linux-kernel - Re: [PATCH 1/2] clocksource: arm_arch_timer: unify sched

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Date:	Thu, 18 Apr 2013 20:37:27 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
CC:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	arm@...nel.org, Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] clocksource: arm_arch_timer: unify sched_clock init

On 04/18/2013 07:00 PM, Stephen Boyd wrote:
> On 04/18/13 12:30, Rob Herring wrote:
>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>> index 122ff05..17ed8e4 100644
>> --- a/drivers/clocksource/arm_arch_timer.c
>> +++ b/drivers/clocksource/arm_arch_timer.c
>> @@ -266,6 +266,15 @@ static struct notifier_block arch_timer_cpu_nb __cpuinitdata = {
>>  	.notifier_call = arch_timer_cpu_notify,
>>  };
>>  
>> +static u64 sched_clock_mult __read_mostly;
>> +
>> +unsigned long long notrace arch_timer_sched_clock(void)
>> +{
>> +	return arch_timer_read_counter() * sched_clock_mult;
>> +}
>> +unsigned long long sched_clock(void) \
>> +	__attribute__((weak, alias("arch_timer_sched_clock")));
> 
> I'm still lost, how does this prevent the timer in ARM's 32 bit
> sched_clock code from getting setup in sched_clock_postinit()? That
> print is still there right? Who owns sched_clock() in multi-target builds?

For arm64, it does not define sched_clock, so it will get
arch_timer_sched_clock.

For arm, sched_clock is defined in arch/arm/kernel/sched_clock.c and the
weak alias is not used. The arm sched_clock function just calls a
function pointer which defaults to sched_clock_32 (which is the original
arm sched_clock implementation). If the arch timer is present, then the
function pointer is set to arch_timer_sched_clock and any calls to
setup_sched_clock and the sched_clock_postinit have no effect.
Otherwise, the functionality is basically unchanged for <=32-bit
sched_clock implementations.

> Why can't we play along with the sched_clock code that lives in arm?
> Maybe we should resurrect those clocksource sched_clock patches again.
> Or maybe we should add support for setup_sched_clock_64() in arm's sched
> clock code.

That's what I originally had which Russell objected to. The needs for
the arch timer is a bit different since we don't need to deal with
wrapping. And we need the same boot time offset and suspend handling in
both arm and arm64.

Rob

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