The patch titled
Subject: include/linux/delay.h: add explanation of udelay() inaccuracy
has been removed from the -mm tree. Its filename was
add-explanation-of-udelay-inaccuracy.patch
This patch was dropped because it was merged into mainline or a subsystem tree
------------------------------------------------------
From: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Subject: include/linux/delay.h: 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%.
Link: http://lkml.kernel.org/r/E1cUAQz-0002HC-EX@xxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx>
Cc: David Riley <davidriley@xxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/delay.h | 11 +++++++++++
1 file changed, 11 insertions(+)
diff -puN include/linux/delay.h~add-explanation-of-udelay-inaccuracy include/linux/delay.h
--- a/include/linux/delay.h~add-explanation-of-udelay-inaccuracy
+++ a/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>
_
Patches currently in -mm which might be from rmk+kernel@xxxxxxxxxxxxxxx are
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