Q: How do various standard software components (ftdi_sio driver in the Linux
   kernel, libftdi etc) identify what kind of FTDI chip they are talking to?
   For example, how do they tell apart between FT2232C/D and FT2232H with their
   respective differences in features and capabilities?

A: They look primarily at the bcdDevice word in the USB device descriptor.  One
   can also look at bNumInterfaces in the configuration descriptor (more
   reliable, as will be seen momentarily) to distinguish between single-channel
   vs dual-channel vs 4-channel devices, but bcdDevice is still the only way to
   tell apart between FT232BM and FT232R, or between FT2232C/D and FT2232H.

However, this all-important bcdDevice word exhibits a very noteworthy quirk: it
is included in FTDI's EEPROM configuration structure, thus it can be overridden
through EEPROM programming.  The result is that at least on some FTDI chips (so
far I've only tested this quirk on FT2232D) one can make the chip report a
different bcdDevice code than its true chip version, and thus fool standard
host software!  Needless to say, doing so is a very bad idea: the actual
behavior of the chip, the way it acts upon various bits in the EEPROM config
structure and the way it reacts to various USB commands, does not change just
because you changed the "device release number" code, thus various host control
functions will start misbehaving and failing in mysterious ways.

Our FTDI EEPROM tools (see FTDI-EEPROM-tools article) often need to know what
kind of FTDI chip you are working with as in reading, programming or preparing
an EEPROM image for.  Most competitor tools (for example, the ones integrated
into libftdi) do the same thing as all other standard host software components:
they look at the reported bcdDevice and use this code to decide what kind of
FTDI chip they are working with - hence they will be misled and broken if this
word is misprogrammed in an EEPROM.  To avoid this issue, our FTDI EEPROM tools
follow a different design: with our tools you as the operator are responsible
for knowing what kind of FTDI chip you are working with - our tools do NOT
attempt to autodetect the chip, as it is impossible to do so in an environment
that may include badly corrupted or misprogrammed EEPROMs.  Specifically, you
are responsible for manually selecting the correct generator tool (for example,
ftee-gen2232c vs ftee-gen2232h in the case FT2232x) and manually specifying
your EEPROM size with -b or -B options if you are working with a board design
that uses a 93C56 or 93C66 EEPROM.

This largely-manual design of our FTDI EEPROM tools makes them less friendly to
casual users, but in return they should be able to recover from any possible
EEPROM corruption or misprogramming, accidental or malicious.