This document describes how to patch the FT2232D EEPROM as well as
the ftdi_sio kernel driver to allow passing a GSM baud rate in the
same manner as the CP2102 with a patched EEPROM.

First, install the libftdi-dev package. Then compile fc-ft2232d-eeprom.c
with -lftdi:

gcc fc-ft2232d-eeprom.c -o fc-ft2232d-eeprom -O2 -lftdi

Run the program with your board connected: the new EEPROM will be flashed
(by default, it is in a blank state; you can reset the EEPROM to its
original blank state by running the program again). Reconnect the device.

Make sure your have the kernel headers for your current kernel version
installed. A simple way to do this, on Debian/Ubuntu, is to install the
module-assistant package, and run "m-a prepare".

Then, get the kernel source for your system. For example with this command:
apt-get source linux-image-extra-4.4.0-59-generic but this will depend on
your kernel version. From drivers/usb/serial, grab ftdi_sio* and copy them
in a new directory named ftdi_sio_fc

Now rename ftdi_sio.c to ftdi_sio_fc.c, and create a simple Makefile that
only contains:

CONFIG_USB_SERIAL_FTDI_SIO_FC=m
obj-$(CONFIG_USB_SERIAL_FTDI_SIO_FC)		+= ftdi_sio_fc.o

The original patch provided by Mychaela for linux 2.6.37.6 has been updated
for newer kernels:

patch -Np0 -i ftdi_sio_fc.patch

Then edit ftdi_sio_fc.c and make the following modification:

- change the structure named "id_table_combined" so that it becomes:
static const struct usb_device_id id_table_combined[] = {
        { USB_DEVICE(FTDI_VID, 0x4643) /* FreeCalypso */ },
        { }                            /* Terminating entry */
};

You're almost done. Compile and install the new kernel module:

make -C /lib/modules/`uname -r`/build M=$PWD
sudo su
make -C /lib/modules/`uname -r`/build M=$PWD modules_install
depmod -a
modprobe ftdi_sio_fc

You should see in your kernel log:
[ 7361.665693] usbserial: USB Serial support registered for FTDI USB Serial Device
[ 7361.665714] ftdi_sio_fc 2-2:1.0: FTDI USB Serial Device converter detected
[ 7361.665765] usb 2-2: Detected FT2232C
[ 7361.666013] usb 2-2: FTDI USB Serial Device converter now attached to ttyUSB0
[ 7361.666044] ftdi_sio_fc 2-2:1.1: FTDI USB Serial Device converter detected
[ 7361.666111] usb 2-2: Detected FT2232C
[ 7361.666544] usb 2-2: FTDI USB Serial Device converter now attached to ttyUSB1