modalias

Modalias is a Linux kernel mechanism that encodes device identity into a string used to match hardware to compatible driver modules. It serves as a compact, machine-readable identifier that enables automatic driver loading without requiring user intervention.

The modalias string is generated for each device by the kernel and reflects the device’s bus type (for example PCI, USB, or platform) along with vendor, device, and subsystem IDs. Drivers declare the hardware they support using macros such as MODULE_DEVICE_TABLE, which causes the kernel to publish one or more alias patterns. Those patterns become part of the module’s set of aliases that can be matched against a device’s modalias.

In practice, the modalias is exposed in sysfs for physical devices, typically under paths like /sys/bus/xxx/devices/yyy/modalias. User-space tools such as udev read this string and consult the kernel’s module alias database to determine if a suitable driver module is already loaded or should be loaded with modprobe. If a match is found and the module is not present, udev or modprobe will load the appropriate driver automatically.

Examples of modalias formats illustrate the concept. For a PCI device, a modalias may encode the vendor and device IDs along with subsystem and class codes in a pattern starting with pci:. For a USB device, the string might begin with usb: and include vendor and product IDs, along with interface details. The exact fields depend on the bus and hardware.

Modalias thus enables reliable, automatic driver binding by translating hardware identifiers into actionable module-loading instructions.