2 files changed, 24 insertions, 6 deletions

diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt
index 239f14b2b55a..6a5a579126b0 100644
--- a/Documentation/trace/ftrace-design.txt
+++ b/Documentation/trace/ftrace-design.txt
@@ -1,5 +1,6 @@
                 function tracer guts
                 ====================
+                By Mike Frysinger
 
 Introduction
 ------------
@@ -173,14 +174,16 @@ void ftrace_graph_caller(void)
 
         unsigned long *frompc = &...;
         unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
-        prepare_ftrace_return(frompc, selfpc);
+        /* passing frame pointer up is optional -- see below */
+        prepare_ftrace_return(frompc, selfpc, frame_pointer);
 
         /* restore all state needed by the ABI */
 }
 #endif
 
-For information on how to implement prepare_ftrace_return(), simply look at
-the x86 version.  The only architecture-specific piece in it is the setup of
+For information on how to implement prepare_ftrace_return(), simply look at the
+x86 version (the frame pointer passing is optional; see the next section for
+more information).  The only architecture-specific piece in it is the setup of
 the fault recovery table (the asm(...) code).  The rest should be the same
 across architectures.
 
@@ -205,6 +208,23 @@ void return_to_handler(void)
 #endif
 
 
+HAVE_FUNCTION_GRAPH_FP_TEST
+---------------------------
+
+An arch may pass in a unique value (frame pointer) to both the entering and
+exiting of a function.  On exit, the value is compared and if it does not
+match, then it will panic the kernel.  This is largely a sanity check for bad
+code generation with gcc.  If gcc for your port sanely updates the frame
+pointer under different opitmization levels, then ignore this option.
+
+However, adding support for it isn't terribly difficult.  In your assembly code
+that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
+Then in the C version of that function, do what the x86 port does and pass it
+along to ftrace_push_return_trace() instead of a stub value of 0.
+
+Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
+
+
 HAVE_FTRACE_NMI_ENTER
 ---------------------
 
diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 6c22d8a2f289..60e2ce0181ee 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -27,9 +27,7 @@ config HAVE_FUNCTION_GRAPH_TRACER
 config HAVE_FUNCTION_GRAPH_FP_TEST
         bool
         help
-         An arch may pass in a unique value (frame pointer) to both the
-         entering and exiting of a function. On exit, the value is compared
-         and if it does not match, then it will panic the kernel.
+          See Documentation/trace/ftrace-design.txt
 
 config HAVE_FUNCTION_TRACE_MCOUNT_TEST
         bool