linux-kernel - [PATCH 2/3] delay: Add explanation of udelay() inaccuracy

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Date:   Fri, 27 Jan 2017 16:59:30 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     lkml <linux-kernel@...r.kernel.org>
Cc:     Russell King <rmk+kernel@...linux.org.uk>,
        David Riley <davidriley@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Richard Cochran <richardcochran@...il.com>,
        Prarit Bhargava <prarit@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        John Stultz <john.stultz@...aro.org>
Subject: [PATCH 2/3] delay: Add explanation of udelay() inaccuracy

From: Russell King <rmk+kernel@...linux.org.uk>

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@...omium.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Russell King <rmk+kernel@...linux.org.uk>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 include/linux/delay.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/delay.h b/include/linux/delay.h
index a6ecb34..2ecb3c4 100644
--- a/include/linux/delay.h
+++ b/include/linux/delay.h
@@ -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>
-- 
2.7.4