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

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Date:   Wed, 15 Feb 2017 10:33:39 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.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

Hi,

On 02/14/2017 05:23 PM, Sergei Shtylyov wrote:
> 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.

Sure.

Best regards,
Lu Baolu

>
>> +    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
>
>