A Beginner's Guide to Writing USB Drivers Without Kernel Code

Writing a USB driver might seem intimidating at first, especially when considering the complexities of Kernel code. However, it's not as daunting as it appears. Crafting a driver for a USB device is akin to writing an application that uses Sockets, requiring no deep hardware expertise. This guide provides a high-level introduction to USB for those unfamiliar with hardware, using an Android phone in Bootloader mode as an example. This device is easily accessible, uses a simple protocol, and typically lacks pre-installed drivers, making it ideal for experimentation.

## Understanding USB Enumeration

Enumeration is the process where the host requests information from the device, determining which driver to load. Standard devices rely on USB Device Class, while vendor-specific devices use Vendor ID (VID) and Product ID (PID). Tools like `lsusb` on Linux help identify these IDs, crucial for driver development. The device class and driver information can be viewed using `lsusb -t`, revealing the device's class and whether a driver is loaded.

## Using libusb for User-Space Applications

Instead of writing Kernel code, a user-space application using the libusb library can be developed. This library provides a straightforward API to communicate with USB devices, allowing for easier debugging and development. The guide includes a program example that initializes libusb, registers a hotplug event handler, and waits for the device to be plugged in, showcasing how to detect a device without Kernel code.

## Communicating with the Device

The next step involves interacting with the device using the Control endpoint, a standardized protocol for requesting data. The guide explains how to send a GET_STATUS request to the device, interpreting the returned data to understand device capabilities like power status and remote wakeup support.

## Requesting Descriptors

Descriptors provide detailed information about the device's capabilities and desired drivers. By sending a GET_DESCRIPTOR request, one can retrieve this information, which is crucial for proper device enumeration and driver loading.

## Exploring USB Endpoints

Endpoints act like network ports, facilitating data transfer between the host and device. The guide details different types of endpoints—Control, Bulk, Interrupt, and Isochronous—and their specific uses. Understanding these is essential for effective communication with the device.

## Implementing the Fastboot Protocol

Finally, the guide delves into the Fastboot protocol, a simple communication method between the host and device. By sending string commands and receiving status codes, users can interact with the device, as demonstrated with a sample code that requests the Fastboot version.

In conclusion, creating a USB driver without touching Kernel code is achievable by understanding USB fundamentals and leveraging user-space libraries like libusb. This approach simplifies the process, making it accessible even to those without extensive hardware knowledge.