Merge branch 'tracing/core' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic...

[linux-2.6-block.git] / Documentation / trace / ftrace-design.txt

diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt
index 239f14b2b55a28213e1a7170db4b870ab82ff1b9..f1f81afee8a04d19bc69a7231c25a91c0e0f17e0 100644 (file)
--- 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
 ---------------------
 
@@ -218,11 +238,10 @@ HAVE_SYSCALL_TRACEPOINTS
 
 You need very few things to get the syscalls tracing in an arch.
 
+- Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).
 - Have a NR_syscalls variable in <asm/unistd.h> that provides the number
   of syscalls supported by the arch.
-- Implement arch_syscall_addr() that resolves a syscall address from a
-  syscall number.
-- Support the TIF_SYSCALL_TRACEPOINT thread flags
+- Support the TIF_SYSCALL_TRACEPOINT thread flags.
 - Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace
   in the ptrace syscalls tracing path.
 - Tag this arch as HAVE_SYSCALL_TRACEPOINTS.