All TI GSM firmwares known to this author (FreeCalypso developer Space Falcon)
implement some kind of flash file system, or FFS.  Several different FFS code
implementations, and correspondingly several different on-flash data formats,
have been used throughout the history of TI's involvement in the wireless
terminal business.  The FFS incarnation of primary interest to the FreeCalypso
project is the one invented by Mads Meisner-Jensen at TI in the early 2000s
(at least according to the comments in the sources available to us), and it is
relevant to us in the following ways:

* When targeting the GSM modem in Openmoko's GTA01/02 smartphones, we need to
  work with the original FFS from the factory (call it MokoFFS), the same FFS
  as used by the mokoN firmwares: this FFS contains the IMEI and the RF
  calibration values from the factory, which we most certainly don't want to go
  without.

* The Leonardo firmware semi-src which we are using as the reference for
  building our own full source, multi-target GSM fw contains a turnkey-working
  implementation of this very FFS, using the on-flash format in question and
  providing run-time APIs expected by the rest of the GSM fw suite.  Following
  the principle of ``if it ain't broke, don't fix it'', we can use this FFS not
  only on the gtamodem target, but also on other targets, including those where
  we would be starting from a blank state and thus have the freedom to use
  whatever FFS we like.

* The original proprietary fw on the Pirelli DP-L10 phone also happens to use
  an FFS in the same format.  Pirelli's FFS does *not* contain the IMEI or any
  of the RF calibration values though, and trying to reuse it directly for our
  own FC GSM fw seems to be more trouble than benefit - so we'll probably have
  our fw start with a blank TIFFS instead - but there is still insight to be
  gained from in-vitro examination of captured Pirelli FFS images.

Naming
======

I have previously referred to the FFS format in question as Mokopir-FFS or
MPFFS, from "Moko" and "Pirelli".  I was originally hesitant to call it TIFFS,
as lacking the source code, I had no way of knowing whether the FFS format and
implementation were of TI's own invention, or something that TI licensed as a
black box from one of their many proprietary software partners.  (I was unable
to identify it as any well-known, industry-standard FFS format, but absence of
evidence is not evidence of absence.)  But now that we have TI's original source
code which implements this FFS (first the MV100-0.1.rar source, then the full
Leonardo one), complete with comments and a HISTORY file, we know that our FFS
was invented and implemented by someone named Mads Meisner-Jensen at TI - I'm
guessing in the SSA group in Nice, France.

I am now making a naming transition from MPFFS to TIFFS: there is really no
link between this FFS format and the Openmoko+Pirelli duo, other than the
happenstance of me having first encountered this FFS on these two GSM device
brands, and the name TIFFS is more neutrally-descriptive.

What it is
==========

In a rare departure from TI's norm (most of TI's GSM firmware and associated
development tools suffer from heavy Windows poisoning), what I call TIFFS is
very Unixy.  It is a file system with a hierarchical directory tree structure
and with Unixy forward-slash-separated, case-sensitive pathnames; the semantics
of "what is a file" and "what is a directory" are exactly the same as in UNIX;
and TIFFS even supports symlinks, although that support is a little under-
developed, and apparently no FFS symlinks were ever used in any production GSM
device.  Thus the FFS implemented in TI-based GSM devices (modems and
"dumbphones") is really no different from, for example, JFFS2 in embedded Linux
systems.

(The only traditional UNIX file system features which are missing in TIFFS are
 the creation/modification/access timestamps and the ownership/permission
 fields.)

The FFS in a GSM device typically stores two kinds of content:

* Factory data: IMEI, RF calibration values, device make/model/revision
  ID strings etc.  These files are expected to be programmed on the factory
  production line and not changed afterward.

* Dynamic data written into the FFS in normal device operation: when you use a
  "dumbphone" running TI-based firmware, every time you store something "on the
  phone" or in "non-volatile memory", that item is actually stored in the FFS.
  (Where else, if you think of it?)  That includes contacts and received SMS
  stored "on the phone" instead of the SIM, any selections you make in the
  settings/preferences menus which persist across reboots (power cycles), call
  history etc.

It needs to be noted that the "dynamic data" aspect of FFS usage applies not
only to complete phones, but also to modems like the one used in the GTA01/02.
One would naively think that non-volatile storage of data in flash outside of
factory programming would be needed only in a device with its own UI, and that
a modem subservient to external AT commands would be completely stateless
across reboot/power cycles; but that is not the case in actuality.  TI's GSM
firmwares, including the Openmoko ones (the "standard" mokoN), are designed to
always "mount" their FFS with read/write access; TI's FFS implementation in the
firmware has no concept of a "read-only mount".

