tracing: Add documentation for trace clock tai

Add documentation for newly introduced trace clock "tai".
This clock corresponds to CLOCK_TAI.

Link: https://lkml.kernel.org/r/20220414091805.89667-4-kurt@linutronix.de
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 45b8c56..b37dc19 100644 (file)
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -517,6 +517,18 @@ of ftrace. Here is a list of some of the key files:
                processing should be able to handle them. See comments in the
                ktime_get_boot_fast_ns() function for more information.
 
+       tai:
+               This is the tai clock (CLOCK_TAI) and is derived from the wall-
+               clock time. However, this clock does not experience
+               discontinuities and backwards jumps caused by NTP inserting leap
+               seconds. Since the clock access is designed for use in tracing,
+               side effects are possible. The clock access may yield wrong
+               readouts in case the internal TAI offset is updated e.g., caused
+               by setting the system time or using adjtimex() with an offset.
+               These effects are rare and post processing should be able to
+               handle them. See comments in the ktime_get_tai_fast_ns()
+               function for more information.
+
        To set a clock, simply echo the clock name into this file::
 
          # echo global > trace_clock