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How hard can it be to implement a 24bit counter in hardware ?

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Make sure we read a likely value from the PIIX timecounter.

This should fix a large fraction of the "calcru: negative time"
warnings produced by SMP machines.

Another hole in one by: bde
Didn't belive Bruce:    phk
This commit is contained in:
Poul-Henning Kamp 1999-11-29 11:27:40 +00:00
parent cd423b136e
commit ab1c0de5ed
1 changed files with 33 additions and 1 deletions
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34
sys/i386/i386/mp_clock.c
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@ -6,6 +6,29 @@
* this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp * this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
* ---------------------------------------------------------------------------- * ----------------------------------------------------------------------------
* *
* Just when we thought life were beautiful, reality pops its grim face over
* the edge again:
*
* ] 20. ACPI Timer Errata
* ]
* ] Problem: The power management timer may return improper result when
* ] read. Although the timer value settles properly after incrementing,
* ] while incrementing there is a 3nS window every 69.8nS where the
* ] timer value is indeterminate (a 4.2% chance that the data will be
* ] incorrect when read). As a result, the ACPI free running count up
* ] timer specification is violated due to erroneous reads. Implication:
* ] System hangs due to the "inaccuracy" of the timer when used by
* ] software for time critical events and delays.
* ]
* ] Workaround: Read the register twice and compare.
* ] Status: This will not be fixed in the PIIX4 or PIIX4E.
*
* The counter is in other words not latched to the PCI bus clock when
* read. Notice the workaround isn't: We need to read until we have
* three monotonic samples and then use the middle one, otherwise we are
* not protected against the fact that the bits can be wrong in two
* directions. If we only cared about monosity two reads would be enough.
*
* $FreeBSD$ * $FreeBSD$
* *
*/ */
@ -63,7 +86,16 @@ SYSCTL_PROC(_machdep, OID_AUTO, piix_freq, CTLTYPE_INT | CTLFLAG_RW,
static unsigned static unsigned
piix_get_timecount(struct timecounter *tc) piix_get_timecount(struct timecounter *tc)
{ {
return (inl(piix_timecounter_address)); unsigned u1, u2, u3;
u2 = inl(piix_timecounter_address);
u3 = inl(piix_timecounter_address);
do {
u1 = u2;
u2 = u3;
u3 = inl(piix_timecounter_address);
} while (u1 > u2 || u2 > u3);
return (u2);
} }
static int static int