I am still investigating just what kinds of data are routinely written into the
non-volatile FFS by the firmware in normal operation on devices like the GTA0x
modem, but there most definitely are some.

There is no hard separation between "static" and "dynamic" data in the file
system structure; TIFFS is thus akin to an embedded Linux system with just a
single root file system containing both "static" files like userland binaries
and "dynamic" ones like configuration files under /etc which the user is
expected to edit with vi after logging into the box, or log and similar files
created by the system itself under /var, for example.

Where it lives
==============

The type of flash memory used in Calypso GSM modems and "dumbphones" is called
NOR flash.  This NOR flash memory is physically divided (by the design of the
flash chip itself) into units called "sectors" or more descriptively, erase
blocks.  The typical NOR flash sector size (in Calypso GSM devices) ranges from
64 KiB in the GTA02 modem's NOR flash (4 MiB total) to 256 KiB in the
S71PL129NC0 flash+RAM chip used in the Pirelli DP-L10 (16 MiB of flash total).
The key physical property is that any bit may be changed from a '1' to a '0' at
any time, in any combination, but resetting of '0' bits back to ones can be
done only on the granularity of these largish sectors, in an operation called
"sector erase".

The location of TIFFS within the flash memory of a given GSM device is defined
by the firmware design of that device, but is always some integral number of
contiguous flash sectors.  Some examples:

* On the GTA01/02 GSM modem, FFS occupies 7 sectors of 64 KiB each, starting at
  flash offset 0x380000.

* On the Pirelli DP-L10, the FFS used by the original proprietary fw occupies
  18 sectors of 256 KiB each (for 4.5 MiB in total), starting at the beginning
  of the 2nd flash chip select (0x02000000 in the ARM7 address space).

* The smallest real FFS configuration called for by the table in dev.c in TI's
  original Leonardo fw source is 3 sectors of 64 KiB each; the same table also
  sports a 4 KiB x 4 configuration for RAM-based testing (emulation of FFS in
  RAM without real flash).

* The largest FFS configuration that has been envisioned by the original
  designers seems to be somewhere around 128 sectors.

Each flash sector used for TIFFS begins with this 6-byte signature:

46 66 73 23 10 02

The first 4 bytes are 'Ffs#' in ASCII, and the following two bytes are the
format version number of 0x0210 in little-endian byte order.  The following two
bytes give a count of how many times that sector has been erased and rewritten
(FF FF in "fresh" or "virgin" FFS images), and the following byte indicates
that block's role and status in the FFS life cycle.

How it works
============

Just like JFFS2 and other high-quality flash file systems, TIFFS is designed to
recover gracefully from any possible power failure or crash: one can yank the
battery from the GSM device (or induce a firmware crash) at the most mis-
opportune moment in the middle of an FFS write operation, and the FFS is
expected to recover on the next boot cycle.  I won't be able to document here
all gory details of exactly how this goal is achieved, partly because I haven't
studied the code to the requisite level of depth myself yet, but all of the
responsible code lives under gsm-fw/services/ffs in this freecalypso-sw source
tree; feel free to study it.

In its "normal" or "clean" state (i.e., when not in the middle of a write
operation or recovery from an ungracefully interrupted one), a TIFFS instance
consists of the following 3 types of blocks:

* One block containing inode records, indicated by AB in its type/flags/status
  byte in the block header;
* N-2 blocks (where N is the total number of flash sectors allocated for the
  FFS) containing (or waiting to be filled with) data chunks - indicated by BD
  in the type/flags/status byte;
* One "free" block, indicated by BF - destined to become a new AB or a new BD
  at some point.

Each object written into the FFS (file, directory or symlink) consists of a
16-byte inode record written into the AB block and a data chunk written into
one of the BD blocks.  The data chunk includes the name of the object, hence
one is required even for directories.  Data chunks are contiguous, uncompressed,
and subject to an upper size limit of 2048 or 8192 bytes, depending on the FFS
configuration.  Files larger than this limit are stored in a "segmented" form,
giving rise to a 4th inode or object type (after file, directory and symlink):
segment.  Each segment of a segmented file consists of not only a data chunk,
but also an inode record for the segment, which gives the location of the data
chunk and ties the segment object into the overall FFS structure, making it
accessible.

