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    SubjectRe: New topic (PowerPC Linux PCI HELL)

    On Fri, 15 Sep 2000, Linus Torvalds wrote:

    > On Fri, 15 Sep 2000, Gérard Roudier wrote:
    > >
    > > > Just as an example: imagine that the IO windows haven't been set up
    > > > correctly. If the low-level driver just blindly enables IO cycles by
    > > > writing to the PCI_COMMAND register, that device may come up in an invalid
    > > > state, and mess up the whole system. The driver simply does not KNOW
    > > > enough. It doesn't know where other devices are, and it _shouldn't_ know.
    > >
    > > How do you want this to happen ? Could you elaborate.
    > It's really easy.
    > Call "pci_enable_device()".
    > What's so hard about that?

    This function delegates too much as a whole to the PCI generic layer, IMO.
    Imagine that for sanity I want to allocate all the device resources, but
    only _enable_ part of device features (for example only memory
    transactions). Imagine some special handling to be necessary due to some
    chip bug.

    > You don't seem to realize, but it's entirely possible to have a setup
    > where some device CANNOT be allocated it's IO region. The BIOS may have
    > left the device disabled on purpose, simply because there wasn't enough
    > free space in the memory map to enable the device anywhere.

    PCI specs said corresponding BAR must be set to ZERO, here.

    > You can't just have the device driver enable such a device. It _has_ to
    > ask the PCI layer to do it for it - because the PCI layer is the only one
    > that can figure out that "Oh, damn, this machine has 3GB of memory, and 4
    > video cards that want a 256MB aperture each, and we don't have any place
    > to map this card any more".

    I want to say the generic layer "What to do" in some more fine-grained way
    than just a single verb, at least. I may accept to delegate it some "How
    to do it".

    > No ifs, why's or buts. A driver that just enables the IO windows is a
    > buggy driver.

    In PCI, you donnot enable windows, but you enable/disable devices to
    generate and/or respond to transactions. OTOH, if you look at all the bits
    in the COMMAND register, you will see that some other features are also to
    be addressed by the enabling/disabling kernel interface.

    > > > In contrast, the general PCI layer _does_ know. It keeps track of
    > > > resources, makes sure that different cards do not have overlapping address
    > > > ranges, knows about PCI bridges (a card behind a PCI bridge can only be
    > > > enabled after the _bridge_ has been enabled and can only be mapped in the
    > > > region that the bridge maps).
    > >
    > > Yes, it is expected to do its work (e.g. assigning ressources to all
    > > agents on the BUS hierachy).
    > Right.
    > And sometimes it CANNOT.
    > End of story.
    > Linus


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