Bluetooth development (9) -- A2DP protocol in combination with code

The last chapter talked about avdtp Connection process , Let's take a look at this chapter btstack Example .

because a2dp Is an audio transmission framework protocol , The specific use has involved the application layer , For example, is our device a speaker device or a sound source device , We are currently a speaker device , So let's see a2dp_sink_deom.c.

First of all, call a2dp_and_avrcp_setup Function performs a series of initializations , From the name of this function, we know , The initialization includes a2dp The protocol and avrcp agreement ,a2dp We have already talked about the basic agreement avdtp,avrcp Is based on avctp Agreed , AVCTP The protocol describes Bluetooth devices Audio/Video The format and mechanism of control signal exchange , It's an overall agreement , The specific control information is specified by the protocol ( Such as AVRCP) Realization ,AVCTP Itself only specifies control command and response The overall format of , For example, how do we pause at the speaker end 、 Play 、 stop it 、 On a 、 Next operation , You have to rely on avrcp, Another example is the volume synchronization function , Is also dependent on avrcp.

1      l2cap_init();

2       // Initialize AVDTP Sink

3       a2dp_sink_init();

4       a2dp_sink_register_packet_handler(&a2dp_sink_packet_handler);

5       a2dp_sink_register_media_handler(&handle_l2cap_media_data_packet);

6

7       avdtp_stream_endpoint_t * local_stream_endpoint = a2dp_sink_create_stream_endpoint(AVDTP_AUDIO,

8           AVDTP_CODEC_SBC, media_sbc_codec_capabilities, sizeof(media_sbc_codec_capabilities),

9           media_sbc_codec_configuration, sizeof(media_sbc_codec_configuration));

10      if (!local_stream_endpoint){

11          printf("A2DP Sink: not enough memory to create local stream endpoint\n");

12          return 1;

13      }

14      a2dp_local_seid = avdtp_local_seid(local_stream_endpoint);

15

16      // Initialize AVRCP service.

17      avrcp_init();

18      avrcp_register_packet_handler(&avrcp_packet_handler);

……

Basically, they are all initialization operations , The first line initializes l2cap, The second line initializes a2dp sink, Which is for l2cap The callback function is registered with the data of

l2cap_register_service(&avdtp_packet_handler, BLUETOOTH_PSM_AVDTP, 0xffff, gap_get_security_level());

We talked about that before ,l2cap Establish logical channel , The basis for distinguishing upper layer agreements is PSM, So here PSM Be sure to fill in BLUETOOTH_PSM_AVDTP.

After all avdtp dependent l2cap Data will enter this avdtp_packet_handler To deal with .

1   void avdtp_packet_handler(uint8_t packet_type, uint16_t channel, uint8_t *packet, uint16_t size){

2       bd_addr_t event_addr;

3       uint16_t psm;

4       uint16_t local_cid;

5       uint8_t  status;

6       uint16_t l2cap_mtu;

7

8       bool accept_streaming_connection;

9       bool outoing_signaling_active;

10      bool decline_connection;

11

12      avdtp_stream_endpoint_t * stream_endpoint = NULL;

13      avdtp_connection_t * connection = NULL;

14

15      switch (packet_type) {

16          case L2CAP_DATA_PACKET:

17              connection = avdtp_get_connection_for_l2cap_signaling_cid(channel);

18              if (connection){

19                  handle_l2cap_data_packet_for_signaling_connection(connection, packet, size);

20                  break;

21              }

22             

23              stream_endpoint = avdtp_get_stream_endpoint_for_l2cap_cid(channel);

24              if (!stream_endpoint){

25                  if (!connection) break;

26                  handle_l2cap_data_packet_for_signaling_connection(connection, packet, size);

27                  break;

28              }

29             

30              if (stream_endpoint->connection){

31                  if (channel == stream_endpoint->connection->l2cap_signaling_cid){

32                      handle_l2cap_data_packet_for_signaling_connection(stream_endpoint->connection, packet, size);

33                      break;

34                  }

35              }

36

37              if (channel == stream_endpoint->l2cap_media_cid){

38                  btstack_assert(avdtp_sink_handle_media_data);

39                  (*avdtp_sink_handle_media_data)(avdtp_local_seid(stream_endpoint), packet, size);

40                  break;

