writing high-performance code. This section describes the places where the
eventdev API and DLB2 misalign.
+Scheduling Domain Configuration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are 32 scheduling domainis the DLB2.
+When one is configured, it allocates load-balanced and
+directed queues, ports, credits, and other hardware resources. Some
+resource allocations are user-controlled -- the number of queues, for example
+-- and others, like credit pools (one directed and one load-balanced pool per
+scheduling domain), are not.
+
+The DLB2 is a closed system eventdev, and as such the ``nb_events_limit`` device
+setup argument and the per-port ``new_event_threshold`` argument apply as
+defined in the eventdev header file. The limit is applied to all enqueues,
+regardless of whether it will consume a directed or load-balanced credit.
+
+Load-Balanced Queues
+~~~~~~~~~~~~~~~~~~~~
+
+A load-balanced queue can support atomic and ordered scheduling, or atomic and
+unordered scheduling, but not atomic and unordered and ordered scheduling. A
+queue's scheduling types are controlled by the event queue configuration.
+
+If the user sets the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag, the
+``nb_atomic_order_sequences`` determines the supported scheduling types.
+With non-zero ``nb_atomic_order_sequences``, the queue is configured for atomic
+and ordered scheduling. In this case, ``RTE_SCHED_TYPE_PARALLEL`` scheduling is
+supported by scheduling those events as ordered events. Note that when the
+event is dequeued, its sched_type will be ``RTE_SCHED_TYPE_ORDERED``. Else if
+``nb_atomic_order_sequences`` is zero, the queue is configured for atomic and
+unordered scheduling. In this case, ``RTE_SCHED_TYPE_ORDERED`` is unsupported.
+
+If the ``RTE_EVENT_QUEUE_CFG_ALL_TYPES`` flag is not set, schedule_type
+dictates the queue's scheduling type.
+
+The ``nb_atomic_order_sequences`` queue configuration field sets the ordered
+queue's reorder buffer size. DLB2 has 4 groups of ordered queues, where each
+group is configured to contain either 1 queue with 1024 reorder entries, 2
+queues with 512 reorder entries, and so on down to 32 queues with 32 entries.
+
+When a load-balanced queue is created, the PMD will configure a new sequence
+number group on-demand if num_sequence_numbers does not match a pre-existing
+group with available reorder buffer entries. If all sequence number groups are
+in use, no new group will be created and queue configuration will fail. (Note
+that when the PMD is used with a virtual DLB2 device, it cannot change the
+sequence number configuration.)
+
+The queue's ``nb_atomic_flows`` parameter is ignored by the DLB2 PMD, because
+the DLB2 does not limit the number of flows a queue can track. In the DLB2, all
+load-balanced queues can use the full 16-bit flow ID range.
+
Flow ID
~~~~~~~
The flow ID field is preserved in the event when it is scheduled in the
DLB2.
+Reconfiguration
+~~~~~~~~~~~~~~~
+
+The Eventdev API allows one to reconfigure a device, its ports, and its queues
+by first stopping the device, calling the configuration function(s), then
+restarting the device. The DLB2 does not support configuring an individual queue
+or port without first reconfiguring the entire device, however, so there are
+certain reconfiguration sequences that are valid in the eventdev API but not
+supported by the PMD.
+
+Specifically, the PMD supports the following configuration sequence:
+1. Configure and start the device
+2. Stop the device
+3. (Optional) Reconfigure the device
+4. (Optional) If step 3 is run:
+
+ a. Setup queue(s). The reconfigured queue(s) lose their previous port links.
+ b. The reconfigured port(s) lose their previous queue links.
+
+5. (Optional, only if steps 4a and 4b are run) Link port(s) to queue(s)
+6. Restart the device. If the device is reconfigured in step 3 but one or more
+ of its ports or queues are not, the PMD will apply their previous
+ configuration (including port->queue links) at this time.
+
+The PMD does not support the following configuration sequences:
+1. Configure and start the device
+2. Stop the device
+3. Setup queue or setup port
+4. Start the device
+
+This sequence is not supported because the event device must be reconfigured
+before its ports or queues can be.
+
+Atomic Inflights Allocation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the last stage prior to scheduling an atomic event to a CQ, DLB2 holds the
+inflight event in a temporary buffer that is divided among load-balanced
+queues. If a queue's atomic buffer storage fills up, this can result in
+head-of-line-blocking. For example:
+
+- An LDB queue allocated N atomic buffer entries
+- All N entries are filled with events from flow X, which is pinned to CQ 0.
+
+Until CQ 0 releases 1+ events, no other atomic flows for that LDB queue can be
+scheduled. The likelihood of this case depends on the eventdev configuration,
+traffic behavior, event processing latency, potential for a worker to be
+interrupted or otherwise delayed, etc.
+
+By default, the PMD allocates 16 buffer entries for each load-balanced queue,
+which provides an even division across all 128 queues but potentially wastes
+buffer space (e.g. if not all queues are used, or aren't used for atomic
+scheduling).
+
+The PMD provides a dev arg to override the default per-queue allocation. To
+increase a vdev's per-queue atomic-inflight allocation to (for example) 64:
+
+ .. code-block:: console
+
+ --vdev=dlb1_event,atm_inflights=64
+
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