Thomson SpeedTouch 546 v6

I have a ADSL2+ Modem/Router, i want to install openwrt on.

I have found multiple information about it.

He has a BCM6338KFBG (CPU), BCM5325EKQMG(Switch), Samsung K4S281632I-UC75(Flash/128MB) and Spansion S29GL032A90TFIR4(Memory/32MB).

I have found this documentation :

Apparently if i press reset longer time, the bootloader run a BOOTP and want a BANT-V file.

I have identify a serial (2 pins) on J3 and a jtag (14 pins) on J6.

I dont have a jtag cable with i can try with serial with RPI3 but i dont know how do this.

Anybody can give me a answer ?