Message-ID: <47640C33.1050106@reed.com>
Date: Sat, 15 Dec 2007 12:17:39 -0500
From: "David P. Reed" <dpreed@reed.com>
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To: "H. Peter Anvin" <hpa@zytor.com>
CC: Alan Cox <alan@lxorguk.ukuu.org.uk>, Pavel Machek <pavel@ucw.cz>,
       Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
       Rene Herman <rene.herman@keyaccess.nl>
Subject: Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80
 on some AMD64x2 laptops, etc.
References: <469578CD.3080609@reed.com>	<1184216528.12353.203.camel@chaos>	<1184218962.12353.209.camel@chaos>	<46964352.7040301@reed.com>	<1184253339.12353.223.camel@chaos>	<469697C6.50903@reed.com>	<1184274754.12353.254.camel@chaos>	<4761F193.7090400@reed.com>	<20071214131502.GA14359@elte.hu>	<4762C551.5070003@zytor.com>	<20071214210652.GB28793@elf.ucw.cz>	<4763001A.1070102@zytor.com> <20071214232955.545ab809@the-village.bc.nu> <4763442A.5020507@reed.com> <476369E2.8050301@zytor.com>
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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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