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Message-ID: <475D2C37.6010809@keyaccess.nl>
Date: Mon, 10 Dec 2007 13:08:23 +0100
From: Rene Herman <rene.herman@keyaccess.nl>
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To: Krzysztof Halasa <khc@pm.waw.pl>
CC: Pavel Machek <pavel@ucw.cz>, Andi Kleen <andi@firstfloor.org>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, "David P. Reed" <dpreed@reed.com>,
       linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
       Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: RFC: outb 0x80 in inb_p, outb_p harmful on some modern AMD64
 with MCP51 laptops
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On 10-12-07 12:30, Krzysztof Halasa wrote:

> Rene Herman <rene.herman@keyaccess.nl> writes:
> 
>> Alan, did you double-check that 8 us? I tried to but I seem to not
>> have trustworthy documentation.
> 
> I remember 16-bit CPU-driven ISA was able to do 2-3 MB/s transfers,
> that means at least 1 Maccesses/second = up to 1 microsecond/access.

Yes, the thing is that I'm fairly convinced that an out to an unused port 
takes sort of exactly 1 us (8 cycles at 8 MHz). That's what I've always 
known it to be at least and have taken as the point. The same's also said 
here for example:

http://tldp.org/HOWTO/IO-Port-Programming-4.html

as well as in the bit of MINIX source in an earlier version of this thread:

http://lkml.org/lkml/2002/3/14/194

Oh, in fact, a few older posts by Alan himself:

http://lkml.org/lkml/2002/3/15/2
http://lkml.org/lkml/2003/9/22/263

I'm not a hardware person as the level of actual electrical signals but if 
an earlier instance of Alan is disagreeing with the current one as well, I 
guess it's halfway safe to join that one...

Rene.