The Design and Implementation of the Anykernel and Rump Kernels

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using an interface called USBDI, or USB Driver Interface. The role of the host
controller, along with ugen, is a detail which makes USB especially suitable for
userspace drivers: we need to implement a host controller which maps USBDI to
the ugen device node instead of having to care about all bus details.
We implemented a host controller called ugenhc. When the kernel’s device autoconfiguration
subsystem calls the ugenhc driver to probe the device, the ugenhc driver
tries to open /dev/ugen on the host. If the open is successful, the host kernel has
attached a device to the respective ugen instance and ugenhc can return a successful
match. Next, the ugenhc driver is attached in the rump kernel, along with a USB
bus and a USB root hub. The root hub driver explores the bus to see which devices
are connected to it, causing the probes to be delivered first to ugenhc and through
/dev/ugen to the host kernel and finally to the actual hardware. Figure 3.24 contains
a “dmesg” of a server with four ugenhc devices configured and one USB mass
media attached.
3.10.2 Defining Device Relations with Config
Device autoconfiguration [108] is used to attach hardware device drivers in the
NetBSD kernel. A configuration file determines the relationship of device drivers
in the system and the autoconfiguration subsystem attempts to attach drivers according
to the configuration. The configuration is expressed in a domain specific
language (DSL). The language is divided into two parts: a global set of descriptions
for which drivers can attach to which busses, and a system-specific configuration
of what hardware is expected to be present and how this particular configuration
allows devices to attach. For example, even though the USB bus allows a USB hub
to be attached to another hub, the device configuration might allow a USB hub to
be attached only to the host controller root hub and not other hubs.