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    SubjectRe: NE2000 slow interrupts safe?
    On Thu, 16 Apr 1998, Richard Gooch wrote:

    > Hi, all. Looking at the code for the NE2000 driver, I note that the
    > interrupt is registered as a fast interrupt. I'm trying to convince
    > people here to change from pSOS to Linux. A problem they have is
    > that the NE2000 interrupt service routine takes up to 2
    > milliseconds. This blocks another interrupt from a custom card for too
    > long (it interrupts every millisecond), causing them to loose
    > interrupts.

    I don't think this is correct. The NE2000 driver just can't take that
    much time. To really test it, set a bit in the printer port when the
    ISR gets control, then reset the bit just before the return. There is
    a relatively fixed overhead from the top-level kernel ISR code that
    will not be shown. Check the bit with a scope.

    I have a NE2 clone on a ISA (slow) bus on this machine (Pentium 166).
    The ISR time is 320 to 540 microseconds, depending upon what it is
    doing. This information was important to me, because I have a custom
    board (DSP) that interrupts 1440 times per second. I don't lose any
    interrupts although I don't do serial I/O at the same time.

    >
    > So my question is whether it is safe to re-enable other interrupts in
    > the Linux NE2000 interrupt handler?
    >

    Even if it is safe, you end up with the interrupt events being stale.
    Handling the new hardware state when the ISR eventually gets the CPU
    back basically changes the ISR into a polling routine which will seriously
    impact performance.

    Cheers,
    Dick Johnson
    ***** FILE SYSTEM MODIFIED *****
    Penguin : Linux version 2.1.92 on an i586 machine (66.15 BogoMips).
    Warning : It's hard to remain at the trailing edge of technology.


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