shared/idl/host_messages.fbs

// Copyright (C) 2017 The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

/// CHRE flat buffers declaration.
///
/// New fields declaration should be added at the end.
/// See https://flatbuffers.dev/md__schemas.html for details.

namespace chre.fbs;

/// Represents a message sent to/from a nanoapp from/to a client on the host
table NanoappMessage {
  app_id:ulong = 0;
  message_type:uint = 0;

  /// Identifies the host-side endpoint on the host that sent or should receive
  /// this message. The default value is a special value defined in the HAL and
  /// elsewhere that indicates that the endpoint is unspecified.
  host_endpoint:ushort = 0xfffe;

  /// Vector containing arbitrary application-specific message data
  message:[ubyte] (required);

  /// List of Android permissions that cover the contents of a message from a
  /// nanoapp to the host.
  /// These permissions are used to record and attribute access to
  /// permissions-controlled resources.
  message_permissions:uint;

  /// List of Android permissions declared by the nanoapp / granted to the host.
  /// For messages from a nanoaapp to the host, this must be a superset of
  /// message_permissions.
  permissions:uint;

  // If true, the message has awakened the host AP (i.e. the AP has transitioned
  // from suspend to awake as a result of this message transfer). This field is
  // only valid for messages originating from a nanoapp.
  woke_host:bool = false;

  // Whether the message is reliable.
  // The receiver acknowledges reliable messages by sending a status back.
  // The status contains the message delivery status.
  is_reliable:bool;

  // The message sequence number used for reliable messages.
  message_sequence_number:uint;
}

// Status of sending a reliable message.
table MessageDeliveryStatus {
  // The message sequence number.
  message_sequence_number:uint;

  // Error code.
  error_code:byte;
}

table HubInfoRequest {}
table HubInfoResponse {
  /// The name of the hub. Nominally a UTF-8 string, but note that we're not
  /// using the built-in "string" data type from FlatBuffers here, because the
  /// generated C++ uses std::string which is not well-supported in CHRE. This
  /// applies for vendor and toolchain as well.
  name:[byte];
  vendor:[byte];
  toolchain:[byte];

  /// Legacy platform version reported in the HAL; semantics not strictly
  /// defined
  platform_version:uint;

  /// Toolchain version reported in the HAL; semantics not strictly defined
  toolchain_version:uint;

  peak_mips:float;
  stopped_power:float;
  sleep_power:float;
  peak_power:float;

  /// Maximum size regular message that can be sent to a nanoapp
  max_msg_len:uint;

  /// @see chreGetPlatformId()
  platform_id:ulong;

  /// @see chreGetVersion()
  chre_platform_version:uint;

  /// Whether reliable messages are supported
  supports_reliable_messages:bool = false;

  // TODO: list of connected sensors
}

table NanoappListRequest {}

/// Metadata regarding a Nanoapp RPC service. See the Android API
/// core/java/android/hardware/location/NanoAppRpcService.java for more details
/// on how this value is used by the Android application.
table NanoappRpcService {
  id: ulong;
  version: uint;
}

table NanoappListEntry {
  app_id:ulong;
  version:uint;
  enabled:bool = true;

  /// Whether the nanoapp is a pre-loaded "system" nanoapp, i.e. one that should
  /// not show up in the list of nanoapps in the context hub HAL. System
  /// nanoapps are typically used to leverage CHRE for some device functionality
  /// and do not interact via the context hub HAL.
  is_system:bool = false;

  /// Nanoapp permissions, if supported. Nanoapp permissions are required on
  /// CHRE API v1.5+, and are defined in chre/util/system/napp_permissions.h
  permissions:uint;

  /// The list of RPC services supported by this nanoapp.
  rpc_services:[NanoappRpcService];

