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Date: Sat, 02 Jun 2007 17:30:10 -0700
From: Nate Lawson <nate@root.org>
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To: Poul-Henning Kamp <phk@phk.freebsd.dk>
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Cc: takawata@freeBSD.org, Dag-Erling Sm??rgrav <des@des.no>,
	current@freeBSD.org, Kris Kennaway <kris@obsecurity.org>
Subject: Re: HPET vs other timers
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Poul-Henning Kamp wrote:
> Nate Lawson wrote:
>>> Anyone able to speculate why though?  HPET only reads 32 bits from a
>>> memory mapped region.  No locking or other requirements.  ACPI_timer
>>> does multiple IO ops, which according to bde@ are much slower than
>>> memory reads.  
> 
> HPET needs to do metastability mitigation and is not "just a read
> from a memory mapped region".

If it does, then it's not implemented yet:

static u_int
hpet_get_timecount(struct timecounter *tc)
{
        struct acpi_hpet_softc *sc;

        sc = tc->tc_priv;
        return (bus_read_4(sc->mem_res, HPET_OFFSET_VALUE));
}

Are you sure you're not thinking of ACPI-safe?

-- 
Nate