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DateSat, 15 Dec 2007 12:17:39 -0500
From"David P. Reed" <>
SubjectRe: [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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