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    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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