  // TODO: memory usage
}

table NanoappListResponse {
  nanoapps:[NanoappListEntry] (required);
}

/// Represents a request for loading a nanoapp.
/// The nanaopp can either be requested to be loaded via a file or via a buffer.
/// For loading via a file, the following steps will be taken:
/// 1. The loader sends a LoadNanoappRequest message to CHRE. app_binary must
///    be set for legacy purposes, but should be empty. Additionally,
///    fragment_id and total_app_size are unused in this request. The loading
///    that happens as part of this request is serialized, but asynchronous
///    meaning that load requests will be processed in the order they are sent
///    but multiple requests can be outstanding at any given time.
/// 2. CHRE stores the filename and waits until its event loop is able to
///    process the request.
/// 3. Once ready, the nanoapp will be loaded from the file specified in the
///    original request and will send a callback indicating the
///    completion/failure of the request.
/// For loading via a buffer, loading may optionally be fragmented into multiple
/// sequential requests, which will follow the following steps:
/// 1. The loader sends a LoadNanoappRequest message to CHRE. If the request
///    is fragmented, then the fields fragment_id and total_app_size must
///    be defined. Once the first fragment is sent to CHRE, all subsequent
///    fragments must be delivered before a new LoadNanoappRequest can be
///    issued. If a new request is received while a current request has
///    outstanding fragments, the current request will be overridden with the
///    new one.
/// 2. CHRE preallocates the required amount of memory, and loads app_binary,
///    appending to already loaded fragments as appropriate.
/// 3. If the request is fragmented, then the requestor must sequentially send
///    multiple LoadNanoappRequest with incremental nanoapp binary fragments.
///    CHRE will respond with LoadNanoappResponse for each request. For
///    requests starting from the second fragment, all fields except
///    fragment_id and app_binary should be ignored by CHRE.
///
///    Once the LoadNanoappRepsonse for the last fragment is received
///    by the HAL, the HAL client will receive a callback indicating the
///    completion/failure of a load request.
///
/// If any request fragment is lost, then the entire load request will be
/// considered to have failed. If the request times out (e.g. the requestor
/// process crashes), then the load request will be cancelled at CHRE and fail.
table LoadNanoappRequest {
  transaction_id:uint;

  app_id:ulong;
  app_version:uint;
  target_api_version:uint;

  app_binary:[ubyte] (required);

  /// Fields that are relevant for fragmented loading
  /// The framgent count starts at 1 and should end at the total number of
  /// fragments. For clients that do not support fragmented loading, the
  /// default behavior should be to assume one fragment.
  fragment_id:uint = 0;
  total_app_size:uint;

  /// Null-terminated ASCII string containing the file name that contains the
  /// app binary to be loaded.
  app_binary_file_name:[byte];

  /// The nanoapp flag values from the nanoapp header defined in
  /// build/build_template.mk. Refer to that file for more details.
  app_flags:uint;

  /// If true and fragmented loading is requested, the LoadNanoappResponse
  /// for the last fragment will be sent after the fragment was confirmed
  /// to be placed in memory and no additional response will be sent after
  /// the nanoapp is linked and started in the framework.
  respond_before_start:bool;
}

table LoadNanoappResponse {
  transaction_id:uint;

  /// Denotes whether a load request succeeded or failed.
  /// If any fragment of a load request fails, the entire load request for
  /// the same transaction will fail.
  success:bool;

  /// The fragment count of the load reponse is for.
  fragment_id:uint = 0;

  // TODO: detailed error code?
}

/// Contains information needed for the host to load the token database for the
/// nanoapp. This message is only sent if a token database section is found in
/// the nanoapp elf binary.
table NanoappTokenDatabaseInfo {
  instance_id:uint;

  app_id:ulong;

  /// The size offset of the token database from the start of the address of
  /// the ELF binary in bytes.
  database_offset_bytes:uint;

  /// The size of the token database section in the ELF binary in bytes.
  database_size_bytes:uint;
}

table UnloadNanoappRequest {
  transaction_id:uint;

  app_id:ulong;

  /// Set to true to allow this request to unload nanoapps identified as "system
  /// nanoapps", i.e. ones with is_system set to true in NanoappListResponse.
  allow_system_nanoapp_unload:bool;
}

table UnloadNanoappResponse {
  transaction_id:uint;
  success:bool;
}

/// Represents log messages from CHRE.
table LogMessage {
  /// A buffer containing formatted log data. A flat array is used here to avoid
  /// overhead in serializing and deserializing. The format is as follows:
  ///
  /// uint8_t                 - log level (1 = error, 2 = warning,
  ///                                      3 = info, 4 = debug)
  /// uint64_t, little-endian - timestamp in nanoseconds
  /// char[]                  - message to log
  /// char, \0                - null-terminator
  ///
  /// This pattern repeats until the end of the buffer for multiple log
  /// messages. The last byte will always be a null-terminator. There are no
  /// padding bytes between these fields. Treat this like a packed struct and be
  /// cautious with unaligned access when reading/writing this buffer.
  buffer:[byte];
}

