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author | 2017-01-19 10:59:13 +0000 | |
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committer | 2017-01-20 14:32:39 -0800 | |
commit | 9f8197980d87a28ec3d0b3b986f770e7e7878485 (patch) | |
tree | 2dbb1d82e7ad26dbdab3be5fa38fd38b18da1140 /include/linux/delay.h | |
parent | timekeeping: Remove unused timekeeping_{get,set}_tai_offset() (diff) | |
download | linux-9f8197980d87a28ec3d0b3b986f770e7e7878485.tar.gz linux-9f8197980d87a28ec3d0b3b986f770e7e7878485.tar.bz2 linux-9f8197980d87a28ec3d0b3b986f770e7e7878485.zip |
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>
Diffstat (limited to 'include/linux/delay.h')
-rw-r--r-- | include/linux/delay.h | 11 |
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> |