linux-kernel - Re: [PATCH 1/2] x86/tsc: Allow quick PIT calibration despite interruptions

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Message-ID: <993ab528-c06f-607c-438d-ec11b47427bf@schnhrr.de>
Date:   Thu, 14 Feb 2019 22:48:35 +0100
From:   Jan H. Schönherr <jan@...nhrr.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     bp@...en8.de, mingo@...hat.com, x86@...nel.org,
        pmenzel@...gen.mpg.de, Thomas.Lendacky@....com, hpa@...or.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/tsc: Allow quick PIT calibration despite
 interruptions

Am 12.02.19 um 12:57 schrieb Thomas Gleixner:
> On Tue, 29 Jan 2019, Thomas Gleixner wrote:
>> On Tue, 29 Jan 2019, Jan H. Schönherr wrote:
>>
>>> Am 29.01.2019 um 11:23 schrieb Jan H. Schönherr:
>>>> +calibrate:
>>>> +	/*
>>>> +	 * Extrapolate the error and fail fast if the error will
>>>> +	 * never be below 500 ppm.
>>>> +	 */
>>>> +	if (d1 + d1 >= (delta * MAX_QUICK_PIT_ITERATIONS) >> 11) {
>>>> +		pr_info("Fast TSC calibration failed (wouldn't work)\n");
>>>> +		return 0;
>>>> +	}
>>>
>>> I messed this check up. "delta" is not the actual tsc delta between the
>>> first two successful reads at this point.
>>>
>>> (If it were, it might also correspond to more than one iteration; not
>>> sure if we care about that aspect, though.)
>>
>> I rather go for correct and if only for the reason that me/you/whoelse
>> doesn't have to scratch the head for an hour when looking at this half a
>> year from now.
> 
> Any update on this?

Second version is out.

Regards
Jan