TI's Calypso GSM/GPRS baseband processor chip has not one but two UART serial
ports, called "MODEM" and "IrDA" in the hardware documentation.  In hardware
terms, both support basic data-leads-only UART operation at a fixed baud rate,
but their extended capabilities differ: the IrDA UART adds IrDA capability (no
surprise), whereas the MODEM UART adds hardware flow control and autobaud.  If
one is not implementing an actual IrDA interface, then the so-called "IrDA"
UART becomes a strict subset of the MODEM one in terms of hw capabilities -
just an extra UART, but a somewhat less capable one.

In a classic modem design such as that present in the GTA0x smartphones made by
FIC/Openmoko, the Calypso presents a standard AT command interface on its MODEM
UART port.  (In the case of GTA0x phones this serial channel is wired to the
phone's application processor; in a standalone modem it would be wired to a
USB-serial chip or even to a classic RS-232 DB25 port.)  However, what is less
known is that the standard firmware for such modems simultaneously presents an
entirely different interface on the IrDA UART - an interface intended for
development, debugging and factory production testing (which includes RF
calibration and IMEI etc programming), rather than for "normal" end users.

Normally this debug/development serial interface (called RVTMUX as will be
explained momentarily) is hidden from "ordinary" users - for example, on FIC
GTA0x phones it is wired to the analog headset jack through a hardware switch
which needs to be enabled through a GPIO signal from the AP.  On Mot C139 and
its siblings the situation is similar: one needs to key in the secret magic
sequence **16379#, and then the firmware presents a hidden menu for switching
the analog headset jack between its "normal" function and the UART carrying
RVTMUX.

But there also exist some oddball devices on which the RVTMUX interface is
presented "in your face".  The Pirelli DP-L10 phone has a USB charging port
which is also wired (through a CP2102 USB-serial chip) to the IrDA UART on the
Calypso - that's right, IrDA, not MODEM - a design decision with which this
hacker strongly disagrees.  (It'll definitely be wired to the MODEM UART
instead on our own semi-clone of this phone, but I digress.)  Apparently Foxconn
(the designers of this phone) had no desire to provide a standard AT command
interface, and instead the only "official" way to use the "data" function of
their USB port (rather than the charging function) is for their "PC sync"
feature, i.e., their proprietary Weendoze software.  And guess what, their
proprietary "PC sync" feature works over TI's RVTMUX interface, as that is
what's presented on Calypso's IrDA UART behind the CP2102!

OK, so what is this RVTMUX?  RV stands for RiViera, an application framework
which TI added to their GSM firmware suite in the early 2000s, T stands for
trace, and MUX stands for multiplexor.  It's a binary packet interface, although
many of these packets contain ASCII debug messages inside.  The framing format
is the same in both directions: each packet begins and ends with an STX (0x02)
byte, all payload bytes except 0x02 and 0x10 are sent literally, and there is a
DLE (0x10) byte prepended before any 0x02 or 0x10 in the payload.  It's the same
general principle as asynchronous HDLC (RFC 1662): packets can contain any
binary data, and the framing provides packet boundaries - although TI's version
is a little less robust than async-HDLC when it comes to recovering after lost
synchronization.

The firmware suite component responsible for actually sending and receiving
these packets over the assigned UART port (usually IrDA, but can be MODEM too,
as on Compal phones for example) is called RVT (RiViera Trace), and it
implements a MUX function.  There are several logical channels multiplexed over
one physical serial port, and the first byte of every packet indicates which
logical channel it belongs to.  Any component within the GSM firmware suite can
send packets to RVT for transmission on this serial interface, and can also
register to receive packets beginning with a particular type ID byte.

Debug trace output
==================

All GSM device firmwares that are based on TI's Calypso chipset reference fw
continuously emit quite voluminous debug trace output on their RVTMUX serial
port, whether it is hidden or exposed on a given device.  Like all RVTMUX
traffic, this debug trace output takes the form of binary packets as explained
above, but the content of these packets is mostly ASCII with some binary header
bytes prepended.  FreeCalypso host utility rvtdump captures all serial output
from a GSM device's RVTMUX port, parses the packet structure and displays this
output in line-oriented pure ASCII with all binary parts decoded.

Test Mode commands
==================

The other major use of the RVTMUX interface is sending so-called Test Mode
commands from an external host to a running GSM device.  Depending on the
firmware version, a GSM device can be commanded to do any of the following
things through this mechanism:

* Exercise RF test modes, e.g., transmit continuously at a set frequency and
  power level;
* Read and write arbitrary memory locations in the Calypso ARM7 address space;
* Read and write ABB chip registers;
* Reboot or power off;
* Access and manipulate the device's flash file system (FFS).

