Commit-ID: 9f8197980d87a28ec3d0b3b986f770e7e7878485 Gitweb: http://git.kernel.org/tip/9f8197980d87a28ec3d0b3b986f770e7e7878485 Author: Russell King <rmk+kernel@xxxxxxxxxxxxxxx> AuthorDate: Thu, 19 Jan 2017 10:59:13 +0000 Committer: John Stultz <john.stultz@xxxxxxxxxx> CommitDate: Fri, 20 Jan 2017 14:32:39 -0800 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@xxxxxxxxxxxx> Cc: John Stultz <john.stultz@xxxxxxxxxx> Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxx> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx> --- 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> -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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