/*
    ChibiOS/RT - Copyright (C) 2006,2007,2008,2009,2010 Giovanni Di Sirio.

    This file is part of ChibiOS/RT.

    ChibiOS/RT is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    ChibiOS/RT is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @page article_events Events Explained
 * Events are an important feature in ChibiOS/RT, most device drivers generate
 * events in order to notify the application that something happened at the
 * I/O level.<br>
 * While event flags are not something unknown in other operating systems,
 * their peculiar implementation in ChibiOS/RT requires a more in depth
 * explanation.<br>
 * Lets start with the events related terminology:
 * - <b>Event Source</b>, an @p EventSource is a system object that can be
 *   @a broadcasted asynchronously in response of a system event, as example,
 *   when the CAN driver receives a packet from the CAN bus it @a broadcasts
 *   an event source in order to inform the registered threads that a packet
 *   has just arrived.
 * - <b>Broadcast</b>, the operation performed on an event source in order
 *   to inform the @a registered threads that an event just occurred.
 *   Broadcasting can happened both in interrupt handlers and in threads.
 * - <b>Event Listener</b>, a system object that associates a @p Thread object
 *   to an event source. The process of associating a @p Thread to an
 *   @p EventSource using an @p EventListener is called @a registration.
 * - <b>Registration</b>, action performed by a thread in order to be informed
 *   of events from a specific event source. Of course a thread can be
 *   @a registered on more than one event source by using multiple
 *   @p EventListener objects. Threads can also @a unregister from an event
 *   source.
 * - <b>Pend</b>, each thread has a mask of @a pending events. The @a broadcast
 *   operation @a pends an event mask on all the @a registered threads.
 * - <b>Wait</b>, synchronous operation performed by a thread in order to
 *   @a wait a specific combination of events. The API offers a variety of
 *   @a wait functions, please refer to the events API documentation.
 * .
 * Note that events are asynchronously generated, as example in an interrupt
 * handler, but are synchronously served.
 *
 * <h2>Events related data structures</h2>
 * The following diagram explains the relationship between an event source,
 * its list of event listeners and the @a registered threads.
 * @dot
  digraph example {
    rankdir="LR";

    node [shape=rectangle, fontname=Helvetica, fontsize=8,
          fixedsize="true", width="1.0", height="0.5"];
    edge [fontname=Helvetica, fontsize=8, sep=3.0];

    es [shape=record, label="EventSource | es_next", style="bold"];

    subgraph cluster_0 {
      fontname=Helvetica;
      label = "Listeners List";
      color = blue;
      node [shape=record, fontname=Helvetica, fontsize=8,
            fixedsize="true", width="1.5", height="1.0"];
      el4 [label="EventListener 4 | el_mask: 0x0001 | el_listener | el_next"];
      el3 [label="EventListener 3 | el_mask: 0x0C00 | el_listener | el_next"];
      el2 [label="EventListener 2 | el_mask: 0x0001 | el_listener | el_next"];
      el1 [label="EventListener 1 | el_mask: 0x0004 | el_listener | el_next"];
      el1 -> el2 -> el3 -> el4 [label=" el_next", constraint=false];
    }

    subgraph cluster_1 {
      fontname=Helvetica;
      label = "Registered Threads";
      color = blue;
      node [shape=record, fontname=Helvetica, fontsize=8,
            fixedsize="true", width="1.5", height="0.8"];
      t1 [label="Thread 1 | p_epending:0x0000 | p_ewmask:0xFFFF"];
      t2 [label="Thread 2 | p_epending:0x000F | p_ewmask:0x0C01"];
      t3 [label="Thread 3 | p_epending:0x0008 | p_ewmask:0x0001"];
      t4 [label="Thread 4 | p_epending:0x0000 | p_ewmask:0xFFFF"];
    }

    es -> el1 [label=" es_next"];
    el4 -> es [label=" el_next"];
    el1 -> t1 [label="el_listener"];
    el2 -> t2 [label="el_listener"];
    el3 -> t3 [label="el_listener"];
    el4 -> t4 [label="el_listener"];
  }
 * @enddot
 * Note that each event listener has a different bit mask to be @a pended on
 * its associated thread when the event source is @a broadcasted, this means
 * that each thread can define its own event identifiers independently. A
 * @a broadcast operation can also @a pend more than one bit on the
 * @a registered threads.<br>
 * The threads have a variety of @a wait primitives, they can @a wait for one
 * or more event flags to become @a pending, and can also specify AND/OR
 * conditions, as example a thread can @a wait for any event to become
 * @a pending or @a wait for all the specified events to become @a pending.<br>
 * The field @p p_epending is the mask of the currently pending events,
 * the field @p p_ewmask is the mask of the events the thread is interested
 * on in that moment (AND or OR condition depending on the invoked
 * @a wait API).
 *
 * <h2>Use Scenarios</h2>
 * Events are best used when one of more of the following conditions are
 * required:
 * - Having to wait on multiple conditions, Events are the only mechanism
 *   that easily allow that.
 * - Synchronous response to one or more asynchronous events.
 * - Single threaded applications working in a event driver environment (but
 *   events are not limited to single threaded applications).
 * .
 */