In the segment of history of interest to us TI has produced two different
target firmware components that can receive, interpret and act upon Test Mode
command packets:

* The original Test Mode component of Layer 1, called L1TM or TML1: this
  component handles all RF test modes (needed for RF calibration on device
  production lines), and originally it also implemented memory and ABB register
  read and write commands, and provided access to TMFFS1 (see below).  In the
  original implementation this component registered itself as the handler for
  the "TM" RVTMUX channel (RVT packet type 0x14), so it would receive all TM
  packets sent to the device.

* Enhanced Test Mode (ETM) is a later invention.  It registers itself (instead
  of the old TM in L1) with RVT as the handler for the "TM" RVTMUX channel, and
  then provides a registration service of its own, such that various components
  in the fw suite can register to receive external command packets passing
  first through RVT, then through ETM, and can send responses passing through
  ETM, then through RVT back to the external host.  If a given fw version
  contains both ETM and L1TM, then L1TM registers itself with ETM; an external
  host would send exactly the same binary command packets to exercise RF test
  modes, but inside the firmware they now pass through ETM on their way to L1TM.

The ETM_CORE module contained within ETM itself provides some low-level debug
commands: by sending the right binary command packets to the GSM device via the
RVTMUX serial channel, an external host can examine or modify any memory
location and any hardware register, cause the device to reset, etc.  Prior to
ETM some of these functions (but not all) could be exercised through older TM3
commands, but in FreeCalypso we became familiar with the ETM versions of these
commands long before the older ones because we got the ETM component in full
source form, whereas the sole surviving copy of TCS211 that serves as our golden
reference came with L1TM in binary object form like the rest of L1, and we got
to source-reconstructing it only much later.

Our TCS211 reference fw has both ETM and L1TM, thus it accepts both ETM and TM3
command packets; the same holds for our current production firmwares that are
based on this TCS211 reference.  ETM commands (including TMFFS2, see below) also
work on Pirelli's fw, but Mot/Compal's original fw for the C139 has only the
original non-enhanced Test Mode, not ETM.

FFS access via TM/ETM
=====================

One of the essential facilities provided in one form or another in all known
incarnations of the Test Mode mechanism is the ability to access and manipulate
the GSM device's flash file system (FFS).  See TIFFS-Overview for a description
of this file system.  TI's TMFFS1 and TMFFS2 protocols provide a command and
response packet interface to the FFS API functions inside the fw, and enable an
external host connected to the GSM device via the RVTMUX channel to perform
arbitrary read and write operations on the device file system.

In the segment of history of interest to us TI has produced two different
and entirely incompatible versions of the TMFFS protocol: TMFFS1 and TMFFS2.
Or rather, what is now called TMFFS1 was originally just TMFFS, and then came
TMFFS2.  TMFFS2 works only through ETM, whereas TMFFS1 predates ETM: in the
original implementation the tm_ffs() function in the FFS code was called from
L1TM code.

Our copy of TCS211 reference fw includes the source for both TMFFS1 and TMFFS2;
it is theoretically possible to build a firmware image that includes both TMFFS
versions (they won't conflict because they respond to different command
packets), but it is pretty clear that TI never intended to have both enabled
at the same time.  Our copy of TCS211 came with TMFFS1 enabled and we didn't
change it when we made the moko12 (leo2moko-r1) fw release for the Openmoko
community (the previous proprietary mokoN firmwares also implement TMFFS1),
but we have subsequently switched to TMFFS2 for our current TCS211-based work.
Our current production firmwares implement TMFFS2.

Pirelli's fw implements TMFFS2: we don't have any source for this fw, but our
FreeCalypso host utilities written to talk the TMFFS2 protocol based on our
available TCS211 source work beautifully when run against Pirelli's fw.

Use in FreeCalypso
==================

Our current production firmwares for FreeCalypso modems faithfully follow the
architecture of TI's TCS211 without any fundamental changes.  Thus the
functionality which we present via RVTMUX is exactly the same as TI's original:
our firmwares emit the same 3 kinds of debug traces (RV, L1 and GPF) as various
pre-existing ones, and for Test Mode functionality we have ETM, L1TM and TMFFS2.

We have adopted ETM and TMFFS2 as the standard combination for FreeCalypso,
i.e., ETM_CORE for memory and ABB register reads and writes and TMFFS2 for
external FFS access.  We needed to develop our own host tools for operating on
GSM device FFS via one of the two TMFFS protocols, and after studying the fw
source implementing both, I (Space Falcon) came to the conclusion that TMFFS2
is both more capable and more reliable; my guess is that TMFFS1 was likely kept
around only because some of TI's crappy Weendoze host software depended on it.
(See chipsetsw/drivers/drv_app/ffs/board/tmffs.c in leo2moko or tcs211-fcmodem
if you would like to judge for yourself.)

