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doc/TIFFS write-up
author Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
date Tue, 14 Jan 2014 02:31:09 +0000
parents 2900fe603f8a
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Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
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1 All TI GSM firmwares known to this author (FreeCalypso developer Space Falcon)
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2 implement some kind of flash file system, or FFS. Several different FFS code
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3 implementations, and correspondingly several different on-flash data formats,
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4 have been used throughout the history of TI's involvement in the wireless
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Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
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5 terminal business. The FFS incarnation of primary interest to the FreeCalypso
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6 project is the one invented by Mads Meisner-Jensen at TI in the early 2000s
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7 (at least according to the comments in the sources available to us), and it is
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8 relevant to us in the following ways:
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9
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10 * When targeting the GSM modem in Openmoko's GTA01/02 smartphones, we need to
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11 work with the original FFS from the factory (call it MokoFFS), the same FFS
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12 as used by the mokoN firmwares: this FFS contains the IMEI and the RF
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13 calibration values from the factory, which we most certainly don't want to go
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14 without.
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15
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16 * The Leonardo firmware semi-src which we are using as the reference for
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17 building our own full source, multi-target GSM fw contains a turnkey-working
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18 implementation of this very FFS, using the on-flash format in question and
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19 providing run-time APIs expected by the rest of the GSM fw suite. Following
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20 the principle of ``if it ain't broke, don't fix it'', we can use this FFS not
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21 only on the gtamodem target, but also on other targets, including those where
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22 we would be starting from a blank state and thus have the freedom to use
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23 whatever FFS we like.
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24
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25 * The original proprietary fw on the Pirelli DP-L10 phone also happens to use
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26 an FFS in the same format. Pirelli's FFS does *not* contain the IMEI or any
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27 of the RF calibration values though, and trying to reuse it directly for our
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28 own FC GSM fw seems to be more trouble than benefit - so we'll probably have
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29 our fw start with a blank TIFFS instead - but there is still insight to be
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30 gained from in-vitro examination of captured Pirelli FFS images.
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31
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32 Naming
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33 ======
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34
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35 I have previously referred to the FFS format in question as Mokopir-FFS or
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36 MPFFS, from "Moko" and "Pirelli". I was originally hesitant to call it TIFFS,
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37 as lacking the source code, I had no way of knowing whether the FFS format and
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38 implementation were of TI's own invention, or something that TI licensed as a
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39 black box from one of their many proprietary software partners. (I was unable
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40 to identify it as any well-known, industry-standard FFS format, but absence of
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41 evidence is not evidence of absence.) But now that we have TI's original source
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42 code which implements this FFS (first the MV100-0.1.rar source, then the full
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43 Leonardo one), complete with comments and a HISTORY file, we know that our FFS
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44 was invented and implemented by someone named Mads Meisner-Jensen at TI - I'm
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45 guessing in the SSA group in Nice, France.
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46
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47 I am now making a naming transition from MPFFS to TIFFS: there is really no
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48 link between this FFS format and the Openmoko+Pirelli duo, other than the
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49 happenstance of me having first encountered this FFS on these two GSM device
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50 brands, and the name TIFFS is more neutrally-descriptive.
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51
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52 What it is
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53 ==========
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54
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55 In a rare departure from TI's norm (most of TI's GSM firmware and associated
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56 development tools suffer from heavy Windows poisoning), what I call TIFFS is
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57 very Unixy. It is a file system with a hierarchical directory tree structure
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58 and with Unixy forward-slash-separated, case-sensitive pathnames; the semantics
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59 of "what is a file" and "what is a directory" are exactly the same as in UNIX;
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60 and TIFFS even supports symlinks, although that support is a little under-
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61 developed, and apparently no FFS symlinks were ever used in any production GSM
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62 device. Thus the FFS implemented in TI-based GSM devices (modems and
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63 "dumbphones") is really no different from, for example, JFFS2 in embedded Linux
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64 systems.
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65
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66 (The only traditional UNIX file system features which are missing in TIFFS are
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67 the creation/modification/access timestamps and the ownership/permission
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68 fields.)
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69
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70 The FFS in a GSM device typically stores two kinds of content:
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71
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72 * Factory data: IMEI, RF calibration values, device make/model/revision
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73 ID strings etc. These files are expected to be programmed on the factory
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74 production line and not changed afterward.
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75
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76 * Dynamic data written into the FFS in normal device operation: when you use a
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77 "dumbphone" running TI-based firmware, every time you store something "on the
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78 phone" or in "non-volatile memory", that item is actually stored in the FFS.
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79 (Where else, if you think of it?) That includes contacts and received SMS
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80 stored "on the phone" instead of the SIM, any selections you make in the
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81 settings/preferences menus which persist across reboots (power cycles), call
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82 history etc.
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83
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84 It needs to be noted that the "dynamic data" aspect of FFS usage applies not
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85 only to complete phones, but also to modems like the one used in the GTA01/02.
