[tip:timers/core] timers: Clarify usleep_range() function comment

[tip-bot for Bjorn Helgaas <tipbot@zytor.com>]

Commit-ID:  b5227d03b7191a9a44bf75a4c228a6a9ddbe781b
Gitweb:     http://git.kernel.org/tip/b5227d03b7191a9a44bf75a4c228a6a9ddbe781b
Author:     Bjorn Helgaas <bhelgaas@google.com>
AuthorDate: Tue, 31 May 2016 16:23:02 -0500
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 10 Jun 2016 00:59:14 +0200

timers: Clarify usleep_range() function comment

Update the usleep_range() function comment to make it clear that it can
only be used in non-atomic context.

Previously we claimed usleep_range() was a drop-in replacement for udelay()
where wakeup is flexible.  But that's only true in non-atomic contexts,
where it's possible to sleep instead of delay.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/20160531212302.28502.44995.stgit@bhelgaas-glaptop2.roam.corp.google.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 kernel/time/timer.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 3a95f97..67dd610 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1702,9 +1702,15 @@ static void __sched do_usleep_range(unsigned long min, unsigned long max)
 }
 
 /**
- * usleep_range - Drop in replacement for udelay where wakeup is flexible
+ * usleep_range - Sleep for an approximate time
  * @min: Minimum time in usecs to sleep
  * @max: Maximum time in usecs to sleep
+ *
+ * In non-atomic context where the exact wakeup time is flexible, use
+ * usleep_range() instead of udelay().  The sleep improves responsiveness
+ * by avoiding the CPU-hogging busy-wait of udelay(), and the range reduces
+ * power usage by allowing hrtimers to take advantage of an already-
+ * scheduled interrupt instead of scheduling a new one just for this sleep.
  */
 void __sched usleep_range(unsigned long min, unsigned long max)
 {