting == 'GPIOFanTemperature' || str_starts_with($setting, 'FanTrip_')) {
+                    if (isset($settings['temperatureInF']) && $settings['temperatureInF'] == 1) {
+                        $origVal = isset($settings[$setting]) ? $settings[$setting] : $default;
+                        $settings[$setting] = round(($origVal * 9 / 5) + 32);
+                        $min = round(($min * 9 / 5) + 32);
+                        $max = round(($max * 9 / 5) + 32);
+                        $default = round(($default * 9 / 5) + 32);
+                        $suffix = ' F';
+                    } else {
+                        // Displaying C: the value may carry the tenth left behind by an
+                        // earlier entry in F, which a step of 1 would flag as invalid.
+                        // "any" still steps the spinner arrows by 1.
+                        $s['step'] = 'any';
+                    }
                 }
 
                 PrintSettingTextSaved($setting, $restart, $reboot, $max, $min, $plugin, $default, $callback, '', 'number', $s);

www/common/settings.php

@@ -291,12 +291,14 @@ function SetGPIOFanProperties()
 {
     global $settings;
     $fanOn = ReadSettingFromFile('GPIOFan');
-    $fanTemp = ReadSettingFromFile('GPIOFanTemperature') . "000";
+    // The overlay wants millidegrees.  Multiply rather than appending "000":
+    // the setting is degrees C to one decimal place so that whole degrees F
+    // survive the conversion both ways (see PutSetting() in
+    // www/api/controllers/settings.php), and "29.4" . "000" is not a number.
+    $fanTempSetting = ReadSettingFromFile('GPIOFanTemperature');
+    $fanTemp = is_numeric($fanTempSetting) ? (string) ((int) round(floatval($fanTempSetting) * 1000)) : '70000';
     $pfx = "";
 
-    if ($fanTemp == '000') {
-        $fanTemp = '70000';
-    }
     if ($fanOn == '0') {
         $pfx = "#";
     }