The Driver Model Core, Part I

The 2.5 kernel implements a unified device driver model that will make driver development for 2.6 easier.

In the 2.5 Linux kernel development series, a unified device driver model framework was created by Pat Mochel. This framework consists of a number of common structures and functions all device driver subsystems have been converted to use. It also consists of some generic structures that are starting to be used outside of the driver code by other parts of the kernel. This article discusses parts of the driver model and provides an example of how to convert a specific device driver subsystem to the driver model.

Buses, Devices and Classes

The driver framework breaks all things down into buses, devices and classes. Using these primitives, it controls how drivers are matched up with physical and virtual devices, and it shows the user how all of these things are interconnected.

A bus can be described as something with devices connected to it. Examples of buses are PCI, USB, i2c, PCMCIA and SCSI. Usually only one bus driver controls the activity on a bus, and it provides a type of bridge from the bus it is on to the bus it controls.

An example of a bridge is a USB controller that lives on the PCI bus. It talks to the PCI bus as a PCI device and looks to the kernel as a PCI driver. But it controls all access to that specific USB bus, talking to the different USB devices plugged in to it.

Buses are represented in the kernel with the struct bus_type definition, found in include/linux/device.h. All buses in the system are shown to the user in subdirectories of the sysfs directory /sys/bus/.

Sidebar: Sysfs

Devices are physical or virtual devices that reside on a bus. They are represented by the struct device definition and are created by the bus when the bus sees they are present in the system. Usually only one driver controls a specific device at one time. They can be seen in the /sys/devices directory as a giant tree of all devices in the system or in the /sys/bus/BUS_TYPE/devices/ directory for a specific type of device.

Devices also have drivers assigned to them that control how to talk to the device across a specific bus. Some drivers know how to talk to multiple buses, such as the Tulip network driver, which can talk to PCI and ISA Tulip devices. All drivers are represented by the struct device_driver definition. They can be seen in sysfs at /sys/bus/BUS_TYPE/drivers/. Drivers register with a specific bus and export a list of different types of devices they can support. The bus matches the devices and drivers based on this list of exported devices. The list also is exported to user space so the /sbin/hotplug tools can be used to match drivers to devices that do not have drivers already loaded. See my article, “Hot Plug”, in the April 2002 issue of Linux Journal for more information on this interface and how it works [also available at www.linuxjournal.com/article/5604].

Classes here do not take the general object-oriented definition but, rather, are things that provide a function to the user. They are not bus- or device-specific things but functionally look to the user as the same type of device. Examples of classes are audio devices; pointing devices, such as mice and touchpads; keyboards; joysticks; IDE disks; and tty devices. The kernel always has had these kinds of devices, and they traditionally have been grouped together by major/minor number range, so the user can access them easily. Classes are represented in the kernel with the struct device_class definition, and they can be seen as subdirectories of the sysfs directory /sys/class/.

For a description of the whole driver model, along with an introduction to the structures below the driver model that do all of the real work, see the thorough document at www.kernel.org/pub/linux/kernel/people/mochel/doc/lca/driver-model-lca2003.tar.gz. It was written by Pat Mochel for the 2003 Linux.Conf.Au conference.

Theory in Action

All of the above descriptions sound great on paper, but how does the driver model actually affect the kernel code? To show this, let us walk through how the i2c driver subsystem was modified to support this driver model.

The i2c code has lived outside of the main kernel tree for a long time, and it was offered as a patch for the 2.0, 2.2 and 2.4 kernels. It also was the subject of “Using the i2c Bus”, by Simon G. Vogl, one of the main authors of the code [LJ, March 1997, www.linuxjournal.com/article/1342]. In the 2.4 development cycle, a number of the i2c core files and a few i2c bus drivers were accepted into the main kernel. In the 2.5 development cycle, a few more drivers were added; hopefully, all of them eventually will migrate into the main tree. For a good description of the i2c code, what devices it supports and how to use it, see the main development site at secure.netroedge.com/~lm78/index.html.

When loaded, the i2c bus drivers, which talk to the i2c controller chips, export a number of files in the /proc/bus directory. When an i2c device driver is loaded and bound to an i2c device, it exports files and directories in the /proc/sys/dev/sensors directory. By moving the representation of the devices and buses to the kernel driver core, all of these separate files can be shown in their proper places in /sys.

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