Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80on some AMD64x2 laptops, etc.

[David P. Reed]

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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