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86 One would naively think that non-volatile storage of data in flash outside of
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87 factory programming would be needed only in a device with its own UI, and that
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88 a modem subservient to external AT commands would be completely stateless
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89 across reboot/power cycles; but that is not the case in actuality. TI's GSM
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90 firmwares, including the Openmoko ones (the "standard" mokoN), are designed to
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91 always "mount" their FFS with read/write access; TI's FFS implementation in the
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92 firmware has no concept of a "read-only mount".
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93
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94 I am still investigating just what kinds of data are routinely written into the
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95 non-volatile FFS by the firmware in normal operation on devices like the GTA0x
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96 modem, but there most definitely are some.
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97
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98 There is no hard separation between "static" and "dynamic" data in the file
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99 system structure; TIFFS is thus akin to an embedded Linux system with just a
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100 single root file system containing both "static" files like userland binaries
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101 and "dynamic" ones like configuration files under /etc which the user is
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102 expected to edit with vi after logging into the box, or log and similar files
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103 created by the system itself under /var, for example.
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104
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105 Where it lives
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106 ==============
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107
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108 The type of flash memory used in Calypso GSM modems and "dumbphones" is called
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109 NOR flash. This NOR flash memory is physically divided (by the design of the
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110 flash chip itself) into units called "sectors" or more descriptively, erase
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111 blocks. The typical NOR flash sector size (in Calypso GSM devices) ranges from
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112 64 KiB in the GTA02 modem's NOR flash (4 MiB total) to 256 KiB in the
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113 S71PL129NC0 flash+RAM chip used in the Pirelli DP-L10 (16 MiB of flash total).
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114 The key physical property is that any bit may be changed from a '1' to a '0' at
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115 any time, in any combination, but resetting of '0' bits back to ones can be
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116 done only on the granularity of these largish sectors, in an operation called
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117 "sector erase".
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118
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119 The location of TIFFS within the flash memory of a given GSM device is defined
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120 by the firmware design of that device, but is always some integral number of
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121 contiguous flash sectors. Some examples:
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122
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123 * On the GTA01/02 GSM modem, FFS occupies 7 sectors of 64 KiB each, starting at
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124 flash offset 0x380000.
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125
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126 * On the Pirelli DP-L10, the FFS used by the original proprietary fw occupies
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127 18 sectors of 256 KiB each (for 4.5 MiB in total), starting at the beginning
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128 of the 2nd flash chip select (0x02000000 in the ARM7 address space).
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129
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130 * The smallest real FFS configuration called for by the table in dev.c in TI's
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131 original Leonardo fw source is 3 sectors of 64 KiB each; the same table also
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132 sports a 4 KiB x 4 configuration for RAM-based testing (emulation of FFS in
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133 RAM without real flash).
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134
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135 * The largest FFS configuration that has been envisioned by the original
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136 designers seems to be somewhere around 128 sectors.
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137
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138 Each flash sector used for TIFFS begins with this 6-byte signature:
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139
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140 46 66 73 23 10 02
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141
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142 The first 4 bytes are 'Ffs#' in ASCII, and the following two bytes are the
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143 format version number of 0x0210 in little-endian byte order. The following two
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144 bytes give a count of how many times that sector has been erased and rewritten
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145 (FF FF in "fresh" or "virgin" FFS images), and the following byte indicates
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146 that block's role and status in the FFS life cycle.
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147
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148 How it works
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149 ============
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150
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151 Just like JFFS2 and other high-quality flash file systems, TIFFS is designed to
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152 recover gracefully from any possible power failure or crash: one can yank the
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153 battery from the GSM device (or induce a firmware crash) at the most mis-
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154 opportune moment in the middle of an FFS write operation, and the FFS is
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155 expected to recover on the next boot cycle. I won't be able to document here
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156 all gory details of exactly how this goal is achieved, partly because I haven't
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157 studied the code to the requisite level of depth myself yet, but all of the
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158 responsible code lives under gsm-fw/services/ffs in this freecalypso-sw source
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159 tree; feel free to study it.
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160
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161 In its "normal" or "clean" state (i.e., when not in the middle of a write
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162 operation or recovery from an ungracefully interrupted one), a TIFFS instance
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163 consists of the following 3 types of blocks:
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164
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165 * One block containing inode records, indicated by AB in its type/flags/status
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166 byte in the block header;
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167 * N-2 blocks (where N is the total number of flash sectors allocated for the
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168 FFS) containing (or waiting to be filled with) data chunks - indicated by BD
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169 in the type/flags/status byte;
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170 * One "free" block, indicated by BF - destined to become a new AB or a new BD
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171 at some point.
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172
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173 Each object written into the FFS (file, directory or symlink) consists of a
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174 16-byte inode record written into the AB block and a data chunk written into
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175 one of the BD blocks. The data chunk includes the name of the object, hence
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176 one is required even for directories. Data chunks are contiguous, uncompressed,
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177 and subject to an upper size limit of 2048 or 8192 bytes, depending on the FFS
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178 configuration. Files larger than this limit are stored in a "segmented" form,
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179 giving rise to a 4th inode or object type (after file, directory and symlink):
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180 segment. Each segment of a segmented file consists of not only a data chunk,
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181 but also an inode record for the segment, which gives the location of the data
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182 chunk and ties the segment object into the overall FFS structure, making it
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183 accessible.
