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

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Message-ID: <fe5b1c72-2371-bdaa-3f56-dd6e49368d35@redhat.com>
Date:	Wed, 13 Jul 2016 15:51:57 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Nicolai Stange <nicstange@...il.com>
Cc:	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



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.

Paolo