usb_set_interface(9)

📦 version 7.3.1, in nixpkgs unstable

💅 usb_set_interface — the Kernel API Manual tea ☕

okay so bestie, you found a usb_set_interface(9) page and you're like "what is this diva doing" — don't worry, I got you. This is a kernel API manual page, it lives in ./drivers/usb/core, and it's literally about making a particular alternate setting be current. That's the whole vibe. Let's get into it 🎀

✨ NAME ✨

usb_set_interface — Makes a particular alternate setting be current 💫 (yes that's the official job description and honestly? iconic.)

🌸 SYNOPSIS 🌸

int usb_set_interface (struct usb_device *dev , int interface , int
alternate );

Like, that's the whole signature. Three arguments, one int coming back at you. No cap. 🫶

🦋 ARGUMENTS 🦋

  • dev — the device whose interface is being updated. it's giving "main character device energy" 👑
  • interface — the interface being updated. the specific one. the chosen one. 💅
  • alternate — the setting being chosen. (yes there's a period after it in the original and yes we're keeping it, we don't erase history here 🙈)

⭐ CONTEXT ⭐

task context, might sleep. 😴 that's it. that's the whole section. Sleep is allowed, but only in task context — do not come in here from an interrupt, bestie, I'm warning you.

💖 DESCRIPTION 💖

So this is used to enable data transfers on interfaces that may not be enabled by default. Not all devices support such configurability — some devices are just like "no 💀" and that's their right.

Only the driver bound to an interface may change its setting. That's the rule. That's the boundary. Respect it. 🚧

Within any given configuration, each interface may have several alternative settings. These are often used to control levels of bandwidth consumption. For example — and this is the good part — the default setting for a high speed interrupt endpoint may not send more than 64 bytes per microframe, while interrupt transfers of up to 3KBytes per microframe are legal. Also, isochronous endpoints may never be part of an interface's default setting. So to access such bandwidth, alternate interface settings must be made current. It's giving "unlock the premium tier" 🎟️✨

Note that in the Linux USB subsystem, bandwidth associated with an endpoint in a given alternate setting is not reserved until an URB is submitted that needs that bandwidth. Some other operating systems allocate bandwidth early, when a configuration is chosen — not us though, we're ✨lazy allocation girlies✨ in the best way.

xHCI reserves bandwidth and configures the alternate setting in usb_hcd_alloc_bandwidth. If it fails, the original interface altsetting may be disabled. Drivers cannot rely on any particular alternate setting being in effect after a failure. Read that again. cannot rely. Do not assume. 🫠

This call is synchronous, and may not be used in an interrupt context. Also, drivers must not change altsettings while urbs are scheduled for endpoints in that interface; all such urbs must first be completed (perhaps forced by unlinking). If a thread in your driver uses this call, make sure your disconnect method can wait for it to complete. fr fr, this is not a drill 💀

🔥 RETURN 🔥

Zero on success, or else the status code returned by the underlying usb_control_msg call. That's it. Zero = slay, anything else = go read usb_control_msg and figure out what happened. 🕵️‍♀️

🌈 SEE ALSO 🌈

Kernel file ./drivers/usb/core/message.c usb_control_msg(9), usb_control_msg_send(9), usb_control_msg_recv(9), usb_interrupt_msg(9), usb_bulk_msg(9), usb_bulk_msg_killable(9), usb_sg_init(9), usb_sg_wait(9), usb_sg_cancel(9), usb_get_descriptor(9), usb_get_string(9), usb_string(9), usb_cache_string(9), usb_get_status(9), usb_clear_halt(9), usb_disable_endpoint(9), usb_reset_endpoint(9), usb_disable_interface(9), usb_disable_device(9), usb_enable_endpoint(9), usb_enable_interface(9), usb_reset_configuration(9), usb_set_wireless_status(9), usb_driver_set_configuration(9), cdc_parse_cdc_header(9)

like, that's a whole friend group right there. the whole USB core squad. say hi to all of them 🫶✨

and that's the page, bestie 🧚‍♀️💜 this iconic little manual lives in ./drivers/usb/core and it's dated October 2026 — yes, from the future, because USB hangs around longer than any of us. mwah 💋

📦 where this page comes from