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    SubjectRe: [PATCH 22/71] ncr5380: Eliminate selecting state

    On Sun, 29 Nov 2015, Geert Uytterhoeven wrote:

    > If an ISA access takes 8 us, while the CPU runs at 1 GHz, i.e. 500M
    > loops/s, the difference will be huge.

    Ondrej showed that an ISA access can take about 1.6 us. I don't know what
    to make of the "8 uS" comment in the mainline driver. Maybe it was an even
    slower ISA card.

    Anyway, I made a measurement on my hardware and confirmed that lpj is a
    very bad proxy for device register access throughput. The "loops per
    access" gap is several orders of magnitude:

    lpj HZ access time (us) lpa
    ------------------------------------------------------------------------
    Ondrej's 5380 ISA card (PC): 4797252 250 1.6 1.9k
    My DMX3191D PCI card (PowerMac): 167079 100 0.42 7.0

    >
    > Perhaps you can calibrate an NCR5380_read() loop at driver init time,
    > and use the calibration value later?

    I had the same idea but I didn't think that the complexity was justified
    by the low precision requirement. But now that I have some timings I have
    to agree.

    --


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