STRUCT TB_XDOMAIN(9)

💜 struct tb_xdomain — Cross-domain (XDomain) connection 💜

okay so this is a page straight out of the Kernel API Manual, and it's all about struct tb_xdomain, bestie. it's giving "two hosts, one very cute connection" energy. let's get into it ✨

---

✨ SYNOPSIS ✨

struct tb_xdomain {
    struct device dev;
    struct tb *tb;
    uuid_t *remote_uuid;
    const uuid_t *local_uuid;
    u64 route;
    u16 vendor;
    u16 device;
    unsigned int local_max_hopid;
    unsigned int remote_max_hopid;
    struct mutex lock;
    const char *vendor_name;
    const char *device_name;
    unsigned int link_speed;
    enum tb_link_width link_width;
    bool link_usb4;
    bool is_unplugged;
    bool removing;
    bool needs_uuid;
    struct ida service_ids;
    struct ida in_hopids;
    struct ida out_hopids;
    u32 *local_property_block;
    u32 local_property_block_gen;
    u32 local_property_block_len;
    struct tb_property_dir *remote_properties;
    u32 remote_property_block_gen;
    int state;
    struct delayed_work state_work;
    int state_retries;
    struct delayed_work properties_changed_work;
    int properties_changed_retries;
    bool bonding_possible;
    u8 target_link_width;
    atomic_t ntunnels;
    u8 link;
    u8 depth;
 };

---

🫶 MEMBERS 🫶

like, every single field, no skips, fr fr 👑

• dev — the XDomain device itself, main character energy
• tb — pointer to the domain
• remote_uuid — the UUID of the remote domain (the host)
• local_uuid — our own UUID, cached so we don't gotta go fetch it again 💅
• route — the route string the other domain can be reached at
• vendor — vendor ID of the remote domain
• device — device ID of the demote domain
• local_max_hopid — maximum input HopID of this host
• remote_max_hopid — maximum input HopID of the remote host
• lock — the lock that serializes access to the following fields of this structure (yes bestie, the following ones — respect the lock or perish 😭)
• vendor_name — name of the vendor (or NULL if it's not known)
• device_name — name of the device (or NULL if it's not known)
• link_speed — speed of the link, in Gb/s
• link_width — width of the downstream facing link
• link_usb4 — is the downstream link USB4? this flag says yes/no ✨
• is_unplugged — the XDomain is unplugged 🫠
• removing — set by tb_xdomain_remove under lock, so concurrent delayed work queueing can't sneak in
• needs_uuid — if the XDomain doesn't have a remote_uuid, it will be queried first
• service_ids — used to generate IDs for the services
• in_hopids — input HopIDs for DMA tunneling
• out_hopids — output HopIDs for DMA tunneling
• local_property_block — local block of properties
• local_property_block_gen — generation of local_property_block
• local_property_block_len — length of the local_property_block, in dwords
• remote_properties — properties exported by the remote domain
• remote_property_block_gen — generation of remote_properties
• state — next XDomain discovery state to run
• state_work — work used to run the next state
• state_retries — number of retries remain for the state
• properties_changed_work — work used to notify the remote domain that our properties have changed
• properties_changed_retries — number of times left to send the properties changed notification
• bonding_possible — true if lane bonding is possible on the local side
• target_link_width — target link width from the remote host
• ntunnels — keeps track of how many tunnels go through this XDomain
• link — root switch link the remote domain is connected to (ICM only)
• depth — depth in the chain the remote domain is connected at (ICM only)

---

🌸 DESCRIPTION 🌸

okay so real talk: this structure represents a connection across two domains (hosts). each XDomain contains zero or more services, and those services get exposed as struct tb_service objects. slay 🌈

service drivers may access this structure if they need to enumerate non-standard properties — but they need to hold lock when doing so, because properties can be changed asynchronously in response to changes in the remote domain. no cap, do not skip the lock 🙈

---

🦋 SEE ALSO 🦋

go read these next, i'm obsessed with the whole crew 💖

Kernel file ./include/linux/thunderbolt.h, enum tb_security_level(9), struct tb(9), struct tb_property_dir(9), struct tb_property(9), enum tb_link_width(9), struct tb_protocol_handler(9), struct tb_service(9), struct tb_service_driver(9), struct tb_nhi(9), struct tb_ring(9), enum ring_desc_flags(9), struct ring_frame(9), tb_ring_rx(9), tb_ring_tx(9), tb_ring_dma_device(9)

---

and that's the whole page bb 🎀 it lives in ./include/linux, dated October 2026 — go forth and tunnel responsibly, ily 💜🦋