linux-kernel - Re: [RFC PATCH v3 03/21] x86/hpet: Calculate ticks-per-second in a separate function

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Date:   Tue, 14 May 2019 07:23:47 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>
Cc:     Ashok Raj <ashok.raj@...el.com>, Joerg Roedel <joro@...tes.org>,
        Andi Kleen <andi.kleen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, 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>,
        Stephane Eranian <eranian@...gle.com>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
Subject: Re: [RFC PATCH v3 03/21] x86/hpet: Calculate ticks-per-second in a
 separate function

On 5/14/19 7:01 AM, Ricardo Neri wrote:
> It is easier to compute the expiration times of an HPET timer by using
> its frequency (i.e., the number of times it ticks in a second) than its
> period, as given in the capabilities register.
> 
> In addition to the HPET char driver, the HPET-based hardlockup detector
> will also need to know the timer's frequency. Thus, create a common
> function that both can use.
> 
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Ashok Raj <ashok.raj@...el.com>
> Cc: Andi Kleen <andi.kleen@...el.com>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: Clemens Ladisch <clemens@...isch.de>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Philippe Ombredanne <pombredanne@...b.com>
> Cc: Kate Stewart <kstewart@...uxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
> Cc: Stephane Eranian <eranian@...gle.com>
> Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
> Cc: "Ravi V. Shankar" <ravi.v.shankar@...el.com>
> Cc: x86@...nel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
>  drivers/char/hpet.c  | 31 ++++++++++++++++++++++++-------
>  include/linux/hpet.h |  1 +
>  2 files changed, 25 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> index d0ad85900b79..bdcbecfdb858 100644
> --- a/drivers/char/hpet.c
> +++ b/drivers/char/hpet.c
> @@ -836,6 +836,29 @@ static unsigned long hpet_calibrate(struct hpets *hpetp)
>  	return ret;
>  }
>  
> +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
> +	 * femtoseconds per second. Thus, prepare a dividend to obtain the

	 * in femtoseconds per second.

> +	 * frequency in ticks per second.
> +	 */
> +
> +	/* 10^15 femtoseconds per second */
> +	ticks_per_sec = 1000000000000000uLL;

	ULL is overwhelmingly used in the kernel.

> +	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);
>  
>  	printk(KERN_INFO "hpet%d: at MMIO 0x%lx, IRQ%s",
>  		hpetp->hp_which, hdp->hd_phys_address,
> diff --git a/include/linux/hpet.h b/include/linux/hpet.h
> index 8604564b985d..e7b36bcf4699 100644
> --- a/include/linux/hpet.h
> +++ b/include/linux/hpet.h
> @@ -107,5 +107,6 @@ static inline void hpet_reserve_timer(struct hpet_data *hd, int timer)
>  }
>  
>  int hpet_alloc(struct hpet_data *);
> +u64 hpet_get_ticks_per_sec(u64 hpet_caps);
>  
>  #endif				/* !__HPET__ */
> 


-- 
~Randy