linux-kernel - Re: [PATCH v3 1/3] arch, x86, tsc deadline clockevent dev: reduce frequency roundoff error

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Message-ID: <8737ndtxrc.fsf@gmail.com>
Date:	Wed, 13 Jul 2016 16:31:51 +0200
From:	Nicolai Stange <nicstange@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Nicolai Stange <nicstange@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Borislav Petkov <bp@...e.de>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Len Brown <len.brown@...el.com>,
	"Christopher S. Hall" <christopher.s.hall@...el.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] arch, x86, tsc deadline clockevent dev: reduce frequency roundoff error

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 13/07/2016 15:49, Peter Zijlstra wrote:
>> On Wed, Jul 13, 2016 at 03:03:42PM +0200, Nicolai Stange wrote:
>> 
>>>  		clockevents_config_and_register(levt,
>>> +						(u32)(((u64)tsc_khz * 1000) /
>>> +							TSC_DIVISOR),
>>>  						0xF, ~0UL);
>> 
>> div_u64() perhaps ?
>
> Or just squash together the two patches and do
>
> 	tsc_khz * (1000 / TSC_DIVISOR)
>
> because with TSC_DIVISOR equal to 2/4/8 there is no error from
> reassociating the operation.

Oh great, I didn't see this. Will resend.

Thanks,

Nicolai