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Subject[PATCH 2/2] ftrace: document basic ftracer/ftracer graph needs
While implementing ftracer and ftracer graph support, I found the exact
arch implementation details to be a bit lacking (and my x86 foo ain't
great). So after pounding out support for the Blackfin arch, document
the requirements.

Signed-off-by: Mike Frysinger <>
Documentation/trace/ftrace.txt | 7 ++++++
kernel/trace/Kconfig | 43 ++++++++++++++++++++++++++++++++++++++++
2 files changed, 50 insertions(+), 0 deletions(-)

diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
index 5ad2ded..e9f0527 100644
--- a/Documentation/trace/ftrace.txt
+++ b/Documentation/trace/ftrace.txt
@@ -27,6 +27,13 @@ disabled, and more (ftrace allows for tracer plugins, which
means that the list of tracers can always grow).

+Implementation Details
+See kernel/trace/Kconfig for details about different options and
+the required architecture pieces.
The File System

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 417d198..be936c9 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -14,9 +14,46 @@ config HAVE_FTRACE_NMI_ENTER

+ help
+ Implement support for the _mcount() function (or whatever your
+ toolchain names it), and the ftrace_stub function (which only
+ does a return).
+ The _mcount() function should check the function pointer
+ ftrace_trace_function to see if it is set to ftrace_stub. If
+ it is, there is nothing for you to do -- return. If it isn't,
+ then call it the same way the _mcount() function normally calls
+ __mcount_internal() -- the first argument is the "frompc" while
+ the second argument is the "selfpc" (adjusted to remove the size
+ of the embedded _mcount() call).
+ For example, if the function foo() calls bar(), when the bar()
+ function calls _mcount(), the arguments to the tracer are:
+ "frompc" - the address bar() will use to return to foo()
+ "selfpc" - the address bar() (with _mcount() size adjustment)

+ help
+ Update your _mcount() function to check the function pointers
+ ftrace_graph_return (compare to ftrace_stub) and ftrace_graph_entry
+ (compare to ftrace_graph_entry_stub). If either of those are not
+ set to the relevant stub function, call the arch-specific function
+ ftrace_graph_caller which in turn calls the arch-specific function
+ prepare_ftrace_return.
+ The arguments to prepare_ftrace_return() are slightly different.
+ The first argument will instead be a pointer to the "frompc" --
+ typically located in the stack. This allows the function to
+ hijack the return address temporarily to have it point to the
+ arch-specific function return_to_handler(). For exact details of
+ how to implement prepare_ftrace_return(), consult an existing port.
+ It is fairly straight forward C code.
+ The return_to_handler() function is simple as well. Have it call
+ the common ftrace_return_to_handler() function. The return value
+ is the original return address, so restore this to the right place
+ and return.

@@ -25,11 +62,17 @@ config HAVE_FUNCTION_TRACE_MCOUNT_TEST
variable at the mcount call site. Otherwise, this variable
is tested by the called function.

+ It is of course optional for arches, but does improve performance
+ a bit as it prevents constant state saving and call overhead when
+ the function tracer is actually disabled.

+ help
+ See scripts/ for more info.


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