linux-kernel - Re: [PATCH v7 1/5] x86: add simple udelay calibration

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Date:   Tue, 14 Feb 2017 12:23:56 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Mathias Nyman <mathias.nyman@...ux.intel.com>, tglx@...utronix.de,
        peterz@...radead.org, linux-usb@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 1/5] x86: add simple udelay calibration

Hello!

On 2/14/2017 5:27 AM, Lu Baolu wrote:

> Add a simple udelay calibration in x86 architecture-specific
> boot-time initializations. This will get a workable estimate
> for loops_per_jiffy. Hence, udelay() could be used after this
> initialization.
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: x86@...nel.org
> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  arch/x86/kernel/setup.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 4cfba94..aab7faa 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -835,6 +835,26 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>  	return 0;
>  }
>
> +static void __init simple_udelay_calibration(void)
> +{
> +	unsigned int tsc_khz, cpu_khz;
> +	unsigned long lpj;
> +
> +	if (!boot_cpu_has(X86_FEATURE_TSC))
> +		return;
> +
> +	cpu_khz = x86_platform.calibrate_cpu();
> +	tsc_khz = x86_platform.calibrate_tsc();
> +
> +	tsc_khz = tsc_khz ? : cpu_khz;

    Why not:

	if (!tsc_khz)
		tsc_khz = cpu_khz;

    It's more clear IMHO.

> +	if (!tsc_khz)
> +		return;
> +
> +	lpj = tsc_khz * 1000;
> +	do_div(lpj, HZ);
> +	loops_per_jiffy = lpj;
> +}
> +
>  /*
>   * Determine if we were loaded by an EFI loader.  If so, then we have also been
>   * passed the efi memmap, systab, etc., so we should use these data structures
[...]

MBR, Sergei