[openib-general] Re: Disabling IRQ #201 message

[Roland Dreier]

    Grant> If the IRQ line is asserted and we get another interrupt
    Grant> for something we've already handled, then it means the
    Grant> regular interrupt code really hasn't cleaned up
    Grant> sufficiently.  There has to be some way to tell the card
    Grant> what work has been done *OR* flush all inflight DMA (MMIO
    Grant> read - Ouch!).  The interrupt handler needs to guarantee it
    Grant> has handled pending work and communicatied that back to the
    Grant> card so IRQ line is not asserted.

Due to the way interrupts are handled in the Mellanox HCA, I think it
will be quite inefficient to guarantee that the interrupt line is not
asserted when the interrupt handler exits, because:

 A. the HCA has multiple "event queues" (interrupt sources)
 B. clearing interrupts is done by writing to a register (no
    "clear-on-read" when reading the interrupt cause register)

So closing the race between clearing the interrupts and reading the
interrupt cause register is very expensive -- we would end up doing
twice as many MMIO reads as necessary in the common case.

However it should be harmless to handle a "spurious" interrupt
occasionally.  The kernel should only complain if 99900 out of the
last 100000 interrupts are spurious, which we shouldn't see.

 - R.