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    SubjectRe: Too much error in __const_udelay() ?
    On Sat, 2004-06-05 at 08:23, Dominik Brodowski wrote:
    > Hi,
    > > However I've started to see some problems w/ 2.6 and USB on x440/x445s,
    > > both of which use the 100Mhz cyclone time source. Further digging has
    > > pointed to the fact that certain important udelay()s in the USB
    > > subsystem aren't actually waiting long enough.
    > Certain? AFAICS _no_ call to a delay routine actually passed a big enough
    > argument. Or am I missing something? Also, __ndelay seems to be affected
    > as well: it returns zero for 550 nsec even for the TSC variant in your
    > test.c.

    Indeed its likely.

    > > So I'm no math wiz. What's the proper fix here?
    > Below are three changes I'd like to discuss. I'll build a fresh kernel with
    > all three changes enabled + PM_TIMER soon.

    Ah, your test output is a bit confusing (changes to __const_udealy
    affect the output of my_udelay), but I think I understand it. Forgive me
    if I miss-interpret.

    > Change 1:
    > Move the multiplication with HZ up into the mull instruction:
    > unsigned long __const_udelay(unsigned long xloops)
    > {
    > int d0;
    > __asm__("mull %0"
    > :"=d" (xloops), "=&a" (d0)
    > :"1" (xloops),"0" (LPJ * HZ));
    > return __delay(xloops);
    > }

    This does make a good bit of difference! Good catch!

    > Change 2:
    > Round up in __udelay. While it can be argued that some time is also
    > spent in the delay functions, it's better to spend _at least_ the specified
    > time sleeping, in my humble opinion.
    > - return __const_udelay2(usecs * 0x000010c6); /* 2**32 / 1000000 */
    > + return __const_udelay2(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up)*/

    This change looks right to me.

    > Change 3:
    > Asserting at least 1 loop is spent: in really small ndelay() calls to
    > low-mhz timers, this might be better.
    > return __delay(xloops ? xloops : 1);

    I agree w/ Pavel that rounding up sounds better, but I can't get the
    math to work, so this may be the best solution.

    I'm also spinning up a patch w/ these changes to test, let me know how
    your testing went and I'll do the same.

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