Re: [PATCH] x86: provide a DMI based port 0x80 I/O delay override.

[H. Peter Anvin]

Rene Herman wrote:
> On 17-12-07 03:05, H. Peter Anvin wrote:
>
> > Incidentally, I had the thought earlier today that port 0xf0 might be 
> > a suitable delay port.  It is used only by the 387-emulating-a-287 
> > hack for IRQ 13, which Linux doesn't use on 486+.
>
> rene@7ixe4:~/src/port80$ su -c ./port80
> cycles: out 2400, in 2400
> rene@7ixe4:~/src/port80$ su -c ./portf0
> cycles: out 2400, in 2400
>
> (Duron 1300)
>
> I suppose you mean using it instead of port 0x80 always and not just as 
> an alternate port? For the latter 0xed is alright I guess...

FWIW, the criterion used in the kernel for when to use IRQ 13 is:

        /*
         * External FPU? Set up irq13 if so, for
         * original braindamaged IBM FERR coupling.
         */
        if (boot_cpu_data.hard_math && !cpu_has_fpu)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                setup_irq(FPU_IRQ, &fpu_irq);

In that case we can't actually use port 0xF0 (it is, however, safe to 
use it during setup, specifically before we can take our first FPU 
exception.)

	-hpa
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