/// Represents a message sent to CHRE to indicate AP timestamp for time sync
table TimeSyncMessage {
  /// Offset between AP and CHRE timestamp
  offset:long;
}

/// A request to gather and return debugging information. Only one debug dump
/// session can be active at a time. Upon accepting a request, zero or more
/// DebugDumpData messages are generated, followed by a DebugDumpResponse
/// indicating the completion of the operation.
table DebugDumpRequest {}

table DebugDumpData {
  /// Null-terminated ASCII string containing debugging information
  debug_str:[byte];
}

table DebugDumpResponse {
  /// true if the request was accepted and a dump was performed, false if it was
  /// rejected or failed to complete for some reason
  success:bool;

  /// The number of DebugDumpData messages sent in this session
  data_count:uint;
}

/// A request from CHRE for host to initiate a time sync message
/// (system feature, platform-specific - not all platforms necessarily use this)
table TimeSyncRequest {}

/// Request from CHRE to enable direct access to data from the low-power
/// microphone. On some systems, coordination via the AP (e.g. with
/// SoundTrigger HAL) is needed to ensure this capability is powered up when
/// CHRE needs it. The host does not send a response.
table LowPowerMicAccessRequest {}

/// Notification from CHRE that it no longer needs direct access to low-power
/// microphone data.
table LowPowerMicAccessRelease {}

/// An enum describing the setting type.
enum Setting : byte {
  LOCATION = 0,
  WIFI_AVAILABLE,
  AIRPLANE_MODE,
  MICROPHONE,
  BLE_AVAILABLE,
}

/// An enum describing the state of a setting.
enum SettingState : byte {
  DISABLED = 0,
  ENABLED,
}

/// Notification from the host that a system setting has changed
table SettingChangeMessage {
  /// The setting that has changed
  setting:Setting = LOCATION;

  /// The new setting value
  state:SettingState = DISABLED;
}

// An enum describing the level of a log.
enum LogLevel : byte {
  ERROR = 1,
  WARNING = 2,
  INFO = 3,
  DEBUG = 4,
  VERBOSE = 5,
}

// An enum describing the type of a log.
enum LogType : byte {
  STRING = 0,
  TOKENIZED = 1,
  BLUETOOTH = 2,
  NANOAPP_TOKENIZED = 3,
}

// An enum indicating the direction of a BT snoop log.
enum BtSnoopDirection : byte {
  INCOMING_FROM_BT_CONTROLLER = 0,
  OUTGOING_TO_ARBITER = 1,
}

/// Represents V2 log messages from CHRE.
table LogMessageV2 {
  /// A buffer containing formatted log data. A flat array is used here to avoid
  /// overhead in serializing and deserializing. The format is as follows:
  ///
  /// uint8_t                 - Log metadata, encoded as follows:
  ///                           [EI(Upper nibble) | Level(Lower nibble)]
  ///                            * Log Type
  ///                              (0 = No encoding, 1 = Tokenized log,
  ///                               2 = BT snoop log, 3 = Nanoapp Tokenized log)
  ///                            * LogBuffer log level (1 = error, 2 = warn,
  ///                                                   3 = info,  4 = debug,
  ///                                                   5 = verbose)
  /// uint32_t, little-endian - timestamp in milliseconds
  /// char[]                  - Log data buffer
  ///
  /// The log data buffer format is as follows:
  /// * Unencoded (string) logs: The log buffer can be interpreted as a NULL
  ///   terminated string (eg: pass to string manipulation functions, get its
  ///   size via strlen(), etc.).
  ///
  /// * Tokenized logs: The first byte of the log buffer indicates the size of
  ///   the actual encoded data to follow. For example, if a tokenized log of
  ///   size 24 bytes were to be represented, a buffer of size 25 bytes would
  ///   be needed to encode this as: [Size(1B) | Data(24B)]. A decoder would
  ///   then have to decode this starting from a 1 byte offset from the
  ///   received buffer.
  ///
  /// * Bt Snoop logs: The first byte of the log buffer indicates the direction
  ///   of the bt snoop log, depending on whether it is incoming for the BT
  ///   controller or outgoing to the arbiter. The second byte indicates the size
  ///   of the actual BT payload followed. For example, if a bt snoop log of
  ///   size 24 bytes were to be represented, a buffer of size 26 bytes would
  ///   be needed to encode this as: [Direction(1B) | Size(1B) | Data(24B)].
  ///
  /// * Tokenized nanoapp logs: This log type is specifically for nanoapps with
  ///   tokenized logs enabled. The first two bytes is the instance ID of the
  ///   nanoapp which sends this tokenized log message. This instance ID will be
  ///   used to map to the corresponding detokenizer in the log message parser.
  ///   The rest is similar to tokenized logs with one byte of the size followed
  ///   by the payload. For example, if a nanoapp tokenized log of size 24 bytes
  ///   were to be sent, a buffer of size 27 bytes would be to encoded as:
  ///   [InstanceId (2B) | Size(1B) | Data(24B)].
  ///
  /// This pattern repeats until the end of the buffer for multiple log
  /// messages. The last byte will always be a null-terminator. There are no
  /// padding bytes between these fields. Treat this like a packed struct and be
  /// cautious with unaligned access when reading/writing this buffer.
  /// Note that the log message might not be null-terminated if an encoding is
  /// used.
  buffer:[byte];

