Documentation: networking: TC queue based filtering

Add tc-queue-filters.rst with notes on TC filters for
selecting a set of queues and/or a queue.
Signed-off-by: Amritha Nambiar <amritha.nambiar@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/index.rst

@@ -104,6 +104,7 @@ Contents:
    switchdev
    sysfs-tagging
    tc-actions-env-rules
+   tc-queue-filters
    tcp-thin
    team
    timestamping

Documentation/networking/tc-queue-filters.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+TC queue based filtering
+=========================
+
+TC can be used for directing traffic to either a set of queues or
+to a single queue on both the transmit and receive side.
+
+On the transmit side:
+
+1) TC filter directing traffic to a set of queues is achieved
+   using the action skbedit priority for Tx priority selection,
+   the priority maps to a traffic class (set of queues) when
+   the queue-sets are configured using mqprio.
+
+2) TC filter directs traffic to a transmit queue with the action
+   skbedit queue_mapping $tx_qid. The action skbedit queue_mapping
+   for transmit queue is executed in software only and cannot be
+   offloaded.
+
+Likewise, on the receive side, the two filters for selecting set of
+queues and/or a single queue are supported as below:
+
+1) TC flower filter directs incoming traffic to a set of queues using
+   the 'hw_tc' option.
+   hw_tc $TCID - Specify a hardware traffic class to pass matching
+   packets on to. TCID is in the range 0 through 15.
+
+2) TC filter with action skbedit queue_mapping $rx_qid selects a
+   receive queue. The action skbedit queue_mapping for receive queue
+   is supported only in hardware. Multiple filters may compete in
+   the hardware for queue selection. In such case, the hardware
+   pipeline resolves conflicts based on priority. On Intel E810
+   devices, TC filter directing traffic to a queue have higher
+   priority over flow director filter assigning a queue. The hash
+   filter has lowest priority.