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

From: [email-masked] (Will Deacon)
Date: Thu, 12 Oct 2017 11:26:12 +0100

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 <[email-masked]>
> >>>Cc: Mark Rutland <[email-masked]>
> >>>Cc: Marc Zyngier <[email-masked]>
> >>>Cc: Russell King <[email-masked]>
> >>>Cc: Catalin Marinas <[email-masked]>
> >>>Cc: Will Deacon <[email-masked]>
> >>>---
> >>>  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.

Will