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

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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...


Well, we probably should leave the possibility in to use 0x80 -- for one thing, we need to use 0x80 on 386, and there is always the possibility that the switch will have different timing properties on some or all machines.

Note that this doesn't require that a machine actually implements port 0xf0 for FERR/IGNNE, it just requires that they don't use it for something else.

I would be rather inclined to try using port 0xf0 by default as long as family > 3[*] (with fallback to port 0x80) at least experimentally (-mm).

We *might* even be able to use port 0xf0 unconditionally in the setup code, since we're not using the FPU there (the only FPU instructions in the setup code are there to detect the FPU.)

One thing: although I believe most actual implementations of port 0xf0 implement it as a strobe alone (data is ignored), all documentation I've found, including "The Undocumented PC" specifically says "write 0x00 to this port." This *could* mean there are platforms which use other values than 0x00 for other hacks.

	-hpa


[*] The following statements are equivalent:
    - family > 3.
    - CR0.NE is settable.
    - EFLAGS.AC is settable.

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