linux-kernel - Re: [PATCH 2/2] x86/tsc: set LAPIC timer frequency to crystal clock frequency

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Message-ID: <20190423103034.GA8216@gmail.com>
Date:   Tue, 23 Apr 2019 12:30:34 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Daniel Drake <drake@...lessm.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        hpa@...or.com, x86@...nel.org,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        Linux Upstreaming Team <linux@...lessm.com>
Subject: Re: [PATCH 2/2] x86/tsc: set LAPIC timer frequency to crystal clock
 frequency

* Daniel Drake <drake@...lessm.com> wrote:

> On Mon, Apr 22, 2019 at 8:04 PM Ingo Molnar <mingo@...nel.org> wrote:
> > Minor style nit: the parentheses are unnecessary, integer expressions
> > like this are evaluated left to right and multiplication and division has
> > the same precedence.
> 
> Fair point, although the same could be said for cpu_khz_from_msr().

Yes, this standardization on mhz, if it makes sense, might have to be 
propagated a bit to function names and any other variables.

Another naming quirk: what unit is 'lapic_timer_frequency' in? AFAICS 
it's a "period" unit (number of clock cycles per jiffy), not a frequency 
(which is number of cycles per second) - so the better name might be 
lapic_timer_period?

> > But it might also make sense to actually store crystal_mhz instead of
> > crystal_khz, because both CPUID 15H and 16H provides MHz values.
> >
> > That way the above expression would simplify to:
> >
> >         lapic_timer_frequency = crystal_mhz / HZ;
> 
> Wouldn't it be
>         lapic_timer_frequency = crystal_mhz * 1000000 / HZ;
> ?

Sorry, what I wanted to suggest is crystal_hz, not crystal_mhz.

I.e. store the raw unit that comes out of the CPUID, which appears to be 
HZ, right?

That would change the calculation to:

         lapic_timer_period = crystal_hz / HZ;

Note how clearly it's visible in that calculation that what we calculate 
there is a 'period', not a frequency.

Thanks,

	Ingo