linux-kernel - Re: [PATCH v2 7/10] x86/tsc: enable APB timer based calibration

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Date:	Wed, 19 Aug 2009 17:51:00 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Pan, Jacob jun" <jacob.jun.pan@...el.com>
cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 7/10] x86/tsc: enable APB timer based calibration

On Thu, 16 Jul 2009, Pan, Jacob jun wrote:

> >From e9264a1e5f8662d64f7d042544a03504b325bac9 Mon Sep 17 00:00:00 2001
> From: Jacob Pan <jacob.jun.pan@...el.com>
> Date: Wed, 15 Jul 2009 14:20:02 -0700
> Subject: [PATCH] x86/tsc: enable APB timer based calibration
> 
> for platform that do not have PIT, we can use APB timer (if available)
> to calibrate TSC by counting the two. APB timer frequency is known to
> the system by SFI table provided by FW.
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@...el.com>
> ---
>  arch/x86/kernel/tsc.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 6e1a368..8b38a9d 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -18,6 +18,9 @@
>  #include <asm/delay.h>
>  #include <asm/hypervisor.h>
>  
> +#include <asm/platform_feature.h>
> +#include <asm/apb_timer.h>
> +
>  unsigned int __read_mostly cpu_khz;	/* TSC clocks / usec, not used here */
>  EXPORT_SYMBOL(cpu_khz);
>  
> @@ -394,6 +397,14 @@ unsigned long native_calibrate_tsc(void)
>  		return hv_tsc_khz;
>  	}
>  
> +	/* first check if apb timer exist and is usable */
> +	if (platform_has(X86_PLATFORM_FEATURE_APBT)) {
> +		local_irq_save(flags);
> +		fast_calibrate = apbt_quick_calibrate();
> +		local_irq_restore(flags);
> +		if (fast_calibrate)
> +			return fast_calibrate;

  And what is calibrating the TSC here. There is not PIT or comes that
  magically when the APBT thing fails ?

> +	}
>  	local_irq_save(flags);
>  	fast_calibrate = quick_pit_calibrate();
>  	local_irq_restore(flags);
> -- 
> 1.5.6.5
> 
> --
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