* __timer_udelay(1) may return immediately
@ 2015-04-02 9:31 Mason
2015-04-02 12:12 ` Mason
0 siblings, 1 reply; 2+ messages in thread
From: Mason @ 2015-04-02 9:31 UTC (permalink / raw)
To: linux-arm-kernel
Hello everyone,
I'm using timer-based delays from arch/arm/lib/delay.c
Consider the following configuration:
HZ=100
timer->freq = 1000000
Thus
UDELAY_MULT = 107374
ticks_per_jiffy = 10000
Thus __timer_udelay(1) =>
__timer_const_udelay(107374) =>
__timer_delay(0) => calls get_cycles() twice then returns prematurely
The issue comes from a tiny rounding error as
107374 * ticks_per_jiffy >> UDELAY_SHIFT = 0,9999983
which is rounded down to 0.
The root of the issue is that mathematically,
UDELAY_MULT = 2199023 * HZ / 2048 = 107374,169921875
which is rounded down to 107374.
It seems to me that a simple solution would be to round
UDELAY_MULT up instead of down.
Thus UDELAY_MULT = 107375
107375 * ticks_per_jiffy >> UDELAY_SHIFT = 1,0000076
We might end up sleeping one cycle more than necessary, but I don't
think spinning a bit longer would be a problem?
Patch provided for illustration purposes.
What do you think?
Regards.
diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index dff714d..873a43e 100644
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -10,7 +10,7 @@
#include <asm/param.h> /* HZ */
#define MAX_UDELAY_MS 2
-#define UDELAY_MULT ((UL(2199023) * HZ) >> 11)
+#define UDELAY_MULT (((UL(2199023) * HZ) >> 11) + 1)
#define UDELAY_SHIFT 30
#ifndef __ASSEMBLY__
--
(Note to self: tangentially relevant discussion)
http://thread.gmane.org/gmane.linux.kernel/1858260
^ permalink raw reply related [flat|nested] 2+ messages in thread
* __timer_udelay(1) may return immediately
2015-04-02 9:31 __timer_udelay(1) may return immediately Mason
@ 2015-04-02 12:12 ` Mason
0 siblings, 0 replies; 2+ messages in thread
From: Mason @ 2015-04-02 12:12 UTC (permalink / raw)
To: linux-arm-kernel
On 02/04/2015 11:31, Mason wrote:
> I'm using timer-based delays from arch/arm/lib/delay.c
>
> Consider the following configuration:
> HZ=100
> timer->freq = 1000000
>
> Thus
> UDELAY_MULT = 107374
> ticks_per_jiffy = 10000
>
> Thus __timer_udelay(1) =>
> __timer_const_udelay(107374) =>
> __timer_delay(0) => calls get_cycles() twice then returns prematurely
>
> The issue comes from a tiny rounding error as
> 107374 * ticks_per_jiffy >> UDELAY_SHIFT = 0,9999983
> which is rounded down to 0.
>
> The root of the issue is that mathematically,
> UDELAY_MULT = 2199023 * HZ / 2048 = 107374,169921875
> which is rounded down to 107374.
>
> It seems to me that a simple solution would be to round
> UDELAY_MULT up instead of down.
>
> Thus UDELAY_MULT = 107375
> 107375 * ticks_per_jiffy >> UDELAY_SHIFT = 1,0000076
>
> We might end up sleeping one cycle more than necessary, but I don't
> think spinning a bit longer would be a problem?
>
> Patch provided for illustration purposes.
>
> What do you think?
>
> Regards.
>
>
> diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
> index dff714d..873a43e 100644
> --- a/arch/arm/include/asm/delay.h
> +++ b/arch/arm/include/asm/delay.h
> @@ -10,7 +10,7 @@
> #include <asm/param.h> /* HZ */
>
> #define MAX_UDELAY_MS 2
> -#define UDELAY_MULT ((UL(2199023) * HZ) >> 11)
> +#define UDELAY_MULT (((UL(2199023) * HZ) >> 11) + 1)
> #define UDELAY_SHIFT 30
>
> #ifndef __ASSEMBLY__
Come to think of it, a closely related issue is: what to do when the
user requests a delay which resolves to a cycle count with a non-zero
fractional part? (e.g. delay for 7.2 cycles)
I think we should round up these values (delay for 8 cycles in the
example). So forget the first patch, keep the rounded down value
for UDELAY_MULT, and round up the cycle count.
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 5306de3..a9b3c75 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -59,7 +59,7 @@ 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)
Also, I was thinking of implementing ndelay() in delay.h
Would it make sense to define
#define NSDELAY_MULT ((UL(281475) * HZ) >> 18) // or perhaps 281474?
and have ndelay(ns) resolve __const_udelay((ns) * NSDELAY_MULT))
Or should I just keep that in platform-specific headers?
Regards.
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