  /// The number of logs dropped since CHRE started
  num_logs_dropped:uint;
}

// A request to perform basic internal self-test in CHRE. The test to be performed
// is platform-dependent, and can be used to check if the system is functioning
// properly. This message should be used for debugging/testing.
table SelfTestRequest {}
table SelfTestResponse {
  // True if the self-test succeeded.
  success:bool;
}

// Message sent whenever a host endpoint has connected with the Context Hub.
// CHRE may receive messages from this host afterwards.
table HostEndpointConnected {
  /// The host-side endpoint that has connected to the framework.
  host_endpoint:ushort;

  /// The type of host endpoint, which should be any of the CHRE_HOST_ENDPOINT_TYPE_*
  /// values defined in the chre_api/chre/event.h.
  type:ubyte;

  /// The (optional) package name associated with the host endpoint.
  package_name:[byte];

  /// The (optional) attribution tag associated with this host.
  attribution_tag:[byte];
}

// Message sent whenever a host endpoint has disconnected from the Context Hub.
table HostEndpointDisconnected {
  /// The host-side endpoint that has disconnected from the framework.
  host_endpoint:ushort;
}

// Represents metric messages from CHRE
table MetricLog {
  // A unique identifier for the encoded metric message.
  id:uint;

  // The metric data, which is encoded using a custom-defined protocol. This
  // same protocol must be used to decode the data at the host side for consumption.
  encoded_metric:[byte];
}

// A container to store batched metrics messages
table BatchedMetricLog {
  // The batched metrics
  metrics: [MetricLog];
}

// NAN enable request sent from CHRE to the host.
table NanConfigurationRequest {
  enable:bool;
}

// NAN status message sent from the host to CHRE whenever a change in the NAN
// enabled state is detected. Note that this update can be sent to CHRE either
// as a response to a configuration request, or from out of band if NAN was
// disabled by an external agent.
table NanConfigurationUpdate {
  enabled:bool;
}

// Debug configurastion that will be send from Android AP to CHRE during boot time
table DebugConfiguration {
  // Should HealthMonitor::onFailure crash when receiving a false condition
  health_monitor_failure_crash:bool;
}

// Pulse messages are used to check if CHRE is up running.
table PulseRequest {}
table PulseResponse {}

// LE L2CAP COC channel information.
table LeCocChannelInfo {
  // Local Channel Identifier.
  localCid:int;

  // Remote Channel Identifier.
  remoteCid:int;

  // Protocol Service Multiplexer.
  psm:int;

  // Maximum Transmission Unit (MTU, max Rx SDU size) that the local device
  // will accept for packets received on this channel.
  localMtu:int;

  // Maximum Transmission Unit (MTU, max Tx SDU size) that the remote device
  // will accept for packets sent on this channel.
  remoteMtu:int;

  // Maximum PDU Payload Size (MPS) that the local device will accept for
  // packets received on this channel.
  localMps:int;

  // Maximum PDU Payload Size (MPS) that the remote device will accept for
  // packets sent on this channel.
  remoteMps:int;

  // The amount of PDUs that the local device will accept from this channel.
  initialRxCredits:int;

  // The amount of PDUs that the remote device will accept from this channel.
  initialTxCredits:int;
}

// Used to specify the channel information of different protocol.
union ChannelInfo {
  LeCocChannelInfo
}

