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

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Message-ID: <20190422120451.GA9499@gmail.com>
Date:   Mon, 22 Apr 2019 14:04:51 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Daniel Drake <drake@...lessm.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org, len.brown@...el.com,
        rafael.j.wysocki@...el.com, linux@...lessm.com
Subject: Re: [PATCH 2/2] x86/tsc: set LAPIC timer frequency to crystal clock
 frequency


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

> +#ifdef CONFIG_X86_LOCAL_APIC
> +	/*
> +	 * The local APIC appears to be fed by the core crystal clock
> +	 * (which sounds entirely sensible). We can set the global
> +	 * lapic_timer_frequency here to avoid having to calibrate the APIC
> +	 * timer later.
> +	 */
> +	lapic_timer_frequency = (crystal_khz * 1000) / HZ;
> +#endif

Minor style nit: the parentheses are unnecessary, integer expressions 
like this are evaluated left to right and multiplication and division has 
the same precedence.

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;

Thanks,

	Ingo