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-rw-r--r--Documentation/trace/ftrace-design.txt14
1 files changed, 7 insertions, 7 deletions
diff --git a/Documentation/trace/ftrace-design.txt b/Documentation/trace/ftrace-design.txt
index 641a1ef2a7ff..239f14b2b55a 100644
--- a/Documentation/trace/ftrace-design.txt
+++ b/Documentation/trace/ftrace-design.txt
@@ -53,14 +53,14 @@ size of the mcount call that is embedded in the function).
53For example, if the function foo() calls bar(), when the bar() function calls 53For example, if the function foo() calls bar(), when the bar() function calls
54mcount(), the arguments mcount() will pass to the tracer are: 54mcount(), the arguments mcount() will pass to the tracer are:
55 "frompc" - the address bar() will use to return to foo() 55 "frompc" - the address bar() will use to return to foo()
56 "selfpc" - the address bar() (with _mcount() size adjustment) 56 "selfpc" - the address bar() (with mcount() size adjustment)
57 57
58Also keep in mind that this mcount function will be called *a lot*, so 58Also keep in mind that this mcount function will be called *a lot*, so
59optimizing for the default case of no tracer will help the smooth running of 59optimizing for the default case of no tracer will help the smooth running of
60your system when tracing is disabled. So the start of the mcount function is 60your system when tracing is disabled. So the start of the mcount function is
61typically the bare min with checking things before returning. That also means 61typically the bare minimum with checking things before returning. That also
62the code flow should usually kept linear (i.e. no branching in the nop case). 62means the code flow should usually be kept linear (i.e. no branching in the nop
63This is of course an optimization and not a hard requirement. 63case). This is of course an optimization and not a hard requirement.
64 64
65Here is some pseudo code that should help (these functions should actually be 65Here is some pseudo code that should help (these functions should actually be
66implemented in assembly): 66implemented in assembly):
@@ -131,10 +131,10 @@ some functions to save (hijack) and restore the return address.
131 131
132The mcount function should check the function pointers ftrace_graph_return 132The mcount function should check the function pointers ftrace_graph_return
133(compare to ftrace_stub) and ftrace_graph_entry (compare to 133(compare to ftrace_stub) and ftrace_graph_entry (compare to
134ftrace_graph_entry_stub). If either of those are not set to the relevant stub 134ftrace_graph_entry_stub). If either of those is not set to the relevant stub
135function, call the arch-specific function ftrace_graph_caller which in turn 135function, call the arch-specific function ftrace_graph_caller which in turn
136calls the arch-specific function prepare_ftrace_return. Neither of these 136calls the arch-specific function prepare_ftrace_return. Neither of these
137function names are strictly required, but you should use them anyways to stay 137function names is strictly required, but you should use them anyway to stay
138consistent across the architecture ports -- easier to compare & contrast 138consistent across the architecture ports -- easier to compare & contrast
139things. 139things.
140 140
@@ -144,7 +144,7 @@ but the first argument should be a pointer to the "frompc". Typically this is
144located on the stack. This allows the function to hijack the return address 144located on the stack. This allows the function to hijack the return address
145temporarily to have it point to the arch-specific function return_to_handler. 145temporarily to have it point to the arch-specific function return_to_handler.
146That function will simply call the common ftrace_return_to_handler function and 146That function will simply call the common ftrace_return_to_handler function and
147that will return the original return address with which, you can return to the 147that will return the original return address with which you can return to the
148original call site. 148original call site.
149 149
150Here is the updated mcount pseudo code: 150Here is the updated mcount pseudo code: