Documentation: tracing: boot: Add an example of tracing function-calls

Add an example of tracing function calls on a specific function.

Link: https://lkml.kernel.org/r/159972816669.428528.12390560334549382316.stgit@devnote2Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/boottime-trace.rst

@@ -168,6 +168,26 @@ is for tracing functions starting with "user\_", and others tracing
 The instance node also accepts event nodes so that each instance
 can customize its event tracing.
 
+With the trigger action and kprobes, you can trace function-graph while
+a function is called. For example, this will trace all function calls in
+the pci_proc_init()::
+
+  ftrace {
+        tracing_on = 0
+        tracer = function_graph
+        event.kprobes {
+                start_event {
+                        probes = "pci_proc_init"
+                        actions = "traceon"
+                }
+                end_event {
+                        probes = "pci_proc_init%return"
+                        actions = "traceoff"
+                }
+        }
+  }
+
+
 This boot-time tracing also supports ftrace kernel parameters via boot
 config.
 For example, following kernel parameters::