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

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Message-ID: <20190515155426.GA10526@ranerica-svr.sc.intel.com>
Date:   Wed, 15 May 2019 08:54:26 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        "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 Tue, May 14, 2019 at 07:23:47AM -0700, Randy Dunlap wrote:
> 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.
> 

Thanks for your review Randy! I'll fix this grammar issue.
> > +	 * frequency in ticks per second.
> > +	 */
> > +
> > +	/* 10^15 femtoseconds per second */
> > +	ticks_per_sec = 1000000000000000uLL;
> 
> 	ULL is overwhelmingly used in the kernel.
> 

Sure, I'll update it.

BR,
Ricardo