From: slash.tmp@free.fr (Mason)
To: linux-arm-kernel@lists.infradead.org
Subject: Guarantee udelay(N) spins at least N microseconds
Date: Fri, 10 Apr 2015 13:25:37 +0200 [thread overview]
Message-ID: <5527B331.5000205@free.fr> (raw)
Hello everyone,
This is take 2 of my tiny delay.c patch
Problem statement
When converting microseconds to timer cycles in __timer_udelay() and
__timer_const_udelay(), the result is rounded down(*), which means the
system will not spin as long as requested (specifically, between
epsilon and 1 cycle shorter).
If I understand correctly, most drivers expect udelay(N) to spin for
at least N ?s. Is that correct? In that use case, spinning less might
introduce subtle heisenbugs.
Typical example
timer->freq = 90 kHz && HZ = 100
(thus UDELAY_MULT = 107374 && ticks_per_jiffy = 900)
udelay(10) => __timer_const_udelay(10*107374)
=> __timer_delay((1073740*900) >> 30)
=> __timer_delay(0)
So udelay(10) resolves to no delay at all.
(*) 2^41 / 10^6 = 2199023,255552
2199023 < 2^41 / 10^6
UDELAY_MULT = 2199023*HZ / 2^11 < 2^30*HZ / 10^6
cycles = N * UDELAY_MULT * freq/HZ / 2^30
< N * 2^30*HZ / 10^6 * freq/HZ / 2^30
< N / 10^6 * freq
Proposed fix
Since results are always rounded down, all we need is to increment
the result by 1 to round it up.
Would someone ACK the patch below?
Regards.
Patch against 4.0-rc4
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 312d43e..3cfbd07 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -66,7 +66,7 @@ static void __timer_const_udelay(unsigned long xloops)
{
unsigned long long loops = xloops;
loops *= arm_delay_ops.ticks_per_jiffy;
- __timer_delay(loops >> UDELAY_SHIFT);
+ __timer_delay((loops >> UDELAY_SHIFT) + 1);
}
static void __timer_udelay(unsigned long usecs)
next reply other threads:[~2015-04-10 11:25 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-10 11:25 Mason [this message]
2015-04-10 11:42 ` Guarantee udelay(N) spins at least N microseconds Willy Tarreau
2015-04-10 14:53 ` Mason
2015-04-10 15:06 ` Willy Tarreau
2015-04-10 11:44 ` Russell King - ARM Linux
2015-04-10 12:41 ` Mason
2015-04-10 15:06 ` Russell King - ARM Linux
2015-04-10 15:30 ` Mason
2015-04-10 16:08 ` Russell King - ARM Linux
2015-04-10 20:01 ` Mason
2015-04-10 20:42 ` Russell King - ARM Linux
2015-04-10 21:22 ` Mason
2015-04-11 7:30 ` Russell King - ARM Linux
2015-04-11 11:57 ` Mason
2015-04-11 12:10 ` Russell King - ARM Linux
2015-04-11 13:45 ` Mason
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