diff --git a/include/services/gtsi_svc.h b/include/services/gtsi_svc.h
new file mode 100644
index 0000000..cb942ed
--- /dev/null
+++ b/include/services/gtsi_svc.h
@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef GTSI_SVC_H
+#define GTSI_SVC_H
+
+/* GTSI error codes. */
+#define GTSI_SUCCESS			0
+#define GTSI_ERROR_NOT_SUPPORTED	-1
+#define GTSI_ERROR_INVALID_ADDRESS	-2
+#define GTSI_ERROR_INVALID_PAS		-3
+
+/* The macros below are used to identify GTSI calls from the SMC function ID */
+#define GTSI_FNUM_MIN_VALUE	U(0x100)
+#define GTSI_FNUM_MAX_VALUE	U(0x101)
+#define is_gtsi_fid(fid) __extension__ ({		\
+	__typeof__(fid) _fid = (fid);			\
+	((GET_SMC_NUM(_fid) >= GTSI_FNUM_MIN_VALUE) &&	\
+	 (GET_SMC_NUM(_fid) <= GTSI_FNUM_MAX_VALUE)); })
+
+/* Get GTSI fastcall std FID from function number */
+#define GTSI_FID(smc_cc, func_num)			\
+	((SMC_TYPE_FAST << FUNCID_TYPE_SHIFT)	|	\
+	 ((smc_cc) << FUNCID_CC_SHIFT)		|	\
+	 (OEN_STD_START << FUNCID_OEN_SHIFT)	|	\
+	 ((func_num) << FUNCID_NUM_SHIFT))
+
+#define GRAN_TRANS_TO_REALM_FNUM	U(0x100)
+#define GRAN_TRANS_TO_NS_FNUM		U(0x101)
+
+#define SMC_ASC_MARK_REALM	GTSI_FID(SMC_64, GRAN_TRANS_TO_REALM_FNUM)
+#define SMC_ASC_MARK_NONSECURE	GTSI_FID(SMC_64, GRAN_TRANS_TO_NS_FNUM)
+
+#define GRAN_TRANS_RET_BAD_ADDR		-2
+#define GRAN_TRANS_RET_BAD_PAS		-3
+
+#endif /* GTSI_SVC_H */