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    SubjectRe: IO delay, port 0x80, and BIOS POST codes
    On Thu, 14 Mar 2002, Linus Torvalds wrote:

    > Date: Thu, 14 Mar 2002 19:23:20 +0000 (UTC)
    > From: Linus Torvalds <torvalds@transmeta.com>
    > To: linux-kernel@vger.kernel.org
    > Subject: Re: IO delay, port 0x80, and BIOS POST codes
    >
    > In article <E16lZg3-0001Ug-00@the-village.bc.nu>,
    > Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
    > >> Unfortunately we can't read this information because Linux uses
    > >> port 80 as "dummy" port for delay operations. (outb_p and friends,
    > >> actually there seem to be a more hard-coded references to port
    > >> 0x80 in the code).
    > >
    > >The dummy port needs to exist. By using 0x80 we have probably the only
    > >port we can safely use in this way. We know it fouls old style POST
    > >boards on odd occasions.
    >
    > In fact, port 0x80 is safe exactly _because_ it is used for BIOS POST
    > codes, which pretty much guarantees that it will never be used for
    > anything else. That tends to not be as true of any other ports.
    >
    > Also, it should be noted that to get the 1us delay, the port should be
    > behind the ISA bridge in a legacy world (in a modern all-PCI system it
    > doesn't really matter, because the ports that need more delays are
    > faster too, so this works out ok). That's why I personally would be
    > nervous about using some of the well-specified (but irrelevant) ports
    > that are on the PCI side of a super-IO controller.
    >
    > I suspect the _real_ solution is to stop using "inb_p/outb_p" and make
    > the delay explicit, although it may be that some drivers depend on the
    > fact that not only is the "outb $0x80" a delay, it also tends to act as
    > a posting barrier.
    >
    > Linus

    No, the better/correct port is 0xED which removes the conflict.

    We've used 0xED w/o problem doing an embedded linux implementation
    at kernel 2.4.1, where SMM issues were involved. (It was recommended
    to me by an x-Phoenix BIOS developer, because of its safety as well as
    conflict resolution,

    Johnh

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    John Heil
    South Coast Software
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