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Does Your Driver Belong in the Kernel?

This document primarily applies to in-kernel device drivers. Not all device drivers in Mac OS X are in the kernel. For example, most human interface devices (keyboards, mice, and so on) and graphics and imaging devices (printers, scanners, and similar) are controlled by user-space drivers.

In general, your driver should be in the kernel if it is used concurrently across multiple applications or if its primary client is in the kernel. Examples include video drivers, disk or disk controller drivers, and so on.

There are a few cases where a driver has to reside in the kernel even though similar devices can reside in user space. In particular, any device on a PCI or AGP bus must be in the kernel because those busses do not export interfaces to user space for security reasons.

For the most part, PCI devices are things that need to be in the kernel anyway. There are a few exceptions, however, such as MIDI device drivers. While MIDI device drivers live in user space, PCI device drivers must reside in the kernel. Thus PCI cards with MIDI interfaces require two drivers: the actual I/O Kit driver for the device (subclassed from the PCI driver family) and a CFPlugIn in user space to provide a driver to CoreMIDI. Interfacing drivers across user-kernel boundaries is beyond the scope of this book. For help creating drivers that span user-kernel boundaries, you should contact Apple Developer Technical Support.

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