STRUCT USB_FUNCTION(9)

💖 NAME 💖

struct usb_function — bestie, this is the little struct that describes one function of a configuration. That's the whole vibe. It's the building block, the main character of a composite gadget. 👑

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✨ SYNOPSIS ✨

struct usb_function {
    const char                      *name;
    struct usb_gadget_strings       **strings;
    struct usb_descriptor_header    **fs_descriptors;
    struct usb_descriptor_header    **hs_descriptors;
    struct usb_descriptor_header    **ss_descriptors;
    struct usb_descriptor_header    **ssp_descriptors;
    struct usb_configuration        *config;
    struct usb_os_desc_table        *os_desc_table;
    unsigned os_desc_n;
    int (*bind)(struct usb_configuration *, struct usb_function *);
    void (*unbind)(struct usb_configuration *, struct usb_function *);
    void (*free_func)(struct usb_function *f);
    struct module           *mod;
    int (*set_alt)(struct usb_function *, unsigned interface, unsigned alt);
    int (*get_alt)(struct usb_function *, unsigned interface);
    void (*disable)(struct usb_function *);
    int (*setup)(struct usb_function *, const struct usb_ctrlrequest *);
    bool (*req_match)(struct usb_function *, const struct usb_ctrlrequest *, bool config0);
    void (*suspend)(struct usb_function *);
    void (*resume)(struct usb_function *);
    int (*get_status)(struct usb_function *);
    int (*func_suspend)(struct usb_function *, u8 suspend_opt);
    bool func_suspended;
    bool func_wakeup_armed;
};

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🧩 Members 🧩

Grab a snack, we're going through every single one of these. No skipping, that's the rule. 💅

• name — for diagnostics, identifies the function. It's the function's name-tag at the party. 🎀
• strings — tables of strings, keyed by identifiers assigned during bind and by language IDs provided in control requests.
• fs_descriptors — table of full (or low) speed descriptors, using interface and string identifiers assigned during bind. If this pointer is null, the function will not be available at full speed (or at low speed). So null = left on read, bestie. 🙈
• hs_descriptors — table of high speed descriptors, using interface and string identifiers assigned during bind. If this pointer is null, the function will not be available at high speed.
• ss_descriptors — table of super speed descriptors, using interface and string identifiers assigned during bind. If this pointer is null after initiation, the function will not be available at super speed.
• ssp_descriptors — table of super speed plus descriptors, using interface and string identifiers assigned during bind. If this pointer is null after initiation, the function will not be available at super speed plus.
• config — assigned when usb_add_function is called; this is the configuration with which this function is associated. It's giving "officially attached". 🫶
• os_desc_table — table of (interface id, os descriptors) pairs. The function can expose more than one interface. If an interface is a member of an IAD, only the first interface of IAD has its entry in the table.
• os_desc_n — number of entries in os_desc_table.
• bind — before the gadget can register, all of its functions bind to the available resources including string and interface identifiers used in interface or class descriptors; endpoints; I/O buffers; and so on. This is the "claim your stuff" step. 🎒
• unbind — reverses bind; called as a side effect of unregistering the driver which added this function.
• free_func — free the struct usb_function.
• mod — (internal) points to the module that created this structure. Don't touch, it's internal, iykyk. 🤫
• set_alt — (REQUIRED) reconfigures altsettings; function drivers may initialize usb_ep.driver data at this time (when it is used). Note that setting an interface to its current altsetting resets interface state, and that all interfaces have a disabled state.
• get_alt — returns the active altsetting. If this is not provided, then only altsetting zero is supported.
• disable — (REQUIRED) indicates the function should be disabled. Reasons include host resetting or reconfiguring the gadget, and disconnection. 😭
• setup — used for interface-specific control requests.
• req_match — tests if a given class request can be handled by this function.
• suspend — notifies functions when the host stops sending USB traffic.
• resume — notifies functions when the host restarts USB traffic. Welcome back, king. 👑
• get_status — returns function status as a reply to GetStatus request when the recipient is Interface.
• func_suspend — callback to be called when SetFeature(FUNCTION_SUSPEND) is reseived (yes, that's the spelling, leave her alone 💀).
• func_suspended — indicates whether the function is in function suspend state.
• func_wakeup_armed — indicates whether the function is armed by the host for wakeup signaling. Armed and ready, literally. ⚔️

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📖 Description 📖

A single USB function uses one or more interfaces, and should in most cases support operation at both full and high speeds. Each function is associated by usb_add_function with a one configuration; that function causes bind to be called so resources can be allocated as part of setting up a gadget driver. Those resources include endpoints, which should be allocated using usb_ep_autoconfig. 🛠️

To support dual speed operation, a function driver provides descriptors for both high and full speed operation. Except in rare cases that don't involve bulk endpoints, each speed needs different endpoint descriptors. No cap, they're genuinely different. 💯

Function drivers choose their own strategies for managing instance data. The simplest strategy just declares it "static', which means the function can only be activated once. If the function needs to be exposed in more than one configuration at a given speed, it needs to support multiple usb_function structures (one for each configuration).

A more complex strategy might encapsulate a usb_function structure inside a driver-specific instance structure to allows multiple activations. An example of multiple activations might be a CDC ACM function that supports two or more distinct instances within the same configuration, providing several independent logical data links to a USB host. Iconic behaviour honestly. 🌈

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👀 SEE ALSO 👀

• Kernel file ./include/linux/usb/composite.h
• struct usb_os_desc_ext_prop(9)
• struct usb_os_desc(9)
• struct usb_os_desc_table(9)
• struct usb_configuration(9)
• struct usb_composite_driver(9)
• module_usb_composite_driver(9)
• struct usb_composite_dev(9)

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That's the whole page, bestie — go read the friends too. 🦋 This little love letter lives in ./include/linux/usb in the Kernel API Manual, dated October 2026, filed under struct usb_function(9). Mwah, study hard, slay, periodt. 💜🎀