diff options
| -rw-r--r-- | sys/i386/i386/mp_clock.c | 34 |
1 files changed, 33 insertions, 1 deletions
diff --git a/sys/i386/i386/mp_clock.c b/sys/i386/i386/mp_clock.c index abe93e282a3b..4345a531d462 100644 --- a/sys/i386/i386/mp_clock.c +++ b/sys/i386/i386/mp_clock.c @@ -6,6 +6,29 @@ * 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. + * * $FreeBSD$ * */ @@ -63,7 +86,16 @@ SYSCTL_PROC(_machdep, OID_AUTO, piix_freq, CTLTYPE_INT | CTLFLAG_RW, static unsigned 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 |