diff --git a/src/soc/intel/cannonlake/fsp_params.c b/src/soc/intel/cannonlake/fsp_params.c
index e3a2310d5c..77d82d64f8 100644
--- a/src/soc/intel/cannonlake/fsp_params.c
+++ b/src/soc/intel/cannonlake/fsp_params.c
@@ -40,22 +40,28 @@ static const int serial_io_dev[] = {
 	PCH_DEVFN_UART2
 };
 
+/*
+ * Given an enum for PCH_SERIAL_IO_MODE, 1 needs to be subtracted to get the FSP
+ * UPD expected value for Serial IO since valid enum index starts from 1.
+ */
+#define PCH_SERIAL_IO_INDEX(x)		((x) - 1)
+
 static uint8_t get_param_value(const config_t *config, uint32_t dev_offset)
 {
 	struct device *dev;
 
 	dev = dev_find_slot(0, serial_io_dev[dev_offset]);
 	if (!dev || !dev->enabled)
-		return PchSerialIoDisabled;
+		return PCH_SERIAL_IO_INDEX(PchSerialIoDisabled);
 
 	if ((config->SerialIoDevMode[dev_offset] >= PchSerialIoMax) ||
 	    (config->SerialIoDevMode[dev_offset] == PchSerialIoNotInitialized))
-		return PchSerialIoPci;
+		return PCH_SERIAL_IO_INDEX(PchSerialIoPci);
 
 	/*
 	 * Correct Enum index starts from 1, so subtract 1 while returning value
 	 */
-	return config->SerialIoDevMode[dev_offset] - 1;
+	return PCH_SERIAL_IO_INDEX(config->SerialIoDevMode[dev_offset]);
 }
 
 #if IS_ENABLED(CONFIG_SOC_INTEL_COMETLAKE)