Refuse assigning SBUS output to a second serial port

docs/Serial.md

@@ -48,6 +48,7 @@ e.g. after configuring a port for GPS enable the GPS feature.
 * All telemetry systems except MSP will ignore any attempts to override the baudrate.
 * MSP/CLI can be shared with EITHER Blackbox OR telemetry (LTM or MAVlink, not RX telemetry).  In shared mode blackbox or telemetry will be output only when armed.
 * Smartport telemetry cannot be shared with MSP.
+* SBUS servo output (`SERVO_SERIAL`) can only be assigned to one port. The CLI refuses a second assignment; previously it was accepted and shown in the port list, but the second port was never opened.
 * No other serial port sharing combinations are valid.
 * You can use as many different telemetry systems as you like at the same time.
 * You can only use each telemetry system once.  e.g.  FrSky telemetry cannot be used on two port, but LTN Telemetry and FrSky on two different ports is fine.

src/main/fc/cli.c

@@ -929,6 +929,12 @@ static void cliSerial(char *cmdline)
         return;
     }
 
+    const uint32_t duplicatedFunctions = serialDuplicatedSinglePortFunctions(&portConfig);
+    if (duplicatedFunctions) {
+        cliPrintErrorLinef("Function %d is already assigned to another port", (int)duplicatedFunctions);
+        return;
+    }
+
     memcpy(currentConfig, &portConfig, sizeof(portConfig));
 }
 

src/main/io/serial.c

@@ -313,6 +313,25 @@ bool isSerialConfigValid(const serialConfig_t *serialConfigToCheck)
     return true;
 }
 
+// Functions that are resolved with findSerialPortConfig() and therefore only ever use the
+// first port they are assigned to. Assigning one of them to a second port has no effect at
+// all, so the configuration layer refuses it instead of ignoring it silently.
+#define SERIAL_SINGLE_PORT_FUNCTIONS (FUNCTION_SERVO_SERIAL)
+
+uint32_t serialDuplicatedSinglePortFunctions(const serialPortConfig_t *portConfigToCheck)
+{
+    uint32_t functionMaskOfOtherPorts = 0;
+
+    for (int index = 0; index < SERIAL_PORT_COUNT; index++) {
+        const serialPortConfig_t *portConfig = &serialConfig()->portConfigs[index];
+        if (portConfig->identifier != portConfigToCheck->identifier) {
+            functionMaskOfOtherPorts |= portConfig->functionMask;
+        }
+    }
+
+    return portConfigToCheck->functionMask & functionMaskOfOtherPorts & SERIAL_SINGLE_PORT_FUNCTIONS;
+}
+
 serialPortConfig_t *serialFindPortConfiguration(serialPortIdentifier_e identifier)
 {
     for (int index = 0; index < SERIAL_PORT_COUNT; index++) {

src/main/io/serial.h

@@ -159,6 +159,7 @@ void serialRemovePort(serialPortIdentifier_e identifier);
 uint8_t serialGetAvailablePortCount(void);
 bool serialIsPortAvailable(serialPortIdentifier_e identifier);
 bool isSerialConfigValid(const serialConfig_t *serialConfig);
+uint32_t serialDuplicatedSinglePortFunctions(const serialPortConfig_t *portConfigToCheck);
 serialPortConfig_t *serialFindPortConfiguration(serialPortIdentifier_e identifier);
 bool doesConfigurationUsePort(serialPortIdentifier_e portIdentifier);
 serialPortConfig_t *findSerialPortConfig(serialPortFunction_e function);