[PATCH v3 7/8] Documentation: tracing: boot: Add an example of tracing function-calls

[Masami Hiramatsu <mhiramat@kernel.org>]

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

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 Documentation/trace/boottime-trace.rst |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/trace/boottime-trace.rst b/Documentation/trace/boottime-trace.rst
index 1341b449acaa..c216f5695ae2 100644
--- a/Documentation/trace/boottime-trace.rst
+++ b/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::