docs: ftrace: Specifies when buffers get clear

Clarify a few places where the ring buffer and the "snapshot" buffer
are cleared as a side effect of an operation.

This will avoid users lost of tracing data because of these so far
undocumented behavior.

Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20191218191553.q4lwyxmquvtjzjfz@frank-laptop
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index d2b5657ed33e973b4bf9e2d9f709acf1cc3647db..46df39300d22a973fa4c4af5711e033a7747ac4a 100644 (file)
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -95,7 +95,8 @@ of ftrace. Here is a list of some of the key files:
   current_tracer:
 
        This is used to set or display the current tracer
-       that is configured.
+       that is configured. Changing the current tracer clears
+       the ring buffer content as well as the "snapshot" buffer.
 
   available_tracers:
 
@@ -126,7 +127,8 @@ of ftrace. Here is a list of some of the key files:
        This file holds the output of the trace in a human
        readable format (described below). Note, tracing is temporarily
        disabled when the file is open for reading. Once all readers
-       are closed, tracing is re-enabled.
+       are closed, tracing is re-enabled. Opening this file for
+       writing with the O_TRUNC flag clears the ring buffer content.
 
   trace_pipe:
 
@@ -490,6 +492,9 @@ of ftrace. Here is a list of some of the key files:
 
          # echo global > trace_clock
 
+       Setting a clock clears the ring buffer content as well as the
+       "snapshot" buffer.
+
   trace_marker:
 
        This is a very useful file for synchronizing user space