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-rw-r--r--Documentation/trace/ftrace.rst14
1 files changed, 11 insertions, 3 deletions
diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index e45f0786f3f9..67d9c38e95eb 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -461,9 +461,17 @@ of ftrace. Here is a list of some of the key files:
and ticks at the same rate as the hardware clocksource.
boot:
- Same as mono. Used to be a separate clock which accounted
- for the time spent in suspend while CLOCK_MONOTONIC did
- not.
+ 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::