// Request from the host to the offload domain to open a BT socket.
table BtSocketOpen {
  // Unique identifier for this socket connection. This ID in the offload
  // domain matches the ID used on the host side. It is valid only while the
  // socket is connected.
  socketId:long;

  // The name of the socket. Nominally a UTF-8 string, but note that we're not
  // using the built-in "string" data type from FlatBuffers here, because the
  // generated C++ uses std::string which is not well-supported in the offload
  // domain. This applies for vendor and toolchain as well.
  name:[byte];

  // ACL connection handle for the socket.
  aclConnectionHandle:int;

  // Channel information of the socket protocol.
  channelInfo:ChannelInfo;

  // The ID of the Hub to which the end point belongs for hardware offload
  // data path.
  hubId:long;

  // The ID of the Hub endpoint for hardware offload data path.
  endpointId:long;
}

// Status of BT socket open request.
enum BtSocketOpenStatus : byte {
  SUCCESS = 0,
  FAILURE,
}

// Callback from the offload domain to the host to acknowledge that a BT socket
// has been opened successfully or has failed to be opened.
table BtSocketOpenResponse {
  // Unique identifier for this socket connection.
  socketId:long;

  // Status indicating success or failure.
  status:BtSocketOpenStatus;

  // Reason string of the operation failure for debugging purposes.
  reason:[byte];
}

// Request from offload domain to host to close a BT socket.
table BtSocketClose {
  // Unique identifier for this socket connection.
  socketId:long;

  // Reason string for closing the socket for debugging purposes
  reason:[byte];
}

// Host callback to acknowledge that a BT socket has been closed.
table BtSocketCloseResponse {
  // Unique identifier for this socket connection.
  socketId:long;
}

table VendorHubInfo {
  /// The name of the hub. Nominally a UTF-8 string, but note that we're not
  /// using the built-in "string" data type from FlatBuffers here, because the
  /// generated C++ uses std::string which is not well-supported in CHRE.
  name:[byte];

  /// Hub version
  version:uint;

  /// Additional vendor-defined data
  extended_info:[ubyte];
}

union MessageHubDetails {
  HubInfoResponse,
  VendorHubInfo,
}

table MessageHub {
  /// The hub id. -1 is reserved and 0 is invalid. 0x416e64726f696400 represents
  /// the ContextHub service.
  id:long;

  /// Details of the message hub.
  details:MessageHubDetails;
}

table RegisterMessageHub {
  hub:MessageHub;
}

table UnregisterMessageHub {
  id:long;
}

table EndpointId {
  /// Id of the hub hosting the endpoint
  hubId:long;

  /// The id of the endpoint scoped to the hub
  id:long;
}

/// An enum describing the type of an endpoint.
enum EndpointType : ubyte {
  INVALID = 0,
  /// The endpoint is part of the Android Framework
  FRAMEWORK = 1,
  /// The endpoint is an Android app
  APP,
  /// The endpoint is a native Android program
  NATIVE,
  /// The endpoint is a nanoapp
  NANOAPP,
  /// A generic, non-nanoapp endpoint
  GENERIC,
}

enum RpcFormat : ubyte {
  /// Fully custom format
  CUSTOM = 0,
  /// Stable AIDL defined interface using Binder marshalling
  AIDL,
  /// Pigweed RPC defined interface using Protobuf marshalling
  PW_RPC,
}

table Service {
  format:RpcFormat;

  /// Service descriptor. Nominally a UTF-8 string, but note that we're not
  /// using the built-in "string" data type from FlatBuffers here, because the
  /// generated C++ uses std::string which is not well-supported in CHRE.
  descriptor:[byte];

  /// Breaking changes should bump the major version.
  major_version:uint;
  /// Monotonically increasing minor version.
  minor_version:uint;
}

table EndpointInfo {
  id:EndpointId;
  type:EndpointType;

  /// Endpoing name. Nominally a UTF-8 string, but note that we're not using
  /// the built-in "string" data type from FlatBuffers here, because the
  /// generated C++ uses std::string which is not well-supported in CHRE.
  name:[byte];
  version:uint;

