linux-kernel - [PATCH] timers: Clarify usleep

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Message-ID: <20160531212302.28502.44995.stgit@bhelgaas-glaptop2.roam.corp.google.com>
Date:	Tue, 31 May 2016 16:23:02 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] 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@...gle.com>
---
 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..c024c9f 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)
 {