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To: Nate Lawson <nate@root.org>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Sat, 02 Jun 2007 17:30:10 MST."
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Date: Sun, 03 Jun 2007 08:55:53 +0000
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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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In message <46620B92.8020608@root.org>, Nate Lawson writes:
>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:

It's implemented in hardware, that's why the read is so slow.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.