linux-kernel - Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80 on some AMD64x2 laptops, etc.

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Date:	Fri, 14 Dec 2007 21:45:06 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"David P. Reed" <dpreed@...d.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, Pavel Machek <pavel@....cz>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Rene Herman <rene.herman@...access.nl>
Subject: Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80
 on some AMD64x2 laptops, etc.

David P. Reed wrote:
> Just a thought for a way to fix the "very early" timing needed to set up 
> udelay to work in a way that works on all machines.  Perhaps we haven't 
> exploited the BIOS enough:  The PC BIOS since the PC AT (286)  has 
> always had a standard "countdown timer" way to delay for n microseconds, 
> which as far as I know still works.   This can be used to measure the 
> speed of a delay loop, without using ANY in/out instructions directly 
> (the ones in the BIOS are presumably correctly delayed...).

If we enter from the 32-bit entrypoint, we already don't have access to 
the BIOS, though.

	-hpa
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