Intel PIIX4 ISA bridges
piixpcib* at pci?
isa* at piixpcib?
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.
cpu(4), ichpcib(4), intro(4), isa(4), pci(4), apmd(8), sysctl(8)
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 displayed incorrectly.