We have the following host tools for communicating with TI-based GSM firmwares
(both our own and some of the existing proprietary ones):

rvtdump		This tool produces a human-readable dump of all output emitted
		by a TI-based GSM fw in the form of RVTMUX binary packets.  It
		can also log this dump to a file.

rvinterf	This tool is a superset of rvtdump: it not only dumps and/or
		logs all output from the GSM fw, but also provides a mechanism
		for sending command packets to it.

Rvinterf is the engine behind the following host tools that send Test Mode
commands to a target:

fc-tmsh		This is our basic tool for sending Test Mode commands to a
		running GSM fw.  It is strictly asynchronous in that commands
		entered by the operator get sent to the target, and any response
		packets received from the target are displayed as they come in.
		The tool has no knowledge of any correspondence between commands
		being sent and whatever responses they should elicit, i.e., it
		is perfectly suited for experimental discovery of firmware
		behaviour in response to Test Mode commands.

		This tool was written before we realized that there was/is an
		older, more basic Test Mode predating ETM, hence in many place
		we say "ETM" when we really should have said "TM".  Oh well...

fc-fsio		This tool speaks the TMFFS2 protocol and allows a user or
		developer to perform a wide range of operations on the file
		system of a GSM device.  It operates synchronously, i.e., it
		sends ETM/TMFFS2 commands and expects responses in strict
		lock-step; a single user command may translate into a large
		number of ETM/TMFFS2 command packet exchanges.

fc-shell	This tool is asynchronous like fc-tmsh, but instead of talking
		and listening on the TM/ETM RVTMUX channel, it talks and listens
		on GPF's channel and on the new AT-over-RVTMUX channel which we
		added in FreeCalypso.  fc-shell can be used to issue system
		primitive commands to GPF (and to see firmware responses to
		them), and to talk AT commands via RVTMUX.

AT commands over RVTMUX
=======================

There is one more use to which we put the RVTMUX debug serial interface that is
an original FreeCalypso invention: communicating with the AT command interpreter
(ATI).  TI's original architecture assumes that if a product is to offer a
standard AT command interface (the product is either a GSM/GPRS modem for which
this AT command interface is the sole mode of usage or a feature phone that
offers a data port as one of its features), then it will be presented on a
dedicated UART separate from RVTMUX.

However, in the case of our FreeCalypso family of projects about 2 years had
passed between our first functional GSM fw attempts in 2015 and us successfully
building our own development board in 2017; during this time we had to work on
various crippled pre-existing Calypso devices, and many of them had only one
UART practically accessible.  In response to this situation we developed a way
to pass AT commands over RVTMUX.  We created a new RVTMUX channel for this
interface and assigned it RVT packet type 0x1A.  Packets sent from an external
host to the GSM device carry AT commands and SMS string input, whereas packets
flowing the other way carry ATI's responses to commands and asynchronous
notifications such as incoming calls.  The host utility for talking AT commands
to a FreeCalypso GSM device via RVTMUX is fc-shell, described above.

Now that we have built a proper FreeCalypso development board with two UARTs,
the use of this AT-over-RVTMUX hack is deprecated for general usage: this hack
does not support any data services (CSD or GPRS), and even for SMS it is
crippled because maximum-length messages cannot be sent in the more capable PDU
mode.  However, it still comes in handy during certain casual testing sessions,
and it is required if one needs to run our FreeCalypso firmware on Mot C1xx or
Pirelli DP-L10 hardware.

Keepalive mechanism
===================

Another FreeCalypso addition to TI's RVTMUX interface is our keepalive mechanism
for the voice pseudo-modem hack on Mot C1xx targets.  The FreeCalypso family
includes many subprojects, and one of these subprojects involves running modem-
like firmware (control via AT commands only, no local UI) on Mot C1xx phones.
Having a device that was originally made to be a phone with LCD and buttons
turn into a serially-controlled pseudo-modem (LCD stays dark, buttons do
nothing) feels quite weird, and this situation is exacerbated by the flashing
requirement: the only way to run our pseudo-modem fw on Mot C1xx phones is to
flash it, there is no way to run it out of RAM without disturbing the phone's
original fw.

When our Magnetite and Selenite firmwares are built for a C1xx target in the
VPM configuration, they implement the following keepalive logic: if the phone
is powered on and running our fw, but the charging power source is not plugged
in, the fw sends periodic keepalive queries out the serial port to see if there
is a running rvinterf process on the other end of the wire, and automatically
powers off if there is no keepalive response.

Code has been added to rvinterf to respond with a keepalive answer packet when
a keepalive query packet is received; the feature has been implemented on the
rvinterf side first, and then subsequently in our Magnetite fw for C1xx targets.