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Using interrupts and scheduling workers increase latency so latency critical applications may want to avoid that. Make this possible in USB4STREAM by adding a new ConfigFS attribute: busy_poll that, when activated switches the rings to polling mode. The cost for lower latency is that this burns more CPU cycles and things like poll(2) cannot be used. Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
99 lines
3.6 KiB
Plaintext
99 lines
3.6 KiB
Plaintext
What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Configuration group for a stream Thunderbolt/USB4
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service. It is possible to create groups even if there
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is no connection yet to the other host. Once a
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connection established and there is stream service on
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the remote side that matches, this configuration is
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applied to it.
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To find the service name you can run tblist from tbtools [1]:
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# tblist -A
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...
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Domain 0 Route 3 Index 0: stream
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[1] https://github.com/intel/tbtools
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Creates new stream with $name and fills it with the
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default values. If there is an advertised remote stream
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with the same name, uses its values as the default.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/busy_poll
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Date: Nov 2026
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KernelVersion: v7.3
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Instead of using interrupts for completing the frames in
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the TX/RX rings, busy poll them directly from the
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read(2) and write(2) calls. This burns more CPU cycles
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but provides lower latency for applications that need it.
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This also makes poll(2) return EPOLLERR because
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interrupts do not provide wakeup anymore. Likewise a
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blocking read(2) without available data busy-spins until
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data arrives or a signal is received.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/index
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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This matches the X in /dev/tbstreamX and allows userspace
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to map the configfs directory to the corresponding character
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device.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/in_hopid
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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In HopID used with the read path of the tunnel. Available HopIDs
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for tunneling start from 8. You can pass also -1 for automatic
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allocation. The allocated value can be read here. Writing 0 will
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de-allocate if the stream is not in use.
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To figure out the maximum HopID you can run tbget from
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tbtools for the lane adapter. For example below we check
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for lane adapter number 1 (first USB4 port):
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# tbget -r 0 -a 1 -D ADP_CS_5.Max\ Input\ HopID
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19
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This allows to use anything between 8 and 19 inclusive.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/out_hopid
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Out HopID used with the write path of the tunnel. Available HopIDs
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for tunneling start from 8. You can pass also -1 for automatic
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allocation. The allocated value can be read here. Writing 0 will
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de-allocate if the stream is not in use. See @in_hopid
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for how to figure out the maximum HopID.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/ring_size
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Size of the TX/RX rings. Can be adjusted between 32 and
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4096. The default is 256.
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What: /sys/kernel/config/thunderbolt/stream/<xdomain>.<service>/$name/throttling
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Date: Sep 2026
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KernelVersion: v7.2
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Contact: Mika Westerberg <mika.westerberg@linux.intel.com>
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Description:
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Interrupt throttling rate in ns. Lower values can give
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better latency. The default is 8192 ns.
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