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arm64: use WFE for long delays 

The current delay implementation uses the yield instruction, which is a
hint that it is beneficial to schedule another thread. As this is a hint,
it may be implemented as a NOP, causing all delays to be busy loops. This
is the case for many existing CPUs.

Taking advantage of the generic timer sending periodic events to all
cores, we can use WFE during delays to reduce power consumption. This is
beneficial only for delays longer than the period of the timer event
stream.

If timer event stream is not enabled, delays will behave as yield/busy
loops.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
This commit is contained in:
Julien Thierry 2017-10-13 14:32:56 +01:00 committed by Will Deacon
parent ec5c8e429d
commit 7b77452ec5
2 changed files with 22 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

23
arch/arm64/lib/delay.c
View File

@ -24,10 +24,28 @@
#include <linux/module.h>
#include <linux/timex.h>
#include <clocksource/arm_arch_timer.h>
#define USECS_TO_CYCLES(time_usecs) \
xloops_to_cycles((time_usecs) * 0x10C7UL)
static inline unsigned long xloops_to_cycles(unsigned long xloops)
{
return (xloops * loops_per_jiffy * HZ) >> 32;
}
void __delay(unsigned long cycles)
{
cycles_t start = get_cycles();
if (arch_timer_evtstrm_available()) {
const cycles_t timer_evt_period =
USECS_TO_CYCLES(ARCH_TIMER_EVT_STREAM_PERIOD_US);
while ((get_cycles() - start + timer_evt_period) < cycles)
wfe();
}
while ((get_cycles() - start) < cycles)
cpu_relax();
}
@ -35,10 +53,7 @@ EXPORT_SYMBOL(__delay);
inline void __const_udelay(unsigned long xloops)
{
unsigned long loops;
loops = xloops * loops_per_jiffy * HZ;
__delay(loops >> 32);
__delay(xloops_to_cycles(xloops));
}
EXPORT_SYMBOL(__const_udelay);

4
include/clocksource/arm_arch_timer.h
View File

@ -67,7 +67,9 @@ enum arch_timer_spi_nr {
#define ARCH_TIMER_USR_VT_ACCESS_EN (1 << 8) /* virtual timer registers */
#define ARCH_TIMER_USR_PT_ACCESS_EN (1 << 9) /* physical timer registers */
#define ARCH_TIMER_EVT_STREAM_FREQ 10000 /* 100us */
#define ARCH_TIMER_EVT_STREAM_PERIOD_US 100
#define ARCH_TIMER_EVT_STREAM_FREQ \
(USEC_PER_SEC / ARCH_TIMER_EVT_STREAM_PERIOD_US)
struct arch_timer_kvm_info {
struct timecounter timecounter;