[PATCH v3 1/2] arm_arch_timer: Expose event stream status

[Julien Thierry]

On 12/10/17 11:26, Will Deacon wrote:
> On Thu, Oct 12, 2017 at 10:40:57AM +0100, Julien Thierry wrote:
>>
>>
>> On 12/10/17 10:30, Will Deacon wrote:
>>> On Thu, Oct 12, 2017 at 10:27:06AM +0100, Suzuki K Poulose wrote:
>>>> On 29/09/17 11:52, Julien Thierry wrote:
>>>>> The arch timer configuration for a CPU might get reset after suspending
>>>>> said CPU.
>>>>>
>>>>> In order to reliably use the event stream in the kernel (e.g. for delays),
>>>>> we keep track of the state where we can safely concider the event stream as
>>>>
>>>> nit: consider
>>>>
>>>>> properly configured.
>>>>>
>>>>> Signed-off-by: Julien Thierry <julien.thierry at arm.com>
>>>>> Cc: Mark Rutland <mark.rutland at arm.com>
>>>>> Cc: Marc Zyngier <marc.zyngier at arm.com>
>>>>> Cc: Russell King <linux at armlinux.org.uk>
>>>>> Cc: Catalin Marinas <catalin.marinas at arm.com>
>>>>> Cc: Will Deacon <will.deacon at arm.com>
>>>>> ---
>>>>>   arch/arm/include/asm/arch_timer.h    |  1 +
>>>>>   arch/arm64/include/asm/arch_timer.h  |  1 +
>>>>>   drivers/clocksource/arm_arch_timer.c | 20 ++++++++++++++++++--
>>>>>   include/clocksource/arm_arch_timer.h |  6 ++++++
>>>>>   4 files changed, 26 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm/include/asm/arch_timer.h b/arch/arm/include/asm/arch_timer.h
>>>>> index d4ebf56..0b6e104 100644
>>>>> --- a/arch/arm/include/asm/arch_timer.h
>>>>> +++ b/arch/arm/include/asm/arch_timer.h
>>>>> @@ -106,6 +106,7 @@ static inline u32 arch_timer_get_cntkctl(void)
>>>>>   static inline void arch_timer_set_cntkctl(u32 cntkctl)
>>>>>   {
>>>>>   	asm volatile("mcr p15, 0, %0, c14, c1, 0" : : "r" (cntkctl));
>>>>> +	isb();
>>>>>   }
>>>>>
>>>>>   #endif
>>>>> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
>>>>> index a652ce0..bdedd8f 100644
>>>>> --- a/arch/arm64/include/asm/arch_timer.h
>>>>> +++ b/arch/arm64/include/asm/arch_timer.h
>>>>> @@ -144,6 +144,7 @@ static inline u32 arch_timer_get_cntkctl(void)
>>>>>   static inline void arch_timer_set_cntkctl(u32 cntkctl)
>>>>>   {
>>>>>   	write_sysreg(cntkctl, cntkctl_el1);
>>>>> +	isb();
>>>>>   }
>>>>>
>>>>>   static inline u64 arch_counter_get_cntpct(void)
>>>>> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
>>>>> index fd4b7f6..47cf15e 100644
>>>>> --- a/drivers/clocksource/arm_arch_timer.c
>>>>> +++ b/drivers/clocksource/arm_arch_timer.c
>>>>> @@ -77,6 +77,7 @@ struct arch_timer {
>>>>>   static bool arch_counter_suspend_stop;
>>>>>   static bool vdso_default = true;
>>>>>
>>>>> +static cpumask_t evtstrm_available;
>>>>>   static bool evtstrm_enable = IS_ENABLED(CONFIG_ARM_ARCH_TIMER_EVTSTREAM);
>>>>>
>>>>>   static int __init early_evtstrm_cfg(char *buf)
>>>>> @@ -740,6 +741,7 @@ static void arch_timer_evtstrm_enable(int divider)
>>>>>   #ifdef CONFIG_COMPAT
>>>>>   	compat_elf_hwcap |= COMPAT_HWCAP_EVTSTRM;
>>>>>   #endif
>>>>> +	cpumask_set_cpu(smp_processor_id(), &evtstrm_available);
>>>>>   }
>>>>>
>>>>>   static void arch_timer_configure_evtstream(void)
>>>>> @@ -864,6 +866,11 @@ u32 arch_timer_get_rate(void)
>>>>>   	return arch_timer_rate;
>>>>>   }
>>>>>
>>>>> +bool arch_timer_evtstrm_available(void)
>>>>> +{
>>>>> +	return !!cpumask_test_cpu(smp_processor_id(), &evtstrm_available);
>>>>> +}
>>>>> +
>>>>
>>>> This could live in the header file as static inlin, if the evtstrm_available
>>>> is made global and could avoid a function call where it is used.
>>>
>>> You might need an EXPORT_SYMBOl as well if you go down that route.
>>
>> Hmmm, I'm not fond of making the cpu mask global. I understand you can avoid
>> the branch + ret of the function, but generally you want to check if the
>> event stream is present to know if you can use wfe and wake up, so I don't
>> think it might be used in a performance critical path.
>>
>> So I'm wondering, is it really worth it?
> 
> I have no strong preference, but I think you'll need o EXPORT something
> either way so that delay and friends can be used from modules.
> 

Turns out this is not the case. arch_timer_evtstrm_available is called 
by __delay which is itself exported and available for module.

So unless a module needs to check availability of the event stream there 
is no need to export it. I'll leave it like this for now unless there is 
a strong case to be able to inline or export arch_timer_evtstrm_available.

Thanks,

-- 
Julien Thierry