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-rw-r--r--Documentation/trace/ftrace.txt20
1 files changed, 20 insertions, 0 deletions
diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
index 185c39fea2a0..5596e2d71d6d 100644
--- a/Documentation/trace/ftrace.txt
+++ b/Documentation/trace/ftrace.txt
@@ -362,6 +362,26 @@ of ftrace. Here is a list of some of the key files:
to correlate events across hypervisor/guest if
tb_offset is known.
+ mono: This uses the fast monotonic clock (CLOCK_MONOTONIC)
+ which is monotonic and is subject to NTP rate adjustments.
+
+ mono_raw:
+ This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
+ which is montonic but is not subject to any rate adjustments
+ and ticks at the same rate as the hardware clocksource.
+
+ boot: This is the boot clock (CLOCK_BOOTTIME) and is based on the
+ fast monotonic clock, but also accounts for time spent in
+ suspend. Since the clock access is designed for use in
+ tracing in the suspend path, some side effects are possible
+ if clock is accessed after the suspend time is accounted before
+ the fast mono clock is updated. In this case, the clock update
+ appears to happen slightly sooner than it normally would have.
+ Also on 32-bit systems, it's possible that the 64-bit boot offset
+ sees a partial update. These effects are rare and post
+ processing should be able to handle them. See comments in the
+ ktime_get_boot_fast_ns() function for more information.
+
To set a clock, simply echo the clock name into this file.
echo global > trace_clock