High Precision Event Timer Driver for Linux

The High Precision Event Timer (HPET) hardware follows a specification by Intel and Microsoft, revision 1.

Each HPET has one fixed-rate counter (at 10+ MHz, hence “High Precision”) and up to 32 comparators. Normally three or more comparators are provided, each of which can generate oneshot interrupts and at least one of which has additional hardware to support periodic interrupts. The comparators are also called “timers”, which can be misleading since usually timers are independent of each other ... these share a counter, complicating resets.

HPET devices can support two interrupt routing modes. In one mode, the comparators are additional interrupt sources with no particular system role. Many x86 BIOS writers don’t route HPET interrupts at all, which prevents use of that mode. They support the other “legacy replacement” mode where the first two comparators block interrupts from 8254 timers and from the RTC.

The driver supports detection of HPET driver allocation and initialization of the HPET before the driver module_init routine is called. This enables platform code which uses timer 0 or 1 as the main timer to intercept HPET initialization. An example of this initialization can be found in arch/x86/kernel/hpet.c.

The driver provides a userspace API which resembles the API found in the RTC driver framework. An example user space program is provided in samples/timers/hpet_example.c

Userspace API and ioctl Commands

  • HPET_IE_ON / HPET_IE_OFF Enables or disables the interrupt generation for the timer channel.

  • HPET_INFO It is used to query the capabilities and current status of the HPET. When the ioctl call is made, the kernel populates the struct hpet_info structure and returns it to userspace:

    struct hpet_info {
    unsigned long hi_ireqfreq;     /* Hz */
    unsigned long hi_flags;        /* information */
    unsigned short hi_hpet;
    unsigned short hi_timer;
};

  • HPET_EPI / HPET_DPI Enables or disables periodic interrupts. When activated, the timer triggers continuously at the specified frequency.

  • HPET_IRQFREQ Sets the interrupt frequency for the periodic timer. This request takes an unsigned long argument specifying the desired frequency in Hz.