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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