  /// Values from CHRE_MESSAGE_PERMISSION_*
  required_permissions:uint;
  services:[Service];
}

table RegisterEndpoint {
  endpoint:EndpointInfo;
}

table UnregisterEndpoint {
  endpoint:EndpointId;
}

/// HAL->CHRE, indicates the HAL is coming up
table GetMessageHubsAndEndpointsRequest {}
table GetMessageHubsAndEndpointsResponse {
  hubs:[MessageHub];
  endpoints:[EndpointInfo];
}

table OpenEndpointSessionRequest {
  id:ushort;
  fromEndpoint:EndpointId;
  toEndpoint:EndpointId;

  /// If present, describes the service definition used over the session
  serviceDescriptor:[byte];
}

table EndpointSessionOpened {
  id:ushort;
}

/// "Reason"s for stopping an endpoint or session over an endpoint.
enum Reason : ubyte {
  /// Unspecified reason.
  UNSPECIFIED = 0,
  /// Out of memory. There's not enough memory to perform this operation.
  OUT_OF_MEMORY,
  /// Timeout. This operation timed out.
  TIMEOUT,
  /// Endpoint rejected this openEndpointSession request.
  OPEN_ENDPOINT_SESSION_REQUEST_REJECTED,
  /// Endpoint requested closeEndpointSession.
  CLOSE_ENDPOINT_SESSION_REQUESTED,
  /// Invalid endpoint.
  ENDPOINT_INVALID,
  /// Endpoint is now stopped.
  ENDPOINT_GONE,
  /// Endpoint crashed.
  ENDPOINT_CRASHED,
  /// Hub was reset or is resetting.
  HUB_RESET,
}

table EndpointSessionClosed {
  id:ushort;
  reason:Reason;
}

table EndpointSessionMessage {
  /// Id of session this message is being sent within
  session_id:ushort;

  /// Type of the message, specific to the Session protocol
  type:uint;

  /// Values from CHRE_MESSAGE_PERMISSION_*. Permissions required to read the
  /// message.
  permissions:uint;
  data:[ubyte];

  /// Bitmask of additional flags applied to the message:
  /// - 0x1: Message delivery status required within 1s
  flags:uint;
  sequence_number:uint;
}

table EndpointSessionMessageDeliveryStatus {
  /// Id of session the message was sent within
  session_id:ushort;
  status:MessageDeliveryStatus;
}

/// A union that joins together all possible messages. Note that in FlatBuffers,
/// unions have an implicit type
union ChreMessage {
  NanoappMessage,

  HubInfoRequest,
  HubInfoResponse,

  NanoappListRequest,
  NanoappListResponse,

  LoadNanoappRequest,
  LoadNanoappResponse,

  UnloadNanoappRequest,
  UnloadNanoappResponse,

  LogMessage,

  TimeSyncMessage,

  DebugDumpRequest,
  DebugDumpData,
  DebugDumpResponse,

  TimeSyncRequest,

  LowPowerMicAccessRequest,
  LowPowerMicAccessRelease,

  SettingChangeMessage,

  LogMessageV2,

  SelfTestRequest,
  SelfTestResponse,

  HostEndpointConnected,
  HostEndpointDisconnected,

  MetricLog,
  BatchedMetricLog,

  NanConfigurationRequest,
  NanConfigurationUpdate,

  DebugConfiguration,

  PulseRequest,
  PulseResponse,

  NanoappTokenDatabaseInfo,

  MessageDeliveryStatus,

  BtSocketOpen,
  BtSocketOpenResponse,
  BtSocketClose,
  BtSocketCloseResponse,

  GetMessageHubsAndEndpointsRequest,
  GetMessageHubsAndEndpointsResponse,

  RegisterMessageHub,
  UnregisterMessageHub,

  RegisterEndpoint,
  UnregisterEndpoint,

  OpenEndpointSessionRequest,
  EndpointSessionOpened,
  EndpointSessionClosed,

  EndpointSessionMessage,
  EndpointSessionMessageDeliveryStatus,
}

struct HostAddress {
  client_id:ushort;
}

/// The top-level container that encapsulates all possible messages. Note that
/// per FlatBuffers requirements, we can't use a union as the top-level
/// structure (root type), so we must wrap it in a table.
table MessageContainer {
  message:ChreMessage (required);

  /// The originating or destination client ID on the host side, used to direct
  /// responses only to the client that sent the request. Although initially
  /// populated by the requesting client, this is enforced to be the correct
  /// value by the entity guarding access to CHRE.
  /// This is wrapped in a struct to ensure that it is always included when
  /// encoding the message, so it can be mutated by the host daemon.
  host_addr:HostAddress (required);
}

root_type MessageContainer;