Introduction(9)

🎀 Introduction 💅

okay so bestie, welcome to Introduction(9) from the Kernel API Manual — and ngl this page is shorter than my attention span but it's packed, so let's get into it ✨

🦋 what this IOCTL actually does (the main character moment)

This IOCTL is used to associate the current file descriptor with a FW Client (given by UUID), and virtual tag (vtag). Like, the glow-up is this: the IOCTL opens a communication channel between a host client and a FW client on a tagged channel. From this point on, every read and write will communicate with the associated FW client on the tagged channel. And bestie, upon close the communication is terminated. that's it, that's the vibe check 💅

🫶 the argument (struct + union energy)

The IOCTL argument is a struct with a union that contains the input parameter and the output parameter for this IOCTL. yep, one struct, one union, both directions — it's giving efficiency 🔥

• input parameter — the UUID of the FW Client, and a vtag [0,255] (yes, that's the whole range, no sneaky extras)
• output parameter — the properties of the FW client, which is the FW protocol version and max message size 👑

😭 when it says no

Clients that do not support tagged connection will respond with -EOPNOTSUPP.

fr fr, if you see -EOPNOTSUPP coming back, it's not you, it's them — that client just doesn't do tagged connections 🙈

👀 SEE ALSO ✨

• Kernel file ./include/uapi/linux/mei.h — the literal source of truth, go read it bestie 💖
• set and unset event notification for a connected client(9)
• retrieve notification(9)
• struct mei_connect_client_vtag(9)
• struct mei_connect_client_data_vtag(9)

okay that's the whole page, you ate that 🍓 this little gem lives in ./include/uapi/linux and it's dated October 2026 — so yeah, from the future, iconic behavior only 👑💜