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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Configuration group for a stream Thunderbolt/USB4
service. It is possible to create groups even if there
is no connection yet to the other host. Once a
connection established and there is stream service on
the remote side that matches, this configuration is
applied to it.
To find the service name you can run tblist from tbtools [1]:
# tblist -A
...
Domain 0 Route 3 Index 0: stream
[1] https://github.com/intel/tbtools
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Creates new stream with $name and fills it with the
default values. If there is an advertised remote stream
with the same name, uses its values as the default.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/busy_poll
Date: Nov 2026
KernelVersion: v7.3
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Instead of using interrupts for completing the frames in
the TX/RX rings, busy poll them directly from the
read(2) and write(2) calls. This burns more CPU cycles
but provides lower latency for applications that need it.
This also makes poll(2) return EPOLLERR because
interrupts do not provide wakeup anymore. Likewise a
blocking read(2) without available data busy-spins until
data arrives or a signal is received.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/index
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
This matches the X in /dev/tbstreamX and allows userspace
to map the configfs directory to the corresponding character
device.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/in_hopid
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
In HopID used with the read path of the tunnel. Available HopIDs
for tunneling start from 8. You can pass also -1 for automatic
allocation. The allocated value can be read here. Writing 0 will
de-allocate if the stream is not in use.
To figure out the maximum HopID you can run tbget from
tbtools for the lane adapter. For example below we check
for lane adapter number 1 (first USB4 port):
# tbget -r 0 -a 1 -D ADP_CS_5.Max\ Input\ HopID
19
This allows to use anything between 8 and 19 inclusive.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/out_hopid
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Out HopID used with the write path of the tunnel. Available HopIDs
for tunneling start from 8. You can pass also -1 for automatic
allocation. The allocated value can be read here. Writing 0 will
de-allocate if the stream is not in use. See @in_hopid
for how to figure out the maximum HopID.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/ring_size
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Size of the TX/RX rings. Can be adjusted between 32 and
4096. The default is 256.
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/throttling
Date: Sep 2026
KernelVersion: v7.2
Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
Description:
Interrupt throttling rate in ns. Lower values can give
better latency. The default is 8192 ns.
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