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Date: Sun, 30 Oct 2005 07:03:57 -0700
From: Scott Long <scottl@samsco.org>
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To: David Xu <davidxu@freebsd.org>
References: <30595.1130493297@critter.freebsd.dk>	<20051028153457.d0wqgn2ask4sgw4k@netchild.homeip.net>	<20051029195703.GB39253@dragon.NUXI.org>
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Cc: Alexander Leidinger <Alexander@Leidinger.net>, freebsd-current@freebsd.org
Subject: Re: TSC instead of ACPI: powerd doesn't work anymore
	(to	be	expected?)
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David Xu wrote:
> David O'Brien wrote:
> 
>> On Fri, Oct 28, 2005 at 03:34:57PM +0200, Alexander Leidinger wrote:
>>  
>>
>>> I don't have the message at hand. I just had time to write the mail, 
>>> but I
>>> don't have my laptop with me to reproduce the message. But it's easy to
>>> reproduce, just take a PC which is able to make use of powerd and 
>>> switch to
>>> using TSC as the timecounter.
>>>   
>>
>>
>> What is the motivation to use the TSC as a timecounter?
>>
>>  
>>
> TSC is faster than any others, on many systems, so-called ACPI-fast 
> timer is
> really a slow chip, at least far slower than reading from RAM, 
> manufactories
> just lie on this.
> 
> Regards,
> David Xu
> 

Kind of.  The TSC is internal to the CPU and can be read without any
memory accesses or synchronization.  The ACPI-faster counter needs to
be read with an ioport instruction, which is exceedingly slow on
modern hardware.  ACPI-safe needs _three_ ioport reads.

Scott