diff options
| -rw-r--r-- | arch/x86/kernel/hpet.c | 43 |
1 files changed, 21 insertions, 22 deletions
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c index 9ae9b0b25795..cd9948e03d37 100644 --- a/arch/x86/kernel/hpet.c +++ b/arch/x86/kernel/hpet.c @@ -380,36 +380,35 @@ static int hpet_next_event(unsigned long delta, struct clock_event_device *evt, int timer) { u32 cnt; + s32 res; cnt = hpet_readl(HPET_COUNTER); cnt += (u32) delta; hpet_writel(cnt, HPET_Tn_CMP(timer)); /* - * We need to read back the CMP register on certain HPET - * implementations (ATI chipsets) which seem to delay the - * transfer of the compare register into the internal compare - * logic. With small deltas this might actually be too late as - * the counter could already be higher than the compare value - * at that point and we would wait for the next hpet interrupt - * forever. We found out that reading the CMP register back - * forces the transfer so we can rely on the comparison with - * the counter register below. If the read back from the - * compare register does not match the value we programmed - * then we might have a real hardware problem. We can not do - * much about it here, but at least alert the user/admin with - * a prominent warning. - * An erratum on some chipsets (ICH9,..), results in comparator read - * immediately following a write returning old value. Workaround - * for this is to read this value second time, when first - * read returns old value. + * HPETs are a complete disaster. The compare register is + * based on a equal comparison and neither provides a less + * than or equal functionality (which would require to take + * the wraparound into account) nor a simple count down event + * mode. Further the write to the comparator register is + * delayed internally up to two HPET clock cycles in certain + * chipsets (ATI, ICH9,10). We worked around that by reading + * back the compare register, but that required another + * workaround for ICH9,10 chips where the first readout after + * write can return the old stale value. We already have a + * minimum delta of 5us enforced, but a NMI or SMI hitting + * between the counter readout and the comparator write can + * move us behind that point easily. Now instead of reading + * the compare register back several times, we make the ETIME + * decision based on the following: Return ETIME if the + * counter value after the write is less than 8 HPET cycles + * away from the event or if the counter is already ahead of + * the event. */ - if (unlikely((u32)hpet_readl(HPET_Tn_CMP(timer)) != cnt)) { - WARN_ONCE(hpet_readl(HPET_Tn_CMP(timer)) != cnt, - KERN_WARNING "hpet: compare register read back failed.\n"); - } + res = (s32)(cnt - hpet_readl(HPET_COUNTER)); - return (s32)(hpet_readl(HPET_COUNTER) - cnt) >= 0 ? -ETIME : 0; + return res < 8 ? -ETIME : 0; } static void hpet_legacy_set_mode(enum clock_event_mode mode, |