https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/net/bluetooth/l2cap_sock.c Per the bluetooth core spec https://www.bluetooth.com/specifications/bluetooth-core-specification Page 2184, section 3.4.2.1 Exchange MTU Request, PDUs may be sent to negotiate MTU size upon connection Line 415 in l2cap_sock.c, if address types being used are Bluetooth 2 types, it looks like setsockopt(xx, SOL_L2CAP, L2CAP_OPTIONS, xx, xx) is INTENDED to fail with EINVAL errno, but for Bluetooth 2.x connections it would work fine. Line 920 - setsockopt(xx,SOL_BLUETOOTH,BT_RCVMTU,xx,xx) looks to be INTENDED to fail if a connection is already established on the socket The legacy kernel code goes and sets imtu and omtu without any other code. Per the bluetooth spec - inbound and outbound MTU should be set to the minimum MTU between the devices during this ATT PDU handshake. As it stands right now, it looks like the kernel would require disconnecting\reconnecting the socket. I bet it could be changed to mirror the old setsockopt SOL_L2CAP code, so BT2 behaves like BT4+ - but it's not my code and I'm not a BT guru so I'd hate to just submit a patch for it. Is this all intentional? It seems like both imtu and omtu should be allowed to be set in the new code, like the legacy code. Thanks! Dylan
