External bluetooth chip problem (suspecting RTS/CTS problem)

I’m trying to setup configuration where Dragonboard 410C (custom Linux distribution) is using external Bluetooth chip. Firstly, I connected DB410C USB to BT chip through FTDI and after running hciattach command everything started works just fine. Here is screenshot how logic analyzer shows transmission:

Request-to-send is in high state every time when there are packets to sent.

Secondly, I connected DB410C serial port with BT chip serial port. Running hciattach with baud rate 1000000, packets start to be unreadable (some of them looks corrupted, looks like one was written over another). Screenshot of logic analyzer on HCI transmission at 1000000 baud rate shows:

There is no RTS high states at all. (It works smooth when the baud rate is around 115200, but flow control still doesn’t work)

RTS/CTS are working, but not properly. Works fine with FTDI, but when transmission is changed to /dev/ttyMSM1, problems start to appear. Recently we upgraded kernel from 4.4.9 to 4.4.23, but it didn’t help. I’m just wondering where is an issue, is it lack of hardware control flow drivers or CTS/RTS pins usage is controlled by another peripheral so using them is blocked? What should I check to find source of the problem?