Add an explanation of the trace variable
trace_clock to the ftrace documentation.
Signed-off-by: Carsten Emde <C.Emde@xxxxxxxxx>
Index: linux-2.6.33-rc2/Documentation/trace/ftrace.txt
===================================================================
--- linux-2.6.33-rc2.orig/Documentation/trace/ftrace.txt
+++ linux-2.6.33-rc2/Documentation/trace/ftrace.txt
@@ -91,6 +91,33 @@ of ftrace. Here is a list of some of the
This file holds the output of the trace in a human
readable format (described below).
+ trace_clock:
+
+ Three different clocks are available for the tracer
+ timestamps, "local", "medium" and "global". For the
+ time being, however, the "medium" clock is disabled
+ in the source code and requires the line
+ { trace_clock, "medium" },
+ to be inserted in kernel/trace/trace.c between
+ { trace_clock_local, "local" },
+ and
+ { trace_clock_global, "global" },
+ - The "local" clock is the simplest and least coherent
+ tracing clock. It is useful for tracing that does not
+ cross to other CPUs nor does it go through idle events.
+ - The "medium" clock is a scalable global clock with
+ some jitter. It is not completely serialized, but not
+ completely incorrect when crossing CPUs either.
+ - The "global" clock is a globally monotonic, serialized
+ clock. It has a higher overhead than the "local" and the
+ "medium" trace clock but is still an order of magnitude
+ faster than a hardware clock based on the gettimeofday()
+ function would be.
+
+ The content of trace_clock displays the available tracer
+ clocks, the name of the currently selected clock is
+ surrounded by square brackets.
+
trace_pipe:
The output is the same as the "trace" file but this
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