diff --git a/plat/st/stm32mp1/include/stm32mp1_smc.h b/plat/st/stm32mp1/include/stm32mp1_smc.h
index 57240bcaf..52088de28 100644
--- a/plat/st/stm32mp1/include/stm32mp1_smc.h
+++ b/plat/st/stm32mp1/include/stm32mp1_smc.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2019, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2016-2021, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -58,4 +58,10 @@
 #define STM32_SMC_WRITE_SHADOW		0x03
 #define STM32_SMC_READ_OTP		0x04
 
+/* SMC error codes */
+#define STM32_SMC_OK			0x00000000U
+#define STM32_SMC_NOT_SUPPORTED		0xFFFFFFFFU
+#define STM32_SMC_FAILED		0xFFFFFFFEU
+#define STM32_SMC_INVALID_PARAMS	0xFFFFFFFDU
+
 #endif /* STM32MP1_SMC_H */
diff --git a/plat/st/stm32mp1/services/bsec_svc.c b/plat/st/stm32mp1/services/bsec_svc.c
index 2a60e4339..a1d7fc64e 100644
--- a/plat/st/stm32mp1/services/bsec_svc.c
+++ b/plat/st/stm32mp1/services/bsec_svc.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2019, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2016-2021, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -28,11 +28,11 @@ uint32_t bsec_main(uint32_t x1, uint32_t x2, uint32_t x3,
 		result = bsec_program_otp(x3, x2);
 		break;
 	case STM32_SMC_WRITE_SHADOW:
-		*ret_otp_value = 0;
+		*ret_otp_value = 0U;
 		result = bsec_write_otp(x3, x2);
 		break;
 	case STM32_SMC_READ_OTP:
-		*ret_otp_value = 0;
+		*ret_otp_value = 0U;
 		result = bsec_read_otp(&tmp_data, x2);
 		if (result != BSEC_OK) {
 			break;
@@ -52,9 +52,8 @@ uint32_t bsec_main(uint32_t x1, uint32_t x2, uint32_t x3,
 		break;
 
 	default:
-		result = BSEC_ERROR;
-		break;
+		return STM32_SMC_INVALID_PARAMS;
 	}
 
-	return result;
+	return (result == BSEC_OK) ? STM32_SMC_OK : STM32_SMC_FAILED;
 }
diff --git a/plat/st/stm32mp1/services/stm32mp1_svc_setup.c b/plat/st/stm32mp1/services/stm32mp1_svc_setup.c
index d4ed44525..ed8a448b3 100644
--- a/plat/st/stm32mp1/services/stm32mp1_svc_setup.c
+++ b/plat/st/stm32mp1/services/stm32mp1_svc_setup.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2019, STMicroelectronics - All Rights Reserved
+ * Copyright (c) 2014-2021, STMicroelectronics - All Rights Reserved
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -75,7 +75,7 @@ static uintptr_t stm32mp1_svc_smc_handler(uint32_t smc_fid, u_register_t x1,
 
 	default:
 		WARN("Unimplemented STM32MP1 Service Call: 0x%x\n", smc_fid);
-		ret1 = SMC_UNK;
+		ret1 = STM32_SMC_NOT_SUPPORTED;
 		break;
 	}