[tip:timers/core] timekeeping/clocksource_cyc2ns: Document intended range limitation

[tip-bot for Chris Metcalf <tipbot@zytor.com>]

Commit-ID:  ec4101e8903e318b9fd4e3bbf72b1eaba53c64e1
Gitweb:     http://git.kernel.org/tip/ec4101e8903e318b9fd4e3bbf72b1eaba53c64e1
Author:     Chris Metcalf <cmetcalf@mellanox.com>
AuthorDate: Mon, 28 Nov 2016 14:35:20 -0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 29 Nov 2016 18:02:58 +0100

timekeeping/clocksource_cyc2ns: Document intended range limitation

The "cycles" argument should not be an absolute clocksource cycle
value, as the implementation's arithmetic will overflow relatively
easily with wide (64 bit) clocksource counters.

For performance, the implementation is simple and fast, since the
function is intended for only relatively small delta values of
clocksource cycles.

[jstultz: Fixed up to merge against HEAD & commit message tweaks,
 also included rewording suggestion by Ingo]
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Link: http://lkml.kernel.org/r/1480372524-15181-4-git-send-email-john.stultz@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 include/linux/clocksource.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 0839818..65602d3 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -169,7 +169,10 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 shift_constant)
  * @mult:	cycle to nanosecond multiplier
  * @shift:	cycle to nanosecond divisor (power of two)
  *
- * Converts cycles to nanoseconds, using the given mult and shift.
+ * Converts clocksource cycles to nanoseconds, using the given @mult and @shift.
+ * The code is optimized for performance and is not intended to work
+ * with absolute clocksource cycles (as those will easily overflow),
+ * but is only intended to be used with relative (delta) clocksource cycles.
  *
  * XXX - This could use some mult_lxl_ll() asm optimization
  */