[PATCH v3 01/11] eventdev: improve doxygen introduction text

[Bruce Richardson <bruce.richardson@intel.com>]

Make some textual improvements to the introduction to eventdev and event
devices in the eventdev header file. This text appears in the doxygen
output for the header file, and introduces the key concepts, for
example: events, event devices, queues, ports and scheduling.

This patch makes the following improvements:
* small textual fixups, e.g. correcting use of singular/plural
* rewrites of some sentences to improve clarity
* using doxygen markdown to split the whole large block up into
  sections, thereby making it easier to read.

No large-scale changes are made, and blocks are not reordered

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>

---
V3: reworked following feedback from Mattias
---
 lib/eventdev/rte_eventdev.h | 132 ++++++++++++++++++++++--------------
 1 file changed, 81 insertions(+), 51 deletions(-)

diff --git a/lib/eventdev/rte_eventdev.h b/lib/eventdev/rte_eventdev.h
index ec9b02455d..a741832e8e 100644
--- a/lib/eventdev/rte_eventdev.h
+++ b/lib/eventdev/rte_eventdev.h
@@ -12,25 +12,33 @@
  * @file
  *
  * RTE Event Device API
+ * ====================
  *
- * In a polling model, lcores poll ethdev ports and associated rx queues
- * directly to look for packet. In an event driven model, by contrast, lcores
- * call the scheduler that selects packets for them based on programmer
- * specified criteria. Eventdev library adds support for event driven
- * programming model, which offer applications automatic multicore scaling,
- * dynamic load balancing, pipelining, packet ingress order maintenance and
- * synchronization services to simplify application packet processing.
+ * In a traditional run-to-completion application model, lcores pick up packets
+ * from Ethdev ports and associated RX queues, run the packet processing to completion,
+ * and enqueue the completed packets to a TX queue. NIC-level receive-side scaling (RSS)
+ * may be used to balance the load across multiple CPU cores.
+ *
+ * In contrast, in an event-driver model, as supported by this "eventdev" library,
+ * incoming packets are fed into an event device, which schedules those packets across
+ * the available lcores, in accordance with its configuration.
+ * This event-driven programming model offers applications automatic multicore scaling,
+ * dynamic load balancing, pipelining, packet order maintenance, synchronization,
+ * and prioritization/quality of service.
  *
  * The Event Device API is composed of two parts:
  *
  * - The application-oriented Event API that includes functions to setup
  *   an event device (configure it, setup its queues, ports and start it), to
- *   establish the link between queues to port and to receive events, and so on.
+ *   establish the links between queues and ports to receive events, and so on.
  *
  * - The driver-oriented Event API that exports a function allowing
- *   an event poll Mode Driver (PMD) to simultaneously register itself as
+ *   an event poll Mode Driver (PMD) to register itself as
  *   an event device driver.
  *
+ * Application-oriented Event API
+ * ------------------------------
+ *
  * Event device components:
  *
  *                     +-----------------+
@@ -75,27 +83,39 @@
  *            |                                                           |
  *            +-----------------------------------------------------------+
  *
- * Event device: A hardware or software-based event scheduler.
+ * **Event device**: A hardware or software-based event scheduler.
  *
- * Event: A unit of scheduling that encapsulates a packet or other datatype
- * like SW generated event from the CPU, Crypto work completion notification,
- * Timer expiry event notification etc as well as metadata.
- * The metadata includes flow ID, scheduling type, event priority, event_type,
- * sub_event_type etc.
+ * **Event**: Represents an item of work and is the smallest unit of scheduling.
+ * An event carries metadata, such as queue ID, scheduling type, and event priority,
+ * and data such as one or more packets or other kinds of buffers.
+ * Some examples of events are:
+ * - a software-generated item of work originating from a lcore,
+ *   perhaps carrying a packet to be processed,
+ * - a crypto work completion notification
+ * - a timer expiry notification.
  *
- * Event queue: A queue containing events that are scheduled by the event dev.
+ * **Event queue**: A queue containing events that are scheduled by the event device.
  * An event queue contains events of different flows associated with scheduling
  * types, such as atomic, ordered, or parallel.
+ * Each event given to an event device must have a valid event queue id field in the metadata,
+ * to specify on which event queue in the device the event must be placed,
+ * for later scheduling.
  *
- * Event port: An application's interface into the event dev for enqueue and
+ * **Event port**: An application's interface into the event dev for enqueue and
  * dequeue operations. Each event port can be linked with one or more
  * event queues for dequeue operations.
- *
- * By default, all the functions of the Event Device API exported by a PMD
- * are lock-free functions which assume to not be invoked in parallel on
- * different logical cores to work on the same target object. For instance,
- * the dequeue function of a PMD cannot be invoked in parallel on two logical
- * cores to operates on same  event port. Of course, this function
+ * Enqueue and dequeue from a port is not thread-safe, and the expected use-case is
+ * that each port is polled by only a single lcore. [If this is not the case,
+ * a suitable synchronization mechanism should be used to prevent simultaneous
+ * access from multiple lcores.]
+ * To schedule events to an lcore, the event device will schedule them to the event port(s)
+ * being polled by that lcore.
+ *
+ * *NOTE*: By default, all the functions of the Event Device API exported by a PMD
+ * are non-thread-safe functions, which must not be invoked on the same object in parallel on
+ * different logical cores.
+ * For instance, the dequeue function of a PMD cannot be invoked in parallel on two logical
+ * cores to operate on same  event port. Of course, this function
  * can be invoked in parallel by different logical cores on different ports.