Because aside from complete sector erasure, flash memory bits can only
transition from '1' to '0' but not the other way around, overwriting an existing
file with some new content (an operation which any reasonable file system must
implement in some way) cannot be done in place.  Instead like most flash file
systems, TIFFS implements this common operation by writing the new version of
the file to a new location (previously blank flash) and then invalidating the
old version - and doing all that while keeping in mind the possibility of an
ungraceful crash or powerdown at any moment, and the requirement of recovering
gracefully from any such event.

Of course as an FFS receives more write activity, even if one keeps overwriting
some existing files with new content of the same size, without adding to the
visible total content size (think du(1) command), eventually all remaining blank
flash space will fill up.  At that point (or at some earlier point, depending
on the FFS design and/or configuration) the FFS has to invoke a compaction or
reclamation or garbage collection procedure: any "mixed" blocks containing both
valid and stale data are transitioned into a "stale-only" state by having the
active data moved to a new block, and then the "all stale" blocks are subjected
to sector erasure, becoming new blank sectors.  The logic responsible for these
operations once again needs to be resilient to the possibility of a crash or
powerdown occurring at the most mis-opportune moment, and it also needs to
implement flash wear leveling: there is a physical limit to how many times a
given flash sector can be erased and rewritten before it goes bad.

All of the above are common and well-known principles, successfully implemented
in well-known flash file systems such as JFFS2 in Linux.  TIFFS is absolutely
no different in this regard; for the implementation details, read the source
code.

How this FFS comes into being
=============================

(This section is only relevant to you if you plan on physically producing your
 own GSM phones or modems on your own factory production line, like this author
 fancies doing in the not-too-distant future, or if you simply enjoy knowing
 how it is done.)

To my knowledge, TI never used or produced a tool akin to mkfs.jffs2 in the
embedded Linux world, which would produce a TIFFS image complete with some
initial directory and file content "in vitro".  Instead it appears that the FFS
instances found in shipped products such as Openmoko phones have been created
"in vivo" by TI's firmware running on the device itself during the "production
test" phase.

The process seems to go like this:

* When the printed circuit board is physically populated with components such
  as the Calypso chip and the flash chip, the latter can be blank - if the
  board design has the nIBOOT pin pulled low, enabling the Calypso boot ROM
  (Openmoko and Pirelli both good on this one, but shame on Compal!), there is
  no need to preprogram the flash chip with anything prior to populating it on
  the board, and the device remains fully unbrickable at all times afterward.

* When the assembled board is powered up for the first time, with completely
  blank flash, the Calypso boot ROM will sit there and patiently wait for a
  code download on either of its two UARTs.

* Using TI's FLUID (Flash Loader Utility Independent of Device) or FreeCalypso's
  fc-loadtool free replacement, the factory production station loads the main
  firmware image into the flash.  Note, it is just the firmware image at this
  step, and the FFS sectors remain blank.

* The board is commanded to reboot (or power-cycled), and the firmware image
  boots for the first time.

* TI's FFS implementation code in their standard firmware reacts to all blank
  flash in the FFS sectors as follows: it performs what they call the preformat
  operation, writing the TIFFS signature and a BF state byte into every FFS
  sector, but the main "format" operation, which sets up the AB/BD block roles,
  creates the root inode and makes the FFS ready to accept the creation of its
  first directories and files, is not done automatically.

In order to perform the FFS format operation and then fill the new FFS with
whatever directories and files are deemed needed to be present in "fresh"
shipping products, the factory production station connects to the just-booted
firmware running on the target via the RVT/ETM protocol (see the RVTMUX
write-up), and sends "test mode" commands to this running firmware.  These
"FFS test mode" (or TMFFS) commands include the format operation, an mkdir
operation to create directories, and a "file write" operation akin to doing
'cat > /dir/whatever/file', creating files in FFS and storing any desired data
in them.

