Hot Plug

Hot-pluggable devices have been created to solve a number of user needs. On laptop computers, PCMCIA devices were designed to allow the user to swap cards while the computer was still running. This allowed people to change network adaptors, memory cards and even disk drives without shutting down the machine.

The success of this led to the creation of the USB and IEEE1394 (FireWire) buses. These designs allow for peripherals to be attached and removed at any point. They also were created to try to move systems away from the ISA bus to a full Plug-and-Play-type system.

From the operating system's point of view, there are many problems with hot plugging devices. In the past, the operating system only had to search for the various devices connected to it on power-up, and once seen, the device would never go away. From the view of the device driver, it never expects to have the hardware that it is trying to control disappear. But with hot-pluggable devices, all of this changes.

Now the operating system has to have a mechanism that constantly detects if a new device appears. This usually is done by a bus-specific manager. This manager handles the scanning for new devices and recognizes this disappearance. It must be able to create system resources for the new device and pass control off to a specific driver. The device driver for a hot-pluggable device has to be able to recover gracefully when the hardware is removed and be able to bind itself to new hardware at any moment. Not only does the kernel need to know when devices are removed or added, but the user also should be notified when this happens. Other kinds of kernel events, such as the creation of network devices or the insertion of a laptop into a docking station, also would be useful for the user to know about.

This article describes the new framework in the Linux kernel for supporting USB and other hot-pluggable devices. It covers how the past implementation of PCMCIA loaded its drivers and the problems of that system. It presents the current method of loading USB and PCI drivers, and how this same framework can handle other kinds of user configuration issues easily.

Linux has had support for PCMCIA since 1995. In order for the PCMCIA core to be able to load drivers when a new device was inserted, it had a user-space program called cardmgr. The cardmgr program would receive notification from the kernel's PCMCIA core when a device had been inserted or removed and use that information to load or unload the proper driver for that card. It used a configuration file located at /etc/pcmcia/config to determine which driver should be used for which card. This configuration file needed to be kept up to date with which driver supported which card, or ranges of cards, and has grown to be over 1,500 lines long. Whenever a driver author added support for a new device, they had to modify two different files to enable the device to work properly.

As the USB core code became mature, the group realized that it also needed something like the PCMCIA system to be able to load and unload drivers dynamically when devices were inserted and removed. The group also noted that since USB and PCMCIA both needed this system, and that other kernel hot-plug subsystems also would use such a system, a generic hot-plug core would be useful. David Brownell posted an initial patch to the kernel (marc.theaimsgroup.com/?l=linux-usb-devel&m=96334011602320), enabling it to call out to a user-space program called /sbin/hotplug. This patch eventually was accepted, and other subsystems were modified to take advantage of it.

All USB and PCI devices contain an identifier that describes either what kind of functions they support (like a USB audio or USB mass storage device), or if they do not support a class specification, they contain a unique vendor and product identifier. PCMCIA devices also contain these same kind of identifiers.

These identifiers are known by the PCI and USB kernel drivers, as they need to know which kind of devices they work properly for. The USB and PCI kernel drivers register with the kernel a list of the different types of devices that they support. This list is used to determine which driver will control which devices.

The kernel knows when and what kind of devices are inserted or removed from the system through the device bus core code (USB, FireWire, PCI, etc.). It can send this information to the user.

Taking these three pieces together (devices tell the computer what they are, drivers know what devices they support and the kernel knows what is going on) provides us with a solution to let the computer automatically load the proper driver whenever a new device is inserted.