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    SubjectRe: [PATCH] VGA arbitration: draft of kernel side
    Date
    On Tuesday, March 8, 2005 9:58 pm, Jon Smirl wrote:
    > On Wed, 09 Mar 2005 16:37:13 +1100, Benjamin Herrenschmidt
    >
    > <benh@kernel.crashing.org> wrote:
    > > On Tue, 2005-03-08 at 23:35 -0500, Jon Smirl wrote:
    > > > This is from /linux-2.5/Documentation/filesystems/sysfs-pci.txt. It
    > > > describes how ia64 is achieving legacy IO. The VGA control code
    > > > probably needs to be coordinated with this.
    > >
    > > This is a different thing, and I will implement it on ppc one of these
    > > days. This is for issuing the IO cycles on the bus. It has nothing
    > > to do with the actual arbitration work.
    >
    > Each one of these legacy spaces corresponds to an allowable
    > simultaneous VGA use. There should be one arbiter per legacy space.

    Jon, I think the arbiters have to be per-resource rather than per-legacy
    space. AIUI, the arbiters are there to deal with multiple devices on the
    same bus that respond to the same cycles, like two VGA cards in one legacy
    I/O domain. You either need to relocate one of them so that they don't have
    overlapping I/O ranges or disable one while you talk to the other.

    IOW, legacy space is the whole I/O window of a given bus or PCI domain
    (granularity defined by the platform--some will only have one I/O space), and
    the arbiter's job is to arbitrate access to subsets of each window. I think
    the the VGA stuff here complements the legacy interface rather than
    conflicting with it.

    Jesse
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