[RFC PATCH v2 01/31] tracing: Add a comment about ftrace_regs definition

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

To clarify what will be expected on ftrace_regs, add a comment to the
architecture independent definition of the ftrace_regs.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v2:
  - newly added.
---
 include/linux/ftrace.h |   25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index e8921871ef9a..b174af91d8be 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -118,6 +118,31 @@ extern int ftrace_enabled;
 
 #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
 
+/**
+ * ftrace_regs - ftrace partial/optimal register set
+ *
+ * ftrace_regs represents a group of registers which is used at the
+ * function entry and exit. There are three types of registers.
+ *
+ * - Registers for passing the parameters to callee, including the stack
+ *   pointer. (e.g. rcx, rdx, rdi, rsi, r8, r9 and rsp on x86_64)
+ * - Registers for passing the return values to caller.
+ *   (e.g. rax and rdx on x86_64)
+ * - Registers for hooking the function return including the frame pointer
+ *   (the frame pointer is architecture/config dependent)
+ *   (e.g. rbp and rsp for x86_64)
+ *
+ * Also, architecture dependent fields can be used for internal process.
+ * (e.g. orig_ax on x86_64)
+ *
+ * On the function entry, those registers will be restored except for
+ * the stack pointer, so that user can change the function parameters.
+ * On the function exit, onlu registers which is used for return values
+ * are restored.
+ *
+ * NOTE: user *must not* access regs directly, only do it via APIs, because
+ * the member can be changed according to the architecture.
+ */
 struct ftrace_regs {
 	struct pt_regs		regs;
 };