linux-kernel - [PATCH 3/3] tracing: Add documentation for trace clock tai

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Message-Id: <20220409081300.4762-4-kurt@linutronix.de>
Date:   Sat,  9 Apr 2022 10:13:00 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>
Cc:     Richard Cochran <richardcochran@...il.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: [PATCH 3/3] tracing: Add documentation for trace clock tai

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

Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
---
 Documentation/trace/ftrace.rst | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 45b8c56af67a..b37dc19e4d40 100644
--- 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
-- 
2.30.2