The patch titled
Subject: include/linux/delay.h: add explanation of udelay() inaccuracy
has been added to the -mm tree. Its filename is
add-explanation-of-udelay-inaccuracy.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/add-explanation-of-udelay-inaccuracy.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/add-explanation-of-udelay-inaccuracy.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/SubmitChecklist when testing your code ***
The -mm tree is included into linux-next and is updated
there every 3-4 working days
------------------------------------------------------
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
add-explanation-of-udelay-inaccuracy.patch
--
To unsubscribe from this list: send the line "unsubscribe mm-commits" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
