No more system devices

By Jonathan Corbet
January 11, 2012

Since the early days of the Linux device model, there has been a special device class for "system devices," typically those built into the platform itself. For almost as long, the driver core developers have felt that there was no real need for this device type, which looks weirdly different from every other type of device. For 3.3, they have actually done something about it; system devices are no more.

All in-tree system device drivers have been fixed up to use regular devices instead. The process is relatively simple; it can be seen in, for example, this commit updating kernel/time/clocksource.c. In short, the embedded struct sys_device becomes a simple struct device instead. Attributes defined with SYSDEV_ATTR() are switched to DEVICE_ATTR(). The sysdev_class structure is turned into a nearly empty bus_type structure instead. That is about all that is required.

These changes, naturally, cause a user-space ABI change; system devices had their own special area under /sys which goes away. That has the potential to break programs and scripts, which would not be a good thing. To avoid this problem, a special function has been added:

    int subsys_system_register(struct bus_type *subsys,
			       const struct attribute_group **groups);

Registering a subsystem in this way will restore its old /sys/devices/system hierarchy. Needless to say, this function exists for backward compatibility purposes only; using it in new drivers is not likely to be received well.