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    SubjectRe: [PATCH] 2.6.18-rt7: PowerPC: fix breakage in threaded fasteoi type IRQ handlers

    Benjamin Herrenschmidt wrote:

    >> I must not that this whole ack() vs eoi() stuff is misleading. For example,
    >>in 8259 driver, mask_ack() method actually sends EOI to PIC, not ACK's an IRQ
    >>-- the actual ACK is implicit on x86 and is used to read the interrupt vector
    >>form 8259 on PPC. So, IMO, there probably should only have been either ack()
    >>or eoi() method in the first place. Though I'm not familiar with ARM from
    >>which genirq stuff originated...

    > They are different concepts. Ack clears the event on the PIC, it's
    > tyically necessary for resetting the edge detection logic for edge
    > interrupts and has to happen before the handler is called.

    I know 8259. :-)
    It also resets the corresponding IRQ bit in IRR, and sets it in ISR where
    it's then cleared on EOI command.

    > On MPIC or XICS, this is implicit by reading the vector. On some more
    > dumb controllers, this has to be done explicitely.

    This is not implicit -- CPU has to read INTACK reg. on OpenPIC. Really
    implicit method is in action on x86 where CPU issues dual ACK bus cycle to get
    the vector form 8259...

    > EOI is a more "high level" thing that some "intelligent" PICs that
    > automatically raise the priority do to restore the priority to what it
    > was before the interrupt occured.

    Thank you, I know. Even 8259 has the notion of priority and EOI works the
    same way here.

    > Ben.

    WBR, Sergei
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