On 01/11/2017 20:38, Marc Gonzalez wrote:
> OK, I'll just send my patch, and then crawl back under my rock.
Linus,
As promised, the patch is provided below. And as promised, I will
no longer bring this up on LKML.
FWIW, I have checked that the computed value matches the expected
value for all HZ and delay_us, and for a few clock frequencies,
using the following program:
$ cat delays.c
#include <stdio.h>
#define MEGA 1000000u
typedef unsigned int uint;
typedef unsigned long long u64;
#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
static const uint HZ_tab[] = { 100, 250, 300, 1000 };
static void check_cycle_count(uint freq, uint HZ, uint delay_us)
{
uint UDELAY_MULT = (2147 * HZ) + (483648 * HZ / MEGA);
uint lpj = DIV_ROUND_UP(freq, HZ);
uint computed = ((u64)lpj * delay_us * UDELAY_MULT >> 31) + 1;
uint expected = DIV_ROUND_UP((u64)delay_us * freq, MEGA);
if (computed != expected)
printf("freq=%u HZ=%u delay_us=%u comp=%u exp=%u\n", freq, HZ,
delay_us, computed, expected);
}
int main(void)
{
uint idx, delay_us, freq;
for (freq = 3*MEGA; freq <= 100*MEGA; freq += 3*MEGA)
for (idx = 0; idx < sizeof HZ_tab / sizeof *HZ_tab; ++idx)
for (delay_us = 1; delay_us <= 2000; ++delay_us)
check_cycle_count(freq, HZ_tab[idx], delay_us);
return 0;
}
-- >8 --
Subject: [PATCH] ARM: Tweak clock-based udelay implementation
In 9f8197980d87a ("delay: Add explanation of udelay() inaccuracy")
Russell pointed out that loop-based delays may return early.
On the arm platform, delays may be either loop-based or clock-based.
This patch tweaks the clock-based implementation so that udelay(N)
is guaranteed to spin at least N microseconds.
Signed-off-by: Marc Gonzalez <marc_gonza...@sigmadesigns.com>
---
arch/arm/lib/delay.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
index 2cef11884857..0a25712077ec 100644
--- a/arch/arm/lib/delay.c
+++ b/arch/arm/lib/delay.c
@@ -58,15 +58,15 @@ static void __timer_delay(unsigned long cycles)
{
cycles_t start = get_cycles();
- while ((get_cycles() - start) < cycles)
+ while ((get_cycles() - start) <= cycles)
cpu_relax();
}
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);
+ u64 tmp = (u64)xloops * arm_delay_ops.ticks_per_jiffy;
+ unsigned long cycles = tmp >> UDELAY_SHIFT;
+ __timer_delay(cycles + 1); /* Round up in 1 instruction */
}
static void __timer_udelay(unsigned long usecs)
@@ -92,7 +92,7 @@ void __init register_current_timer_delay(const struct
delay_timer *timer)
if (!delay_calibrated && (!delay_res || (res < delay_res))) {
pr_info("Switching to timer-based delay loop, resolution
%lluns\n", res);
delay_timer = timer;
- lpj_fine = timer->freq / HZ;
+ lpj_fine = DIV_ROUND_UP(timer->freq, HZ);
delay_res = res;
/* cpufreq may scale loops_per_jiffy, so keep a private copy */
--
2.15.0
