diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index c8d61ed..9798566 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -1014,6 +1014,21 @@
 next image. This function is currently invoked in BL2 to flush this information
 to the next BL image, when LOAD\_IMAGE\_V2 is enabled.
 
+Function : plat\_log\_get\_prefix()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : unsigned int
+    Return   : const char *
+
+This function defines the prefix string corresponding to the `log_level` to be
+prepended to all the log output from ARM Trusted Firmware. The `log_level`
+(argument) will correspond to one of the standard log levels defined in
+debug.h. The platform can override the common implementation to define a
+different prefix string for the log output.  The implementation should be
+robust to future changes that increase the number of log levels.
+
 Modifications specific to a Boot Loader stage
 ---------------------------------------------