Add an explanation of the trace variable
trace_clock to the ftrace documentation.

Signed-off-by: Carsten Emde <C.Emde@osadl.org>

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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