Linux – How to get a list of major number -> driver associations

For the first part of the question, I've looked and couldn't find a better way to detach a USB driver than what you're already doing with libusb.

As for the second part of the question, udev can react to driver loading, but not force a specific driver to be assigned to a device.

Every driver in the Linux kernel is responsible for one or more devices. The driver itself chooses what devices it supports. It does this programmatically, i.e. by checking the device's vendor and product ID, or, if those aren't available (e.g. an old device), performing some auto-detection heuristics and sanity checks. Once the driver is confident it's found a device it supports, it attaches itself to it. In short, you often can't force a particular driver to use a particular device. Sometimes, however, a device driver is generous with what it accepts, and a device can work that it doesn't know about. Your mileage will vary! In the past, I've had to manually add weird PCI device/vendor IDs to drivers that should support them, with mixed success and a few amusing kernel crashes.

Now, in the case of modules, there's an extra step. The module loader is woken up by the kernel when a new device is detected. It's passed a ‘modalias’ string, which identifies the device and looks something like this for USB devices:

usb:v046DpC221d0170dc00dsc00dp00ic03isc00ip00

This string contains the device class (usb) and class-specific information (vendor/device/serial number, device class, etc). Each kernel driver contains a line such as:

MODULE_ALIAS("usb:...")

Which must match the usbalias (wildcards are used to match multiple devices). If the modalias matches one that the driver supports, this driver is loaded (or notified of the new device, if it's there already).

You can see the supported devices (by modalias) and their associated modules with

less /lib/modules/`uname -r`/modules.alias

If you grep for the usb-storage device driver, you'll see it has some specific devices it supports by vendor and device ID, and will also attempt to support any device with the right (storage) class, no matter the vendor/device.

You can influence this using userspace mechanisms on your OS (/etc/modprobe.d/ on Debian and friends). You can blacklist modules, or you can specify modules to be loaded by modalias, just like the modules.alias file (and using the same syntax). depmod -a will then regenerate the module loader's patterns.

However, even though you can lead this particular horse to water, but you can't make him drink. If the driver has no support for your device, it should ignore it.

This is the theory in the general case.

In practice, and in the case of USB, I see your device appears to have two interfaces, of which storage is one. The kernel will attach to the storage interface of the overall device. If the other interface has the right class, the usbnet driver could attach to it. Yes, you can have multiple drivers attached to the same physical device, because a USB device exports multiple interfaces (e.g. my Logitech G15 keyboard exports two because it has a keyboard device and an LCD screen, each of which is handled by a separate driver).

The fact that the second interface of your USB device isn't detected is indicative of lack of support in the kernel. Whatever the case, you can list the device interfaces/endpoints in excruciating detail using lsusb -v | less, then scroll down to your particular device (you can limit the output by device:vendor ID or USB path if you're so inclined).

Please note: I'm oversimplifying a bit here with respect to the logical structure of USB devices. Blame the USB consortium. :)

A device driver is a piece of software that operates or controls a particular type of device. On modern, monolithic kernel operating systems these are typically part of the kernel. Many monolithic kernels, including Linux, have a modular design, allowing for executable modules to be loaded at runtime. Device drivers commonly utilize this feature, although nothing prevents the device drivers to be compiled into the kernel image.

A device file is an interface for a device driver that appears in a file system as if it were an ordinary file. In Unix-like operating systems, these are usually found under the /dev directory and are also called device nodes. A device file can represent character devices, which emit a stream data one character at a time, or block devices which allow random access to blocks of data.

Device nodes are created by the mknod system call. The kernel resource exposed by the device node is identified by a major and minor number. Typically the major number identifies the device driver and the minor number identifies a particular device the driver controls.

What the device file appears to contain depends on what the device drivers exposes through the device file. For instance, the character device file which represents the mouse, /dev/input/mice exposes the movement of the mouse as a character stream, whereas the block device file representing a hard disk, such as /dev/sda, exposes the addressable regions of memory of the device. Some devices files also take input, allowing user-space applications to communicate with the device by writing to its device file.