41              }

42

43              if (channel == stream_endpoint->l2cap_reporting_cid){

44                  log_info("L2CAP_DATA_PACKET for reporting: NOT IMPLEMENTED");

45              } else if (channel == stream_endpoint->l2cap_recovery_cid){

46                  log_info("L2CAP_DATA_PACKET for recovery: NOT IMPLEMENTED");

47              } else {

48                  log_error("avdtp packet handler L2CAP_DATA_PACKET: local cid 0x%02x not found", channel);

49              }

50              break;

51             

52          case HCI_EVENT_PACKET:

53              switch (hci_event_packet_get_type(packet)) {

54

55                  case L2CAP_EVENT_INCOMING_CONNECTION:

56                      l2cap_event_incoming_connection_get_address(packet, event_addr);

57                      local_cid = l2cap_event_incoming_connection_get_local_cid(packet);

58                     

59                      outoing_signaling_active = false;

60                      accept_streaming_connection = false;

61                     

62                      connection = avdtp_get_connection_for_bd_addr(event_addr);

……

With the previous chapter , It is easy to understand the classification of various data packets here ,handle_l2cap_data_packet_for_signaling_connection Is dealing with signal channel The data of , The first 39 Yes (*avdtp_sink_handle_media_data)(avdtp_local_seid(stream_endpoint), packet, size) Is dealing with stream channel The data of , The first 52 Yes HCI_EVENT_PACKET It's not really hci event, It is btstack A kind of virtual software internal hci event, Look at number 55 Just know , This kind of hci event root According to the event type To perform different operations ,l2cap Of signal channel To establish a logical channel connection , There is also a set of state machines , There are different states , This L2CAP_EVENT_INCOMING_CONNECTION Is the state of being ready to establish a connection .

Let's go back to a2dp_and_avrcp_setup,

The first 4 That's ok a2dp_sink_register_packet_handler(&a2dp_sink_packet_handler); Except for avdtp The agreement is internal to a2dp In addition to the processing of each sub state , And registered a2dp_sink_packet_handler This application layer callback function , Mainly in a2dp When connecting sub States , Yes codec Layer needs to do some processing , for instance sbc Encoder initialization , Specific processing of audio , These all depend on the implementation of the upper layer .

The first 5 That's ok a2dp_sink_register_media_handler(&handle_l2cap_media_data_packet);

This handle_l2cap_media_data_packet Is the application layer for stream The package's handler , Before avdtp_packet_handler function , Upon receipt of l2cap Layer of avdtp When the package , Will analyze is signal channel My bag or stream channel My bag , If so stream channel If you have a bag , to glance at 39 That's ok , The callback function registered here is executed . Generally speaking , The work that should be done here is to perform the decoding of the decoder , What are you going to do after decoding ？ Send it to the player to play ？ It depends on the application layer .

The first 7 That's ok

avdtp_stream_endpoint_t * local_stream_endpoint = a2dp_sink_create_stream_endpoint(AVDTP_AUDIO,

        AVDTP_CODEC_SBC, media_sbc_codec_capabilities, sizeof(media_sbc_codec_capabilities),

        media_sbc_codec_configuration, sizeof(media_sbc_codec_configuration));

Create a sep, It can also be seen that a2dp in sep It is indeed a virtual concept , Software can be created if you want to .

Take a closer look at the code , Look at this sep Distribute seid When ：

static uint16_t avdtp_get_next_local_seid(void){

    if (stream_endpoints_id_counter == 0xffff) {

        stream_endpoints_id_counter = 1;

    } else {

        stream_endpoints_id_counter++;

    }

    return stream_endpoints_id_counter;

}

You can see seid It's just a plus 1 Dynamic allocation of actions .

The first 14 That's ok

a2dp_local_seid = avdtp_local_seid(local_stream_endpoint);

to a2dp_local_seid This important variable is assigned , because avdtp Is based on seid Of , Therefore, the connection process will be frequently used in the future , It is not too much to save with a global variable .

Next avrcp A series of initialization .

Whole a2dp_and_avrcp_setup After the function is executed , The upper level services are set up ,a2dp_sink_demo.c Support to input instructions from the upper computer to execute corresponding programs , for instance , Input ‘b’, Will call a2dp_sink_establish_stream perform a2dp Connection operation , Next , Input b Give it a try , Let the state machine run .