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    SubjectRe: [PATCH] ALSA: hda-intel - Disable MSI support by default
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     > Now, I think that a MSI thing should look like a PCI write to a magic 
    > address (I'm really not very up on it, so correct me if I'm wrong), and
    > thus maybe bridges are bound to get it right, and the only thing we really
    > need to worry about is the host bridge. Maybe.

    Yes, an MSI message really is just a PCI write cycle (which is why it
    obeys all the PCI ordering as DaveM pointed out, and also can travel
    on different PCIe VCs, etc). So it's hard to see how a bridge could
    mess it up without screwing up lots of other stuff. The big thing is
    the host bridge that has to turn the PCI write into an interrupt, and
    that's where all the chipset problems that I know of are.

    But on the other hand I don't think we have much experience with
    MSI-capable devices below multiple levels of bridges, so you're right,
    there is an unknown risk here.

    > But right now I'm not convinced we really know what all goes wrong. Maybe
    > it's just broken NVidia and AMD bridges. But maybe it's also individual
    > devices that continue to (for example) raise _both_ the legacy IRQ line
    > _and_ send an MSI request.

    Well, since we now require drivers to "opt in" to MSI, I'm not
    convinced that broken devices are a huge issue. We can take things
    case-by-case and, obviously, if some random subset of devices are
    broken, then clearly the driver should be really careful about
    enabling MSI. But usually there's something like a device ID,
    revision, FW version, etc. that we can test.

    > together with certain Intel MSI-capable chips (ie e1000, Intel HDA etc).

    Just for the record -- many e1000 chips that have an MSI capability in
    their PCI config actually do not have working MSI. Hence the

    if (adapter->hw.mac_type > e1000_82547_rev_2) {
    adapter->have_msi = TRUE;

    in the e1000 driver I guess.

    BTW, at the end of the day I agree with the "MSI off by default"
    policy, especially for the sound driver where the only realy gain is
    saving IRQ routing hassles as you said. Even for the mthca InfiniBand
    driver, where MSI-X with multiple vectors is a huge performance win, I
    still have the user explicitly enable it.

    - R.
    -
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