linux-kernel - Re: [PATCH v4 2/4] Always running timer correlated clocksource

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Message-ID: <20151118155353.4b37a06f@yairi>
Date:	Wed, 18 Nov 2015 15:53:53 -0800
From:	Jacob Pan <jacob.jun.pan@...el.com>
To:	"Hall, Christopher S" <christopher.s.hall@...el.com>
Cc:	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Stanton, Kevin B" <kevin.b.stanton@...el.com>,
	jacob.jun.pan@...el.com
Subject: Re: [PATCH v4 2/4] Always running timer correlated clocksource

On Mon, 12 Oct 2015 11:45:20 -0700
"Hall, Christopher S" <christopher.s.hall@...el.com> wrote:
took a while to read the code, i have a few comments/questions
>
> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index c3f7602..c3f098c 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -820,7 +820,7 @@ int recalibrate_cpu_khz(void)
>  #ifndef CONFIG_SMP
This code is used by old p4/k7 clock modulation driver, why do we care?

>         unsigned long cpu_khz_old = cpu_khz;
> 
> -       if (cpu_has_tsc) {
> +       if (boot_cpu_has(X86_FEATURE_ART)) {
>                 tsc_khz = x86_platform.calibrate_tsc();
>                 cpu_khz = tsc_khz;
>                 cpu_data(0).loops_per_jiffy =


> 
> +/*
> + * Convert ART to TSC given numerator/denominator found in
> detect_art()
> + */
> +static u64 convert_art_to_tsc(struct correlated_cs *cs, u64 cycles)
> +{
> +       u64 tmp, res;
> +
> +       res = (cycles / art_to_tsc_denominator) *
> art_to_tsc_numerator;
> +       tmp = (cycles % art_to_tsc_denominator) *
> art_to_tsc_numerator;
> +       res += tmp / art_to_tsc_denominator;
> +
iirc there are some issues with 32bit, better use div64_u64()
--
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