usb4_port_sb_write(9)

💅 usb4_port_sb_write — a kernel API manual moment

okay so first things first: this is a page from the Kernel API Manual, and today's main character is usb4_port_sb_write(9). she writes to sideband registers. that's it, that's the vibe. let's get into it bestie 🎀

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✨ NAME ✨

usb4_port_sb_write — Write to sideband register

like, literally just write to a sideband register. no cap.

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🌸 SYNOPSIS 🌸

int usb4_port_sb_write (struct tb_port *port , enum usb4_sb_target
target , u8 index , u8 reg , const void *buf , u8 size );

six args, one int coming back at you. let's meet the cast 🫶

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🦋 ARGUMENTS 🦋

• port — the USB4 port you're writing to. the whole point, honestly.
• target — which sideband target we're talking to.
• index — the Retimer index, but only if target is USB4_SB_TARGET_RETIMER. otherwise she's just sitting there, ignore her.
• reg — the sideband register index. this is where the data goes, iykyk.
• buf — the data you're writing. the good stuff.
• size — size of buf. yes, she needs to know how much.

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💜 DESCRIPTION 💜

Writes buf to sideband register reg.

that's the whole description. short, sweet, iconic. main character energy, zero filler ✨

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🔥 RETURN 🔥

0 on success, negative errno otherwise. so if you get a negative number back, that's not a slay, that's a vibe check failure 😭 check the errno.

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👑 SEE ALSO 👑

Kernel file ./drivers/thunderbolt/usb4.c — and honestly this list is giving "whole extended friend group," so here's everyone:

usb4_switch_check_wakes(9), usb4_switch_setup(9), usb4_switch_configuration_valid(9), usb4_switch_read_uid(9), usb4_switch_drom_read(9), usb4_switch_lane_bonding_possible(9), usb4_switch_set_wake(9), usb4_switch_set_sleep(9), usb4_switch_nvm_sector_size(9), usb4_switch_nvm_read(9), usb4_switch_nvm_set_offset(9), usb4_switch_nvm_write(9), usb4_switch_nvm_authenticate(9), usb4_switch_nvm_authenticate_status(9), usb4_switch_credits_init(9), usb4_switch_query_dp_resource(9), usb4_switch_alloc_dp_resource(9), usb4_switch_dealloc_dp_resource(9), usb4_port_index(9), usb4_switch_map_pcie_down(9), usb4_switch_map_usb3_down(9), usb4_switch_add_ports(9), usb4_switch_remove_ports(9), usb4_port_unlock(9), usb4_port_hotplug_enable(9), usb4_port_reset(9), usb4_port_configure(9), usb4_port_unconfigure(9), usb4_port_configure_xdomain(9), usb4_port_unconfigure_xdomain(9), usb4_port_sb_read(9), usb4_port_router_offline(9), usb4_port_router_online(9), usb4_port_enumerate_retimers(9), usb4_port_clx_supported(9), usb4_port_asym_supported(9), usb4_port_asym_set_link_width(9), usb4_port_asym_start(9), usb4_port_margining_caps(9), usb4_port_hw_margin(9), usb4_port_sw_margin(9), usb4_port_sw_margin_errors(9), usb4_port_retimer_set_inbound_sbtx(9), usb4_port_retimer_unset_inbound_sbtx(9), usb4_port_retimer_is_last(9), usb4_port_retimer_is_cable(9), usb4_port_retimer_nvm_sector_size(9), usb4_port_retimer_nvm_set_offset(9), usb4_port_retimer_nvm_write(9), usb4_port_retimer_nvm_authenticate(9), usb4_port_retimer_nvm_authenticate_status(9), usb4_port_retimer_nvm_read(9), usb4_usb3_port_max_link_rate(9), usb4_usb3_port_allocated_bandwidth(9), usb4_usb3_port_allocate_bandwidth(9), usb4_usb3_port_release_bandwidth(9), usb4_dp_port_set_cm_id(9), usb4_dp_port_bandwidth_mode_supported(9), usb4_dp_port_bandwidth_mode_enabled(9), usb4_dp_port_set_cm_bandwidth_mode_supported(9), usb4_dp_port_group_id(9), usb4_dp_port_set_group_id(9), usb4_dp_port_nrd(9), usb4_dp_port_set_nrd(9), usb4_dp_port_granularity(9), usb4_dp_port_set_granularity(9), usb4_dp_port_set_estimated_bandwidth(9), usb4_dp_port_allocated_bandwidth(9), usb4_dp_port_allocate_bandwidth(9), usb4_dp_port_requested_bandwidth(9), usb4_pci_port_set_ext_encapsulation(9), usb4_pci_port_ltssm_state(9)

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and that's the page!! 🧚 credits roll: this came out of ./drivers/thunderbolt, dated October 2026, and it's filed under usb4_port_sb_write(9). mwah 💖 go write some sideband registers responsibly bestie 🍓