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authorRussell King <rmk+kernel@armlinux.org.uk>2017-01-19 10:59:13 +0000
committerJohn Stultz <john.stultz@linaro.org>2017-01-20 14:32:39 -0800
commit9f8197980d87a28ec3d0b3b986f770e7e7878485 (patch)
tree2dbb1d82e7ad26dbdab3be5fa38fd38b18da1140
parent40d9f82750044f846005d2ac4eec65e39c1c0f7c (diff)
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>
-rw-r--r--include/linux/delay.h11
1 files changed, 11 insertions, 0 deletions
diff --git a/include/linux/delay.h b/include/linux/delay.h
index a6ecb34cf547..2ecb3c46b20a 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>