Sebastian Urban's Rust Library Makes It Easier to Experiment with a Raspberry Pi's USB Gadget Mode
Library comes with predefined network, serial, Human Interface Device, and Mass Storage Device functions, and you can add your own too.
Developer Sebastian Urban is making it easier to turn a Raspberry Pi 4 into a Rust-powered USB gadget — using, appropriately enough, his usb-gadget library.
"I just wanted to share a library I've been working on: usb-gadget," Urban explains of his creation. "If you've ever tinkered with implementing USB gadgets, i.e. USB peripherals, on Linux, I think you might find it handy!"
Typically, a Raspberry Pi single-board computer (SBC) is used as as a USB host: you connect a power cable to its USB Type-C port and then whatever USB devices you want to use to its two USB 2.0 and two USB 3.0 Type-A ports. Sometimes, though, you want things to work differently, and that's where the USB "gadget" mode comes in: turning the SBC into a USB device compatible with any USB host you'd care to name.
"In a nutshell, [usb-gadget can]: configure pre-defined USB functions like network interfaces, serial ports, HID, etc.; create fully custom USB functions using user-mode Rust code; tweak & play with WebUSB and OS-specific descriptors."
Using USB gadgets, it's possible to expose a Raspberry Pi's serial console over USB — or have it create a network connection to a host machine. You can tap into the hardware random number generator (HRNG) in the board's system-on-chip and use it as a source of entropy for a server, or use it to interface with the general-purpose input/output (GPIO) pins.
The source code for the library has been published to GitHub under the permissive Apache 2.0 license; documentation is available on crates.io. "A USB device controller (UDC) supported by Linux is require," Urban notes. "Normally, standard PCs do not include an UDC. A Raspberry Pi 4 contains an UDC, which is connected to its USB-C port."
More information is available in Urban's Reddit post.