diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 03be786..818fd0b 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -627,6 +627,10 @@
    translation library (xlat tables v2) must be used; version 1 of translation
    library is not supported.
 
+-  ``IMPDEF_SYSREG_TRAP``: Numeric value to enable the handling traps for
+   implementation defined system register accesses from lower ELs. Default
+   value is ``0``.
+
 -  ``INVERTED_MEMMAP``: memmap tool print by default lower addresses at the
    bottom, higher addresses at the top. This build flag can be set to '1' to
    invert this behavior. Lower addresses will be printed at the top and higher
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index bc9c00f..0619119 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -3530,6 +3530,38 @@
 
 This function needs to be implemented by a platform if it enables FEAT_RNG_TRAP.
 
+Function : plat_handle_impdef_trap
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : uint64_t
+    Argument : cpu_context_t *
+    Return   : int
+
+This function is invoked by BL31's exception handler when there is a synchronous
+system register trap caused by access to the implementation defined registers.
+It allows platforms enabling ``IMPDEF_SYSREG_TRAP`` to emulate those system
+registers choosing to program bits of their choice.
+
+The first parameter (``uint64_t esr_el3``) contains the content of the ESR_EL3
+syndrome register, which encodes the instruction that was trapped.
+
+The second parameter (``cpu_context_t *ctx``) represents the CPU state in the
+lower exception level, at the time when the execution of the ``mrs`` instruction
+was trapped.
+
+The return value indicates how to proceed:
+
+-  When returning ``TRAP_RET_UNHANDLED`` (-1), the machine will panic.
+-  When returning ``TRAP_RET_REPEAT`` (0), the exception handler will return
+   to the same instruction, so its execution will be repeated.
+-  When returning ``TRAP_RET_CONTINUE`` (1), the exception handler will return
+   to the next instruction.
+
+This function needs to be implemented by a platform if it enables
+IMPDEF_SYSREG_TRAP.
+
 Build flags
 -----------