The IMEI is assigned and written into FFS in this process, but it is not the
only data item that will be unique for each individual device made.  Much more
important are the RF calibration values: I have yet to learn exactly what is
being (or needs to be) measured, how these measurements are performed (under
what conditions; what external test equipment is needed), and how these measured
and recorded RF calibration values affect GSM device operation, but this TI
presentation gives some clues:

ftp://ftp.ifctf.org/pub/GSM/Calypso/rf_calibration.pdf

All of these calibration values are stored in a bunch of files under the
/gsm/rf subtree, and these files seem to be "owned" by the L1 code.  The latter
has RAM data structures which correspond to these files; upon normal boot the
initialization code looks in FFS, and if it finds any of the RF calibration
files, it reads each present file into the corresponding RAM data structure,
overwriting the compiled-in defaults.  It appears (slightly uncertain because I
have not yet reintegrated the code in question into our own gsm-fw) that the RF
calibration files in FFS come into being as follows:

* The RF calibration code in L1 (i.e., part of the main GSM fw) performs the
  measurements and stores results in its RAM data structures as commanded by
  the production test station through the "test mode" interface;

* A final test mode command directs the above L1 code to write its RAM data
  structures into FFS.

Once I actually learn this RF calibration process properly in connection with
building my own Calypso-based GSM "dumbphone", I'll be able to say exactly what
it would take to recreate these RF calibration values if they are lost.  But
until then the only advice I can give is to make a backup copy of your modem
FFS with fc-loadtool, and to save it securely.

FreeCalypso support for TIFFS
=============================

Aside from implementing and using it in our own gsm-fw, FreeCalypso will offer
the following support for TIFFS:

1. A tiffs host utility is being written which will allow a user to list and
   extract content from saved FFS images (read out of flash with fc-loadtool)
   "in vitro".  It will be a restructured and (hopefully) improved version of
   the mpffs-* tools released back in the summer of SE52 (A.D. 2013); the latter
   already perform the advertised function, but I seek to integrate some other
   functionality which I developed in an ad hoc side project ("pirollback"),
   and I'm taking the opportunity to make the MPFFS->TIFFS renaming.

(The mpffs-* tools mentioned above have been written based on reverse eng only,
 before I found any source code for TI's FFS firmware implementation!  Now that
 we have the source, some terminological and other inevitable misunderstandings
 can be corrected.)

2. A number of FC tools may be strung together into a kit for editing the FFS
   content of a GSM device, e.g., for changing the IMEI.  The following pieces
   will be involved:

* What is destined to eventually become our totally free GSM fw (the gsm-fw
  source subtree at the top of freecalypso-sw) does not contain any of the
  actual GSM protocol stack (or even L1) functionality yet, but it already
  contains both the FFS code and those components (ETM and TMFFS[12]) which
  are needed for interfacing an external "test mode shell" to this FFS
  implementation through the RVTMUX interface.  And when our gsm-fw does gain
  the actual GSM functionality, the ability to build a minimal FFS+ETM-only
  configuration will still be retained.

* The minimal FFS+ETM subset of gsm-fw can be built into a ramImage (runs
  entirely from RAM via fc-xram, no flashing), and run on a physical device
  such as the GTA0x GSM modem via the fc-xram host utility;

* After loading the ramImage, fc-xram will immediately exec our rvinterf host
  utility described in the RVTMUX write-up;

* Once the GSM device is running what is effectively an FFS editing agent out
  of RAM, accessed via rvinterf over the serial channel, the user will be able
  to run fc-tmsh (or perhaps the FFS operations will be implemented in some
  other utility, we'll see), and that "test mode shell" will provide commands
  for writing things to FFS exactly like one would do in the factory production
  line environment for which TI taylored their tools.

The "in vivo" method of editing the FFS content of a GSM device described above
will probably sound very convoluted, and you may find yourself asking for a way
to do it "in vitro" instead: read the FFS out of flash with fc-loadtool, edit
that image "in vitro" with some utility on your PC, and then use fc-loadtool
again to program it back into your device.  But consider that an "in vitro" FFS
modification would involve erasing and rewriting all sectors of your FFS,
whereas an "in vivo" modification of some small file like the IMEI would be
just a short flash write operation without any erasures at all, i.e., kinder
on the flash.

In any case, the "in vivo" method will definitely be available soon because all
of the components involved therein are also needed for other development uses
in the FreeCalypso project, whereas developing a fully-functional "in vitro"
alternative (one that can create an FFS image "de novo" from a tree of files
and directories a la mkfs.jffs2, or add new files to an existing TIFFS image
etc) would be a good amount of extra work which we otherwise don't need - hence
the latter is not very likely to be written any time soon.

However, if the "in vitro" modification you seek is something trivial like
changing the byte content of a file such as /pcm/IMEI or /gsm/com/rfcap without
changing its length, you will be able to use the "in vitro, read-only" tiffs
host utility to find the exact byte location of the file data within the TIFFS
image, and use your favourite hex editor to whack whatever new byte content you
like at that offset.