linux-kernel - Re: [RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a separate function

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Date:   Fri, 14 Jun 2019 17:54:05 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi.kleen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        Stephane Eranian <eranian@...gle.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Ricardo Neri <ricardo.neri@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>,
        Clemens Ladisch <clemens@...isch.de>,
        Arnd Bergmann <arnd@...db.de>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a
 separate function

On Thu, 23 May 2019, Ricardo Neri wrote:
>  
> +u64 hpet_get_ticks_per_sec(u64 hpet_caps)
> +{
> +	u64 ticks_per_sec, period;
> +
> +	period = (hpet_caps & HPET_COUNTER_CLK_PERIOD_MASK) >>
> +		 HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
> +
> +	/*
> +	 * The frequency is the reciprocal of the period. The period is given
> +	 * in femtoseconds per second. Thus, prepare a dividend to obtain the
> +	 * frequency in ticks per second.
> +	 */
> +
> +	/* 10^15 femtoseconds per second */
> +	ticks_per_sec = 1000000000000000ULL;
> +	ticks_per_sec += period >> 1; /* round */
> +
> +	/* The quotient is put in the dividend. We drop the remainder. */
> +	do_div(ticks_per_sec, period);
> +
> +	return ticks_per_sec;
> +}
> +
>  int hpet_alloc(struct hpet_data *hdp)
>  {
>  	u64 cap, mcfg;
> @@ -844,7 +867,6 @@ int hpet_alloc(struct hpet_data *hdp)
>  	struct hpets *hpetp;
>  	struct hpet __iomem *hpet;
>  	static struct hpets *last;
> -	unsigned long period;
>  	unsigned long long temp;
>  	u32 remainder;
>  
> @@ -894,12 +916,7 @@ int hpet_alloc(struct hpet_data *hdp)
>  
>  	last = hpetp;
>  
> -	period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
> -		HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
> -	temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */
> -	temp += period >> 1; /* round */
> -	do_div(temp, period);
> -	hpetp->hp_tick_freq = temp; /* ticks per second */
> +	hpetp->hp_tick_freq = hpet_get_ticks_per_sec(cap);

Why are we actually computing this over and over?

In hpet_enable() which is the first function invoked we have:

        /*
         * The period is a femto seconds value. Convert it to a
         * frequency.
         */
        freq = FSEC_PER_SEC;
        do_div(freq, hpet_period);
        hpet_freq = freq;

So we already have ticks per second, aka frequency, right? So why do we
need yet another function instead of using the value which is computed
once? The frequency of the HPET channels has to be identical no matter
what. If it's not HPET is broken beyond repair.

Thanks,

	tglx