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:	Sat, 15 Dec 2007 12:17:39 -0500
From:	"David P. Reed" <dpreed@...d.com>
To:	"H. Peter Anvin" <hpa@...or.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.



H. Peter Anvin wrote:
> 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.
>
My understanding is that the linux starts in real mode, and uses the 
BIOS for such things as reading the very first image.   
arch/x86/boot/main.c seems to use BIOS calls, and one can do what I 
wrote in C or asm.  Good place to measure the appropriate delay timing, 
and pass it on forward.  That's what I was suggesting, which is why I 
copied the ASM routine from my old code listing as I did.

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