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    SubjectRe: [ACPI] Re: Legacy IO spaces (was Re: [RFC] Reliable video POSTing on resume)

    > If they all point to the same space, I can't tell whether I have three
    > legacy spaces or one. I need to know how many legacy spaces there are
    > in order to know how many VGA cards can simultaneously be enabled.

    You don't need to care about this, at least in userland. All you need
    is proper primitives for locking/unlocking access to a given device.
    Wether you have another one to arbitrate with on the same PCI bus or not
    is almost irrelevant. That is, it is the job of the kernel driver that
    ultimately will do this arbitration (we really need that), and we can
    prefectly well survive a long time with a very simple implementation
    taht always disable all other VGA devies in the system, not caring about
    "simultaneous" access. That implementation can be then improved on

    My point is what we really need to define is the in-kernel and userland
    API to do this basic VGA access arbitration in the first place. I though
    you did something like that a while ago Jon, didn't you ?

    I think it could be as simple as an additional sysfs entry
    "legacy_enabled" added to all "VGA" devices in the system at the PCI
    layer level. Toggling it triggers the "untoggling" of all others,
    including VGA forwarding on bridges, and enables the path to that
    device. For in-kernel users, a pci_* API would work.

    The problem I see though is that it should all be synchronous &
    spinlocked since the vgacon could want to grab at interrupt time (unless
    it's locked by userland, in which case, vgacon should cache & trigger an
    update later).


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