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The HIL bus and HIL drivers

HIL overview

HIL stands for "Human Interface Loop" and was used on older PARISC machines and HP300 (m68k processor) series machines to connect keyboards, mice and other kind of input devices to the machine.

Nowadays you maybe could compare the HIL bus with an very ancient predecessor of the USB bus.

The PARISC kernel provides two different inplementations to support the HIL bus, which are

  • a very simple HIL keyboard-only driver (no other HIL devices supported)
  • a complete HIL stack which supports HIL keyboards, HIL mice, HIL tablets and other HIL devices.

HIL technical overview

The HIL bus is pretty complex, and accessing the HIL devices in a sane manner from Linux is very hard. Especially correct timing on the HIL bus needs much care.

The HIL bus consists of the following components:

  • SDC - System Device Controller
  • MLC - Machine Loop Controller
  • HIL devices

The simple HIL keyboard-only driver

The simple HIL keyboard-only driver just consists of one single kernel module which is named "hilkbd" (note that there is no underscore in the driver name). This driver is a very basic HP Human Interface Loop (HIL) driver which handles the keyboard on HP300 (m68k) and on some HP700 (parisc) series machines. This driver is pretty stable, reliable and has very little performance overhead. If you don't need to support HIL mice or want to use your machine as a desktop machine, you should prefer this driver. If you need mouse support, use the full HIL driver instead.

The full HIL-driver for keyboards, mice, tablets and others

The full HIL-drivers were originally written by Brian S. Julin <bri@calyx.com>.

The following Linux kernel modules implement the HIL bus on PARISC:

  • hp_sdc - driver for the HP System Domain Controller.
  • hp_sdc_mlc - provides access to the HIL MLC through the HP System Device Controller.
  • hil_mlc - provides HIL MLC state machine and serio interface driver
  • hil_kbd - HIL keyboard driver
  • hil_ptr - HIL mouse and tablet driver

hp_sdc, hp_sdc_mlc and hil_mlc are the main drivers which need to be loaded first. After that, any of the other drivers may be loaded at any time.

Debian Linux distribution and HIL devices

All Debian Linux distributions up to and including "Lenny" have had problems with HIL devices. Lenny for example still uses the simple HIL keyboard-only driver for installation, because the autodetection of attached HIL devices was missing in the Linux kernel 2.6.28. With kernel 2.6.29 we hope to have all missing pieces in the kernel implemented, so that further Debian releases will work out of the box with the full HIL stack instead.

If you want to use HIL mice as well, you need to configure Debian to load the full HIL driver instead (after installation).

High CPU load with HIL drivers, ksoftirqd uses 70-100 percent of the CPU time

This is a known problem and we are working on it. The problem is related to how the HIL drivers poll the HIL bus, and how the interrupts and kernel tasklets interact with each other. Currently we try to solve this issue by migrating the HIL drivers to use kernel workqueues and kernel threads instead.

Guy Martin posted once a patch to the parisc mailing list, which temporarily solves this issue, but this patch will not be accepted upstream.

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