linux-kernel - Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers when requested

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Message-ID: <53FEF50F.8030206@arm.com>
Date:	Thu, 28 Aug 2014 10:23:27 +0100
From:	Marc Zyngier <marc.zyngier@....com>
To:	Sonny Rao <sonnyrao@...omium.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Russell King <linux@....linux.org.uk>,
	Sudeep Holla <Sudeep.Holla@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dianders@...omium.org" <dianders@...omium.org>,
	Olof Johansson <olof@...om.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] clocksource: arch_timer: Fix code to use physical timers
 when requested

[cc-ing Will for the VDSO bits]

Hi Sonny,

On 27/08/14 22:03, Sonny Rao wrote:
> This is a bug fix for using physical arch timers when
> the arch_timer_use_virtual boolean is false.  It restores the
> arch_counter_get_cntpct() function after removal in
> 
> 0d651e4e "clocksource: arch_timer: use virtual counters"

This isn't a bug, but rather a feature. It allows the kernel to
consistently use the virtual counter everywhere (including userspace),
no matter if it behaves as a host or a guest, without worrying about the
accessibility of the physical counter (hint: you have no way of knowing
if it is accessible or not).

> and completes the implementation of memory mapped access for physical
> timers, so if a system is trying to use physical timers, it will
> function properly.
> 
> Signed-off-by: Sonny Rao <sonnyrao@...omium.org>
> ---
>  arch/arm/include/asm/arch_timer.h    |  9 +++++++++
>  arch/arm64/include/asm/arch_timer.h  | 10 ++++++++++
>  drivers/clocksource/arm_arch_timer.c | 30 ++++++++++++++++++++++++++----
>  3 files changed, 45 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/include/asm/arch_timer.h b/arch/arm/include/asm/arch_timer.h
> index 0704e0c..e72aa4d 100644
> --- a/arch/arm/include/asm/arch_timer.h
> +++ b/arch/arm/include/asm/arch_timer.h
> @@ -78,6 +78,15 @@ static inline u32 arch_timer_get_cntfrq(void)
>  	return val;
>  }
>  
> +static inline u64 arch_counter_get_cntpct(void)
> +{
> +	u64 cval;
> +
> +	isb();
> +	asm volatile("mrrc p15, 0, %Q0, %R0, c14" : "=r" (cval));
> +	return cval;
> +}
> +
>  static inline u64 arch_counter_get_cntvct(void)
>  {
>  	u64 cval;
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 9400596..58657c4 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -135,6 +135,16 @@ static inline void arch_timer_evtstrm_enable(int divider)
>  #endif
>  }
>  
> +static inline u64 arch_counter_get_cntpct(void)
> +{
> +	u64 cval;
> +
> +	isb();
> +	asm volatile("mrs %0, cntpct_el0" : "=r" (cval));
> +
> +	return cval;
> +}
> +
>  static inline u64 arch_counter_get_cntvct(void)
>  {
>  	u64 cval;
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index 5163ec1..ad723cb 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -30,6 +30,8 @@
>  #define CNTTIDR		0x08
>  #define CNTTIDR_VIRT(n)	(BIT(1) << ((n) * 4))
>  
> +#define CNTPCT_LO	0x00
> +#define CNTPCT_HI	0x04
>  #define CNTVCT_LO	0x08
>  #define CNTVCT_HI	0x0c
>  #define CNTFRQ		0x10
> @@ -386,6 +388,19 @@ static u64 arch_counter_get_cntvct_mem(void)
>  	return ((u64) vct_hi << 32) | vct_lo;
>  }
>  
> +static u64 arch_counter_get_cntpct_mem(void)
> +{
> +	u32 pct_lo, pct_hi, tmp_hi;
> +
> +	do {
> +		pct_hi = readl_relaxed(arch_counter_base + CNTPCT_HI);
> +		pct_lo = readl_relaxed(arch_counter_base + CNTPCT_LO);
> +		tmp_hi = readl_relaxed(arch_counter_base + CNTPCT_HI);
> +	} while (pct_hi != tmp_hi);
> +
> +	return ((u64) pct_hi << 32) | pct_lo;
> +}
> +
>  /*
>   * Default to cp15 based access because arm64 uses this function for
>   * sched_clock() before DT is probed and the cp15 method is guaranteed
> @@ -429,10 +444,17 @@ static void __init arch_counter_register(unsigned type)
>  	u64 start_count;
>  
>  	/* Register the CP15 based counter if we have one */
> -	if (type & ARCH_CP15_TIMER)
> -		arch_timer_read_counter = arch_counter_get_cntvct;
> -	else
> -		arch_timer_read_counter = arch_counter_get_cntvct_mem;
> +	if (type & ARCH_CP15_TIMER) {
> +		if (arch_timer_use_virtual)
> +			arch_timer_read_counter = arch_counter_get_cntvct;
> +		else
> +			arch_timer_read_counter = arch_counter_get_cntpct;
> +	} else {
> +		if (arch_timer_use_virtual)
> +			arch_timer_read_counter = arch_counter_get_cntvct_mem;
> +		else
> +			arch_timer_read_counter = arch_counter_get_cntpct_mem;
> +	}
>  
>  	start_count = arch_timer_read_counter();
>  	clocksource_register_hz(&clocksource_counter, arch_timer_rate);
> 

What will be the effect on arm64 where the VDSO uses the virtual counter
to implement gettimeofday?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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