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l2cap-ertm: try to negotiate 'No FCS' in order to work around iOS bug

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Issue: iOS (e.g. 10.2) uses 'No FCS' as default while Core 5.0 specifies 'FCS' as default
Workaround: try to actively negotiate 'No FCS' and fall back to 'FCS' if 'No FCS' is rejected
This works as iOS accepts the 'No FCS' request, hence the default value is only used on non-iOS devices
This commit is contained in:
Matthias Ringwald 2017-11-10 16:03:53 +01:00
parent e30788a349
commit 6574158ae6
2 changed files with 42 additions and 14 deletions
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53
src/l2cap.c
View File

@ -387,7 +387,11 @@ static uint16_t l2cap_setup_options_ertm_request(l2cap_channel_t * channel, uint
little_endian_store_16( config_options, 5, channel->local_retransmission_timeout_ms);
little_endian_store_16( config_options, 7, channel->local_monitor_timeout_ms);
little_endian_store_16( config_options, 9, channel->local_mps);
return 11;
//
config_options[11] = 0x05; // FCS OPTION
config_options[12] = 1; // length
config_options[13] = channel->fcs_option;
return 14;
}
static uint16_t l2cap_setup_options_ertm_response(l2cap_channel_t * channel, uint8_t * config_options){
@ -406,7 +410,11 @@ static uint16_t l2cap_setup_options_ertm_response(l2cap_channel_t * channel, uin
little_endian_store_16( config_options, 7, channel->local_monitor_timeout_ms);
// less or equal to remote mps
little_endian_store_16( config_options, 9, btstack_min(channel->local_mps, channel->remote_mps));
return 11;
//
config_options[11] = 0x05; // FCS OPTION
config_options[12] = 1; // length
config_options[13] = channel->fcs_option;
return 14;
}
static int l2cap_ertm_send_supervisor_frame(l2cap_channel_t * channel, uint16_t control){
@ -480,6 +488,11 @@ static void l2cap_ertm_configure_channel(l2cap_channel_t * channel, l2cap_ertm_c
channel->rx_packets_data = &buffer[pos];
pos += ertm_config->num_rx_buffers * channel->local_mps;
channel->tx_packets_data = &buffer[pos];
// Issue: iOS (e.g. 10.2) uses "No FCS" as default while Core 5.0 specifies "FCS" as default
// Workaround: try to actively negotiate "No FCS" and fall back to "FCS" if "No FCS" is rejected
// This works as iOS accepts the "No FCS" request, hence the default value is only used on non-iOS devices
channel->fcs_option = 0;
}
uint8_t l2cap_create_ertm_channel(btstack_packet_handler_t packet_handler, bd_addr_t address, uint16_t psm,
@ -1078,7 +1091,7 @@ int l2cap_send_prepared(uint16_t local_cid, uint16_t len){
int fcs_size = 0;
#ifdef ENABLE_L2CAP_ENHANCED_RETRANSMISSION_MODE
if (channel->mode == L2CAP_CHANNEL_MODE_ENHANCED_RETRANSMISSION){
if (channel->mode == L2CAP_CHANNEL_MODE_ENHANCED_RETRANSMISSION && channel->fcs_option){
fcs_size = 2;
}
#endif
@ -2290,6 +2303,13 @@ static void l2cap_signaling_handle_configure_request(l2cap_channel_t *channel, u
default:
break;
}
}
if (option_type == L2CAP_CONFIG_OPTION_TYPE_FRAME_CHECK_SEQUENCE && length == 1){
// "FCS" has precedence over "No FCS"
if (command[pos] != 0){
log_info("ertm: accept remote request for FCS");
channel->fcs_option = 1;
}
}
#endif
// check for unknown options
@ -2981,17 +3001,22 @@ static void l2cap_acl_classic_handler(hci_con_handle_t handle, uint8_t *packet,
#ifdef ENABLE_L2CAP_ENHANCED_RETRANSMISSION_MODE
if (l2cap_channel->mode == L2CAP_CHANNEL_MODE_ENHANCED_RETRANSMISSION){
// assert control + FCS fields are inside
if (size < COMPLETE_L2CAP_HEADER+2+2) break;
int fcs_size = l2cap_channel->fcs_option ? 2 : 0;
// verify FCS (required if one side requests it and BTstack does)
uint16_t fcs_calculated = crc16_calc(&packet[4], size - (4+2));
uint16_t fcs_packet = little_endian_read_16(packet, size-2);
log_info("Packet FCS 0x%04x, calculated FCS 0x%04x", fcs_packet, fcs_calculated);
if (fcs_calculated != fcs_packet){
log_error("FCS mismatch! Packet 0x%04x, calculated 0x%04x", fcs_packet, fcs_calculated);
// TODO: trigger retransmission or something like that
break;
// assert control + FCS fields are inside
if (size < COMPLETE_L2CAP_HEADER+2+fcs_size) break;
if (l2cap_channel->fcs_option){
// verify FCS (required if one side requests it and BTstack does)
uint16_t fcs_calculated = crc16_calc(&packet[4], size - (4+2));
uint16_t fcs_packet = little_endian_read_16(packet, size-2);
if (fcs_calculated == fcs_packet){
log_info("Packet FCS 0x%04x verified", fcs_packet);
} else {
log_error("FCS mismatch! Packet 0x%04x, calculated 0x%04x", fcs_packet, fcs_calculated);
// TODO: trigger retransmission or something like that
break;
}
}
// switch on packet type
@ -3088,7 +3113,7 @@ static void l2cap_acl_classic_handler(hci_con_handle_t handle, uint8_t *packet,
// get SDU
const uint8_t * sdu_data = &packet[COMPLETE_L2CAP_HEADER+2];
uint16_t sdu_len = size-(COMPLETE_L2CAP_HEADER+2+2);
uint16_t sdu_len = size-(COMPLETE_L2CAP_HEADER+2+fcs_size);
// assert SDU size is smaller or equal to our buffers
if (sdu_len > l2cap_channel->local_mps) break;

3
src/l2cap.h
View File

@ -247,6 +247,9 @@ typedef struct {
// if ertm is not mandatory, allow fallback to L2CAP Basic Mode - flag
uint8_t ertm_mandatory;
// Frame Chech Sequence (crc16) is present in both directions
uint8_t fcs_option;
// sender: max num of stored outgoing frames
uint8_t num_tx_buffers;