- * 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.
- *
- * In fact the write to the comparator register is delayed up
- * to two HPET cycles so the workaround we tried to restrict
- * the readback to those known to be borked ATI chipsets
- * failed miserably. So we give up on optimizations forever
- * and penalize all HPET incarnations unconditionally.
+ * 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.