linux-kernel - Re: [PATCH] clocksource: shift helper

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Message-Id: <20080508.012354.82382504.anemo@mba.ocn.ne.jp>
Date:	Thu, 08 May 2008 01:23:54 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	dwalker@...sta.com
Cc:	tglx@...utronix.de, akpm@...ux-foundation.org, johnstul@...ibm.com,
	ralf@...ux-mips.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clocksource: shift helper

On Tue, 06 May 2008 09:39:14 -0700, Daniel Walker <dwalker@...sta.com> wrote:
> > > This is a little helper I pulled from the mips tree.
> > ... 
> > > Signed-off-by: Daniel Walker <dwalker@...sta.com>
> > 
> > And who is the original author of this little helper in the MIPS tree?
> 
> commit 16b7b2ac0148e839da86af8747b6fa4aad43a9b7
> Author: Atsushi Nemoto <anemo@....ocn.ne.jp>

I think Ralf's version is better starting point.

commit 93c846f9047f392fc2335668a5234edfbddb7cdc
Author: Ralf Baechle <ralf@...ux-mips.org>
Date:   Fri Oct 19 08:13:08 2007 +0100

    [MIPS] time: Helpers to compute clocksource/event shift and mult values.

These helpers are live in arch/mips/kernel/time.c now.

void __init clocksource_set_clock(struct clocksource *cs, unsigned int clock)
{
	u64 temp;
	u32 shift;

	/* Find a shift value */
	for (shift = 32; shift > 0; shift--) {
		temp = (u64) NSEC_PER_SEC << shift;
		do_div(temp, clock);
		if ((temp >> 32) == 0)
			break;
	}
	cs->shift = shift;
	cs->mult = (u32) temp;
}

BTW, comparing this with clocksource_hz2mult(), inserting

	temp += clock / 2;

to just before do_div() line might be better.

---
Atsushi Nemoto
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