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    SubjectRe: [patch non-x86] PCI-PCI bridges fix
    On Fri, Nov 23, 2001 at 08:49:30AM +1100, Paul Mackerras wrote:
    > There is code in pci_read_bridge_bases which will decide under some
    > circumstances that a PCI-PCI bridge is transparent and set
    > child->resource[i] = child->parent->resource[i];
    > for i = 0, 1 or 2. What will happen with your request_resource in
    > this case is that the resource will end up being its own parent and
    > its own child and all allocations against it will fail. :)

    These code paths shouldn't be mixed in the first place, i.e. if
    you use stuff in setup-bus.c, don't use pci_read_bridge_bases.
    Basically, these are two different approaches to PCI setup:
    "We trust our firmware" (i386, ia64) and "We are the firmware"
    (alpha, arm, partially parisc).

    > I hit this just yesterday on PPC. On our RS/6000 boxes, if you have a
    > PCI-PCI bridge with nothing behind it, the firmware will configure it
    > with all the apertures closed, i.e. with base set larger than limit.

    Right, as required by PCI-PCI bridge specs.

    > Then pci_read_bridge_bases goes and decides that the bridge is
    > transparent and consequently everything stops working. :(

    I think that "assuming transparent" code is not correct. Yes, PCI-PCI
    bridge specs allow "transparent" (i.e. subtractive decoding) bridges,
    but such bridges must have a class code 0x60401. IMO, we need something
    like this instead:

    if (!dev) /* It's a host bus, nothing to read */

    + if (dev->class & 1) {
    + printk("Subtractive decoding bridge %s: assuming transparent\n",
    + dev->name);
    + for(i=0; i<3; i++)
    + child->resource[i] = child->parent->resource[i];
    + return;
    + }

    Of course, this requires testing on problematic machines.
    BTW, currently almost every PC user sees those "Unknown bridge resource"
    messages just because a graphic card behind the AGP bridge doesn't use
    IO or prefetchable memory...

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