usb_interface_id(9)

💻 usb_interface_id(9) — the Kernel API Manual glow-up ✨

Okay bestie, so this is the usb_interface_id(9) page from the Kernel API Manual, and it's literally all about snagging yourself an unused interface ID. No cap, it's more iconic than it sounds. Let's get into it 💅

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🌸 NAME 🌸

usb_interface_id — allocate an unused interface ID 🎀

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✨ SYNOPSIS ✨

int usb_interface_id (struct usb_configuration *config , struct
usb_function *function );

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🫶 ARGUMENTS 🫶

• config — the configuration associated with the interface, iykyk 💖
• function — the function handling the interface, main character energy fr fr

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🧚 CONTEXT 🧚

Single threaded during gadget setup. That's the vibe check — don't come at it with concurrency, bestie 🙈

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💜 DESCRIPTION 💜

So usb_interface_id gets called from usb_function.bind callbacks to allocate new interface IDs. The function driver will then store that ID in interface, association, CDC union, and other descriptors — like, it's giving very organized queen 👑. It will also handle any control requests targeted at that interface, especially changing its altsetting via set_alt. There may also be class-specific or vendor-specific requests to handle. Slay, honestly.

All interface identifiers should be allocated using this routine, to ensure that for example different functions don't wrongly assign different meanings to the same identifier. Lowkey important. Note that since interface identifiers are configuration-specific, functions used in more than one configuration (or more than once in a given configuration) need multiple versions of the relevant descriptors. Periodt. 💅

Returns the interface ID which was allocated; or -ENODEV if no more interface IDs can be allocated 😭. That's the sad girl moment.

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🦋 SEE ALSO 🦋

Kernel file ./drivers/usb/gadget/composite.c and all these iconic functions, no cap:

struct usb_os_string(9), function_descriptors(9), next_desc(9), config_ep_by_speed_and_alt(9), config_ep_by_speed(9), usb_add_function(9), usb_function_deactivate(9), usb_function_activate(9), usb_func_wakeup(9), bos_desc(9), usb_add_config(9), usb_string_id(9), usb_string_ids_tab(9), usb_gstrings_attach(9), usb_string_ids_n(9), usb_composite_probe(9), usb_composite_unregister(9), usb_composite_setup_continue(9)

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💖 That's the whole page, bestie — go forth and allocate those interface IDs like the icon you are 💖

P.S. This absolute gem comes from ./drivers/usb/gadget and it's dated October 2026 — write that down, it's giving future vibes 🦋✨🎀