Commit 9f819798 authored Jan 19, 2017 by Russell King Committed by John Stultz Jan 20, 2017

delay: Add explanation of udelay() inaccuracy

There seems to be some misunderstanding that udelay() and friends will
always guarantee the specified delay.  This is a false understanding.
When udelay() is based on CPU cycles, it can return early for many
reasons which are detailed by Linus' reply to me in a thread in 2011:

  http://lists.openwall.net/linux-kernel/2011/01/12/372



However, a udelay test module was created in 2014 which allows udelay()
to only be 0.5% fast, which is outside of the CPU-cycles udelay()
results I measured back in 2011, which were deemed to be in the "we
don't care" region.

test_udelay() should be fixed to reflect the real allowable tolerance
on udelay(), rather than 0.5%.

Cc: David Riley <davidriley@chromium.org>
Cc: John Stultz <john.stultz@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: John Stultz <john.stultz@linaro.org>

parent 40d9f827

include/linux/delay.h

+11 −0

Original line number	Diff line number	Diff line
		@@ -5,6 +5,17 @@
		* Copyright (C) 1993 Linus Torvalds
		*
		* Delay routines, using a pre-computed "loops_per_jiffy" value.
		*
		* Please note that ndelay(), udelay() and mdelay() may return early for
		* several reasons:
		* 1. computed loops_per_jiffy too low (due to the time taken to
		* execute the timer interrupt.)
		* 2. cache behaviour affecting the time it takes to execute the
		* loop function.
		* 3. CPU clock rate changes.
		*
		* Please see this thread:
		* http://lists.openwall.net/linux-kernel/2011/01/09/56
		*/

		#include <linux/kernel.h>