| Message ID | 1364849914-26420-4-git-send-email-robherring2@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, Apr 01, 2013 at 03:58:34PM -0500, Rob Herring wrote: > From: Rob Herring <rob.herring@calxeda.com> > > Only 32-bits of the arch timer were being used and wrapping was needlessly > being done in s/w. By using the full counter (56-64 bits), we don't need > to deal with wrapping and can simplify the implementation when using > arch timer. Remember that you _must_ provide the full 64-bit nsec value to the scheduler. A 56-bit value will not do, it has to be the full 64-bits of nsecs. So: > +static unsigned long long notrace arch_timer_sched_clock(void) > { > - return arch_timer_read_counter(); > + return arch_timer_read_counter() * sched_clock_mult; ... > + /* Cache the sched_clock multiplier to save a divide in the hot path. */ > + sched_clock_mult = NSEC_PER_SEC / arch_timer_rate; If the counter is 56-bit and the multiplier is not larger than 255, then you have a potential problem here. That basically translates into a clock rate of no more than 3.9MHz for the architected timer if it is 56-bit.
diff --git a/arch/arm/kernel/arch_timer.c b/arch/arm/kernel/arch_timer.c index d957a51..a7536d4 100644 --- a/arch/arm/kernel/arch_timer.c +++ b/arch/arm/kernel/arch_timer.c @@ -22,9 +22,11 @@ static unsigned long arch_timer_read_counter_long(void) return arch_timer_read_counter(); } -static u32 arch_timer_read_counter_u32(void) +static u32 sched_clock_mult __read_mostly; + +static unsigned long long notrace arch_timer_sched_clock(void) { - return arch_timer_read_counter(); + return arch_timer_read_counter() * sched_clock_mult; } static struct delay_timer arch_delay_timer; @@ -52,10 +54,13 @@ int __init arch_timer_of_register(void) int __init arch_timer_sched_clock_init(void) { - if (arch_timer_get_rate() == 0) + u32 arch_timer_rate = arch_timer_get_rate(); + + if (arch_timer_rate == 0) return -ENXIO; - setup_sched_clock(arch_timer_read_counter_u32, - 32, arch_timer_get_rate()); + /* Cache the sched_clock multiplier to save a divide in the hot path. */ + sched_clock_mult = NSEC_PER_SEC / arch_timer_rate; + sched_clock_func = arch_timer_sched_clock; return 0; }