Part 14: USB Drivers

USB drivers connect the kernel to USB devices. This part covers USB device drivers (not host controller drivers).

USB Model

USB is hierarchical: Host → Hub(s) → Device → Interface(s) → Endpoint(s)

flowchart TB
    subgraph Host["USB Host"]
        HC["Host Controller"]
    end

    subgraph Device["USB Device"]
        DEV["Device<br/>(VID:PID)"]
        CFG["Configuration"]
        IF1["Interface 0<br/>(class/subclass)"]
        IF2["Interface 1"]
        EP1["EP1 IN<br/>(Bulk)"]
        EP2["EP2 OUT<br/>(Bulk)"]
        EP3["EP0<br/>(Control)"]
    end

    HC --> DEV
    DEV --> CFG
    CFG --> IF1
    CFG --> IF2
    IF1 --> EP1
    IF1 --> EP2
    DEV --> EP3

    style Host fill:#738f99,stroke:#0277bd
    style Device fill:#8f8a73,stroke:#f9a825

Key insight: Your driver binds to an interface, not the whole device. A USB device can have multiple interfaces (e.g., keyboard + media keys).

Transfer Types

Type	Use Case	Characteristics
Control	Configuration, commands	Guaranteed delivery, bidirectional
Bulk	Data transfer (storage)	Reliable, no bandwidth guarantee
Interrupt	Small periodic data (HID)	Guaranteed latency, small packets
Isochronous	Streaming (audio/video)	Guaranteed bandwidth, no retry

Most drivers use bulk (data) or interrupt (events) transfers.

Linux USB Architecture

flowchart TB
    subgraph Drivers["USB Drivers"]
        YOUR["Your Driver"]
        HID["HID Driver"]
        STORAGE["Storage Driver"]
    end

    subgraph Core["USB Core"]
        USBCORE["USB Core"]
        USBBUS["USB Bus"]
    end

    subgraph HCD["Host Controller"]
        EHCI["EHCI/XHCI"]
        HW["USB Hardware"]
    end

    YOUR --> USBCORE
    HID --> USBCORE
    STORAGE --> USBCORE
    USBCORE --> USBBUS
    USBBUS --> EHCI
    EHCI --> HW

    style Drivers fill:#7a8f73,stroke:#2e7d32
    style Core fill:#738f99,stroke:#0277bd
    style HCD fill:#8f8a73,stroke:#f9a825

Chapters

Chapter	What You’ll Learn
Concepts	USB model, endpoints, and URBs
Driver Skeleton	usb_driver structure and probe/disconnect
Transfers	Control, bulk, and interrupt URBs
Error Handling	Disconnect races, URB errors, recovery
Suspend and Resume	Autosuspend, PM callbacks
User Space Interface	Character device, file operations, ioctl
Porting Guide	Migrating from libusb, vendor SDKs
WinUSB and libusb	Cross-platform compatibility, usbfs
USB Class Drivers	ACM, mass storage, network (NCM/RNDIS), MCTP
USB Gadget Drivers	Device-side drivers, ConfigFS, FunctionFS
Dynamic Gadget Configuration	Runtime mode switching from kernel code

Example

USB Device Driver - Bulk transfer example with simple protocol

Prerequisites

Kernel module basics (Part 1-2)
Device model (Part 6) - probe/remove pattern
[Managed resources (devm_)](/linux-driver-guide-tutorial/part6/05-devres.html) - USB drivers use usb_ and devm_* functions
Understanding of asynchronous I/O