Intel PIIX4 ISA bridges
piixpcib* at pci?
isa* at piixpcib?
driver supports Intel 82371AB and 82440MX
PIIX4 ISA bridges.
Besides the core functionality, the
piixpcib driver provides support for the first
generation of Intel's SpeedStep technology present in some Pentium III CPUs.
It allows the user to manually control CPU frequency with the
sysctl(8) program. The
apmd(8) daemon can be used to automatically
control CPU frequency. SpeedStep provides two CPU power states, low and
high. The driver will switch the CPU into low power state if the
hw.setperf sysctl is less than or equal to 50, and
into high power state if it is greater than 50.
piixpcib driver first appeared in
The driver sometimes fails to update the hw.cpuspeed sysctl correctly after
switching power state. The reason is that there seems to be no reliable way to
determine CPU frequencies corresponding to low and high power states, so the
driver has to rely on the p3_update_cpuspeed function to dynamically determine
CPU speed after switching power state. This seems to fail occasionally,
especially if interrupt load of the system is high. Note that this bug is
purely cosmetic. Switching power state still works even if the CPU speed is