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    SubjectRe: One of these things (CONFIG_HZ) is not like the others..
    On 01/21/2013 01:12 PM, Russell King - ARM Linux wrote:
    > On Mon, Jan 21, 2013 at 01:00:15PM -0800, John Stultz wrote:
    >> So if you can not get actual timer ticks any faster then 200 HZ on that
    >> hardware, setting HZ higher could cause some jiffies related timer
    >> trouble
    > Err, no John. It's the other way around - especially on some platforms
    > which are incapable of being converted to the clock source support.
    >
    > EBSA110 has _one_ counter. It counts down at a certain rate, and when
    > it rolls over from 0 to FFFF, it produces an interrupt and continues
    > counting down from FFFF.
    >
    > To produce anything close to a reasonable regular tick rate from that,
    > the only way to do it is - with interrupts disabled - read the current
    > value to find out how far the timer has rolled over, and set it so that
    > the next event will expire as close as possible to the desired HZ rate.
    >
    > So, none of the clcokevent stuff can be used; and we rely _purely_ on
    > counting interrupts in jiffy based increments to provide any reference
    > of time.
    > Moreover, because the counter is only 16-bit, and it's clocked from
    > something around 7MHz, well, maths will tell you why 200Hz had to be
    > chosen rather than 100Hz.

    Ah, so the counter can't do anything *lower* then ~107HZ, right? (7MHZ/2^16)

    So we used to have the ACTHZ code to handle error from the HZ rate
    requested and the HZ rate possible given the underlying hardware. That's
    been moved to the register_refined_jiffies(), but do you have a sense if
    there a reason it couldn't be used? I don't quite recall the bounds at
    this second, so ~7% error might very well be too large.

    So yes, I suspect these sorts of platforms, where there are no modern
    clocksource/clockevent driver, as well as further constraints (like
    specific HZ) are likely not good candidates for a multi-arch build.

    thanks
    -john


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