HPET(4)

💖 HPET(4) — High Precision Event Timer driver 💖

Straight out of the Kernel Interfaces Manual, bestie. 💅

🎀 NAME 🎀

hpet — High Precision Event Timer driver. Okay so this is literally the driver for the High Precision Event Timer. ✨

🌸 SYNOPSIS 🌸

To compile this driver into the kernel, place the following lines in your kernel configuration file:

device acpi

The following tunables are settable from loader(8), and they are literally your knobs:

• hint.hpet.X.allowed_irqs — this is a 32bit mask. Each set bit lets the driver use the respective IRQ, but only if the BIOS also set the respective capability bit in the comparator's configuration register. Default value is 0xffff0000, except on some known broken hardware. 💀
• hint.hpet.X.clock — controls event timers functionality support. Set it to 0 and it's disabled. Default value is 1. 🕰️
• hint.hpet.X.legacy_route — controls "LegacyReplacement Route" mode. If enabled, HPET will steal IRQ0 of i8254 timer and IRQ8 of RTC. Before using it, make sure that the respective drivers are not using interrupts, by setting also:

hint.attimer.0.clock=0
hint.atrtc.0.clock=0

Default value is 0. ⚠️ this one needs care, bestie — do not skip the “make sure” part.

• hint.hpet.X.per_cpu — controls how many per-CPU event timers the driver should attempt to register. This functionality requires every comparator in a group to have its own unshared IRQ, so it depends on hardware capabilities and interrupts configuration. Default value is 1.

🦋 DESCRIPTION 🦋

Okay so this driver uses High Precision Event Timer hardware — part of the chipset, usually enumerated via ACPI — to supply the kernel with one time counter and several event timers (usually from 3 to 8). ✨ This hardware includes a single main counter with a known increment frequency (10MHz or more), plus several programmable comparators (optionally with automatic reload feature). When the value of the main counter matches the current value of any comparator, an interrupt can be generated. 🎯

Depending on hardware capabilities and configuration, the interrupt can be delivered as a regular I/O APIC interrupt (ISA or PCI) in range from 0 to 31, or as a Front Side Bus interrupt, alike to PCI MSI interrupts, or — in so called "LegacyReplacement Route" — HPET can steal IRQ0 of i8254 and IRQ8 of the RTC. Interrupt can be either edge- or level-triggered. In the level-triggered case they could be safely shared with PCI IRQs. Driver prefers to use FSB interrupts, if supported, to avoid sharing. If it is not possible, it uses a single sharable IRQ from PCI range. Other modes (LegacyReplacement and ISA IRQs) require special care to setup, but could be configured manually via device hints. 🛠️

Event timers provided by the driver support both one-shot and periodic modes, and they're irrelevant to CPU power states. ⚡ Depending on hardware capabilities and configuration, the driver can expose each comparator as a separate event timer or group them into one or several per-CPU event timers. In the last case, the interrupt of every one of those comparators within the group is bound to a specific CPU core. This is possible only when each of these comparators has its own unsharable IRQ. 🧚

🍓 SEE ALSO 🍓

• acpi(4)
• apic(4)
• atrtc(4)
• attimer(4)
• eventtimers(4)
• timecounters(4)

🌈 HISTORY 🌈

The hpet driver first appeared in FreeBSD 6.3. Support for event timers was added in FreeBSD 9.0. 👑

🎀 cute sign-off: Debian source · HPET(4) · September 14, 2010 — k bye bestie, go check your IRQs 💅🫶