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    SubjectRe: [PATCH 2.6.21-rt2] PowerPC: decrementer clockevent driver

    Benjamin Herrenschmidt wrote:

    >>>>Yes, on some implementations there can be other conditions that
    >>>>make a decrementer exception go away; there is no contradiction
    >>>>here (thankfully). My wording was sloppy.

    >>>Some CPUs have the DEC exceptions basically edge triggered (yeah I know

    >>for example?

    >>>it sucks). That's why, among others, the IRQ soft-disable code has code
    >>>to re-trigger DEC exceptions ASAP (by setting it to 1.. note that we
    >>>could probably use 0 here, we've been a bit conservative).

    Yeah, the classic decrementer is programmed off-by-one.

    > I'm not 100% certain... Paulus thinks all the old 6xx are like that, and
    > maybe POWER4. If I look at the oldest BookIV I can find (the 601), it

    From the "PowerPC Operating Environment Architecture" that I've already
    quoated t follows that POWER4-compatible decremented exception *must* be edge

    > says that an exception is generated when the MSB transitions from 0 to
    > 1. It's not clear wether the exception sticks while that bit is 1 or is

    Freescale MPC 7450 manual says the same, for example.

    > indeed considered as an "edge" event that gets cleared as soon as
    > delivered.

    > Ben.

    WBR, Sergei
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      Last update: 2007-05-18 15:43    [W:0.021 / U:2.436 seconds]
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