I would like to enable the data length extended (eL2CAP) functionality which is normally possible according to Bluetooth 4.2 Core Specifications.
Unfortunately, I didn't found any informations about that. I check then the Bluetooth stack version of the CYW43455 chip and it returns:
pi@raspberrypi:~$ sudo hciconfig -a hci0 version
hci0: Type: Primary Bus: UART
BD Address: B8:27:EB:27:0B:7F ACL MTU: 1021:8 SCO MTU: 64:1
HCI Version: 4.1 (0x7) Revision: 0x8b
LMP Version: 4.1 (0x7) Subversion: 0x6119
Manufacturer: Broadcom Corporation (15)
The datasheet of the chip says Bluetooth 4.2, but it seems the Bluetooth stack inside is 4.1.
CYW43455 Datasheet: http://www.cypress.com/file/358916/download
Bluez driver: version 5.49
Operating system: Raspbian Stretch Lite