  * It is the responsibility of the upper level application to enforce this rule.
  *
@@ -107,22 +127,19 @@
  *
  * Event devices are dynamically registered during the PCI/SoC device probing
  * phase performed at EAL initialization time.
- * When an Event device is being probed, a *rte_event_dev* structure and
- * a new device identifier are allocated for that device. Then, the
- * event_dev_init() function supplied by the Event driver matching the probed
- * device is invoked to properly initialize the device.
+ * When an Event device is being probed, an *rte_event_dev* structure is allocated
+ * for it and the event_dev_init() function supplied by the Event driver
+ * is invoked to properly initialize the device.
  *
- * The role of the device init function consists of resetting the hardware or
- * software event driver implementations.
+ * The role of the device init function is to reset the device hardware or
+ * to initialize the software event driver implementation.
  *
- * If the device init operation is successful, the correspondence between
- * the device identifier assigned to the new device and its associated
- * *rte_event_dev* structure is effectively registered.
- * Otherwise, both the *rte_event_dev* structure and the device identifier are
- * freed.
+ * If the device init operation is successful, the device is assigned a device
+ * id (dev_id) for application use.
+ * Otherwise, the *rte_event_dev* structure is freed.
  *
  * The functions exported by the application Event API to setup a device
- * designated by its device identifier must be invoked in the following order:
+ * must be invoked in the following order:
  *     - rte_event_dev_configure()
  *     - rte_event_queue_setup()
  *     - rte_event_port_setup()
@@ -130,10 +147,15 @@
  *     - rte_event_dev_start()
  *
  * Then, the application can invoke, in any order, the functions
- * exported by the Event API to schedule events, dequeue events, enqueue events,
- * change event queue(s) to event port [un]link establishment and so on.
- *
- * Application may use rte_event_[queue/port]_default_conf_get() to get the
+ * exported by the Event API to dequeue events, enqueue events,
+ * and link and unlink event queue(s) to event ports.
+ *
+ * Before configuring a device, an application should call rte_event_dev_info_get()
+ * to determine the capabilities of the event device, and any queue or port
+ * limits of that device. The parameters set in the various device configuration
+ * structures may need to be adjusted based on the max values provided in the
+ * device information structure returned from the info_get API.
+ * An application may use rte_event_[queue/port]_default_conf_get() to get the
  * default configuration to set up an event queue or event port by
  * overriding few default values.
  *
@@ -145,7 +167,11 @@
  * when the device is stopped.
  *
  * Finally, an application can close an Event device by invoking the
- * rte_event_dev_close() function.
+ * rte_event_dev_close() function. Once closed, a device cannot be
+ * reconfigured or restarted.
+ *
+ * Driver-Oriented Event API
+ * -------------------------
  *
  * Each function of the application Event API invokes a specific function
  * of the PMD that controls the target device designated by its device
@@ -163,11 +189,14 @@
  * performs an indirect invocation of the corresponding driver function
  * supplied in the *event_dev_ops* structure of the *rte_event_dev* structure.
  *
- * For performance reasons, the address of the fast-path functions of the
- * Event driver is not contained in the *event_dev_ops* structure.
+ * For performance reasons, the addresses of the fast-path functions of the
+ * event driver are not contained in the *event_dev_ops* structure.
  * Instead, they are directly stored at the beginning of the *rte_event_dev*
  * structure to avoid an extra indirect memory access during their invocation.
  *
+ * Event Enqueue, Dequeue and Scheduling
+ * -------------------------------------
+ *
  * RTE event device drivers do not use interrupts for enqueue or dequeue
  * operation. Instead, Event drivers export Poll-Mode enqueue and dequeue
  * functions to applications.
@@ -179,21 +208,22 @@
  * crypto work completion notification etc
  *
  * The *dequeue* operation gets one or more events from the event ports.
- * The application process the events and send to downstream event queue through
- * rte_event_enqueue_burst() if it is an intermediate stage of event processing,
- * on the final stage, the application may use Tx adapter API for maintaining
- * the ingress order and then send the packet/event on the wire.
+ * The application processes the events and sends them to a downstream event queue through
+ * rte_event_enqueue_burst(), if it is an intermediate stage of event processing.
+ * On the final stage of processing, the application may use the Tx adapter API for maintaining
+ * the event ingress order while sending the packet/event on the wire via NIC Tx.
  *
  * The point at which events are scheduled to ports depends on the device.
  * For hardware devices, scheduling occurs asynchronously without any software
  * intervention. Software schedulers can either be distributed
  * (each worker thread schedules events to its own port) or centralized
  * (a dedicated thread schedules to all ports). Distributed software schedulers
- * perform the scheduling in rte_event_dequeue_burst(), whereas centralized
- * scheduler logic need a dedicated service core for scheduling.
- * The RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED capability flag is not set
- * indicates the device is centralized and thus needs a dedicated scheduling
- * thread that repeatedly calls software specific scheduling function.
+ * perform the scheduling inside the enqueue or dequeue functions, whereas centralized
+ * software schedulers need a dedicated service core for scheduling.
+ * The absence of the RTE_EVENT_DEV_CAP_DISTRIBUTED_SCHED capability flag
+ * indicates that the device is centralized and thus needs a dedicated scheduling
+ * thread (generally an RTE service that should be mapped to one or more service cores)
+ * that repeatedly calls the software specific scheduling function.
  *
  * An event driven worker thread has following typical workflow on fastpath:
  * \code{.c}
-- 
2.40.1