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    SubjectRe: Multiple MSI
    On Wed, 2008-07-02 at 21:59 -0600, Matthew Wilcox wrote:
    > This is true and worth considering carefully. Are IRQ numbers a scarce
    > resource on PowerPC? They are considerably less scarce than interrupt
    > vectors are on x86-64. How hard is it to make IRQ numbers an abundent
    > resource? Is it simply a question of increasing NR_IRQS?

    Yes, indeed, they aren't really scarce... actually less than the
    underlying HW vectors in most cases, so it isn't a big issue to add some
    kind of constraint to the allocator.

    > This cost should be traded off against the cost of allocating something
    > like the msix_entry array in each driver that wants to use multiple MSIs,
    > passing that array around, using it properly, etc.
    > It would make some sense to pass nr_irqs all the way down to arch code
    > and let arch code take care of reserving the block of vectors (aligned
    > appropriately). That would conserve IRQ numbers, though not vectors.
    > I think we have to consider excess vectors reserved. If we don't, we
    > could get into the situation where a device uses more interrupts than
    > the driver thinks it will and problems ensue.

    Ok, so I lift my objection there in the sense that allocating a linear
    array of virtual numbers shouldn't be a problem (somebody remind me to
    make NR_IRQS a config option one of these days on ppc, or help with just
    getting rid of irq_desc array alltogether :-)

    However, do you want to still keep the fact that they are power-of-2
    aligned up to the API or can I just do a linear block allocation for
    virtual number sand require drivers to do the appropriate
    addition/subtraction to get the N'th one ? I will need to allocate
    appropriately aligned HW numbers but that's done via different
    mechanisms (and in some case not even under full linux control, ie,
    hypervisor/firmware does it on pSeries).

    > By the way, would people be interested in changing the MSI-X API to get
    > rid of the msix_entry array? If allocating consecutive IRQs isn't a
    > problem, then we could switch the MSI-X code to use consecutive IRQs.

    It would make a lot of code simpler...


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