diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index 3ea86b0..6244a63 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -841,6 +841,33 @@
 represents the power domain topology and how this relates to the linear CPU
 index, please refer `Power Domain Topology Design`_.
 
+Function : plat_get_mbedtls_heap() [when TRUSTED_BOARD_BOOT == 1]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Arguments : void **heap_addr, size_t *heap_size
+    Return    : int
+
+This function is invoked during Mbed TLS library initialisation to get a heap,
+by means of a starting address and a size. This heap will then be used
+internally by the Mbed TLS library. Hence, each BL stage that utilises Mbed TLS
+must be able to provide a heap to it.
+
+A helper function can be found in `drivers/auth/mbedtls/mbedtls_common.c` in
+which a heap is statically reserved during compile time inside every image
+(i.e. every BL stage) that utilises Mbed TLS. In this default implementation,
+the function simply returns the address and size of this "pre-allocated" heap.
+For a platform to use this default implementation, only a call to the helper
+from inside plat_get_mbedtls_heap() body is enough and nothing else is needed.
+
+However, by writting their own implementation, platforms have the potential to
+optimise memory usage. For example, on some Arm platforms, the Mbed TLS heap is
+shared between BL1 and BL2 stages and, thus, the necessary space is not reserved
+twice.
+
+On success the function should return 0 and a negative error code otherwise.
+
 Common optional modifications
 -----------------------------
 
@@ -1054,29 +1081,6 @@
 the log output. The implementation should be robust to future changes that
 increase the number of log levels.
 
-Function : plat_get_mbedtls_heap()
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-::
-
-    Arguments : void **heap_addr, size_t *heap_size
-    Return    : int
-
-This function is invoked during Mbed TLS library initialisation to get
-a heap, by means of a starting address and a size. This heap will then be used
-internally by the Mbed TLS library. The heap is requested from the current BL
-stage, i.e. the current BL image inside which Mbed TLS is used.
-
-In the default implementation a heap is statically allocated inside every image
-(i.e. every BL stage) that utilises Mbed TLS. So, in this case, the function
-simply returns the address and size of this "pre-allocated" heap. However, by
-overriding the default implementation, platforms have the potential to optimise
-memory usage. For example, on some Arm platforms, the Mbed TLS heap is shared
-between BL1 and BL2 stages and, thus, the necessary space is not reserved
-twice.
-
-On success the function should return 0 and a negative error code otherwise.
-
 Modifications specific to a Boot Loader stage
 ---------------------------------------------