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184
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185 Because aside from complete sector erasure, flash memory bits can only
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186 transition from '1' to '0' but not the other way around, overwriting an existing
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187 file with some new content (an operation which any reasonable file system must
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188 implement in some way) cannot be done in place. Instead like most flash file
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189 systems, TIFFS implements this common operation by writing the new version of
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190 the file to a new location (previously blank flash) and then invalidating the
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191 old version - and doing all that while keeping in mind the possibility of an
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192 ungraceful crash or powerdown at any moment, and the requirement of recovering
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193 gracefully from any such event.
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194
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195 Of course as an FFS receives more write activity, even if one keeps overwriting
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196 some existing files with new content of the same size, without adding to the
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197 visible total content size (think du(1) command), eventually all remaining blank
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198 flash space will fill up. At that point (or at some earlier point, depending
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199 on the FFS design and/or configuration) the FFS has to invoke a compaction or
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200 reclamation or garbage collection procedure: any "mixed" blocks containing both
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201 valid and stale data are transitioned into a "stale-only" state by having the
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202 active data moved to a new block, and then the "all stale" blocks are subjected
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203 to sector erasure, becoming new blank sectors. The logic responsible for these
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204 operations once again needs to be resilient to the possibility of a crash or
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diff changeset
205 powerdown occurring at the most mis-opportune moment, and it also needs to
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206 implement flash wear leveling: there is a physical limit to how many times a
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207 given flash sector can be erased and rewritten before it goes bad.
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208
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209 All of the above are common and well-known principles, successfully implemented
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210 in well-known flash file systems such as JFFS2 in Linux. TIFFS is absolutely
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211 no different in this regard; for the implementation details, read the source
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212 code.
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213
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214 How this FFS comes into being
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215 =============================
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216
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217 (This section is only relevant to you if you plan on physically producing your
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218 own GSM phones or modems on your own factory production line, like this author
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219 fancies doing in the not-too-distant future, or if you simply enjoy knowing
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220 how it is done.)
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221
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222 To my knowledge, TI never used or produced a tool akin to mkfs.jffs2 in the
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223 embedded Linux world, which would produce a TIFFS image complete with some
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224 initial directory and file content "in vitro". Instead it appears that the FFS
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diff changeset
225 instances found in shipped products such as Openmoko phones have been created
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226 "in vivo" by TI's firmware running on the device itself during the "production
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227 test" phase.
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228
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diff changeset
229 The process seems to go like this:
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230
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231 * When the printed circuit board is physically populated with components such
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diff changeset
232 as the Calypso chip and the flash chip, the latter can be blank - if the
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233 board design has the nIBOOT pin pulled low, enabling the Calypso boot ROM
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234 (Openmoko and Pirelli both good on this one, but shame on Compal!), there is
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235 no need to preprogram the flash chip with anything prior to populating it on
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diff changeset
236 the board, and the device remains fully unbrickable at all times afterward.
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237
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238 * When the assembled board is powered up for the first time, with completely
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diff changeset
239 blank flash, the Calypso boot ROM will sit there and patiently wait for a
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240 code download on either of its two UARTs.
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241
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242 * Using TI's FLUID (Flash Loader Utility Independent of Device) or FreeCalypso's
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243 fc-loadtool free replacement, the factory production station loads the main
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244 firmware image into the flash. Note, it is just the firmware image at this
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245 step, and the FFS sectors remain blank.
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246
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247 * The board is commanded to reboot (or power-cycled), and the firmware image
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248 boots for the first time.
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249
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250 * TI's FFS implementation code in their standard firmware reacts to all blank
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251 flash in the FFS sectors as follows: it performs what they call the preformat
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252 operation, writing the TIFFS signature and a BF state byte into every FFS
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diff changeset
253 sector, but the main "format" operation, which sets up the AB/BD block roles,
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254 creates the root inode and makes the FFS ready to accept the creation of its
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diff changeset
255 first directories and files, is not done automatically.
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256
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257 In order to perform the FFS format operation and then fill the new FFS with
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diff changeset
258 whatever directories and files are deemed needed to be present in "fresh"
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259 shipping products, the factory production station connects to the just-booted
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260 firmware running on the target via the RVT/ETM protocol (see the RVTMUX
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261 write-up), and sends "test mode" commands to this running firmware. These
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262 "FFS test mode" (or TMFFS) commands include the format operation, an mkdir
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263 operation to create directories, and a "file write" operation akin to doing
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264 'cat > /dir/whatever/file', creating files in FFS and storing any desired data
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265 in them.
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266
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267 The IMEI is assigned and written into FFS in this process, but it is not the
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268 only data item that will be unique for each individual device made. Much more
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269 important are the RF calibration values: I have yet to learn exactly what is
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270 being (or needs to be) measured, how these measurements are performed (under
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271 what conditions; what external test equipment is needed), and how these measured
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272 and recorded RF calibration values affect GSM device operation, but this TI
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273 presentation gives some clues:
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274
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275 ftp://ftp.ifctf.org/pub/GSM/Calypso/rf_calibration.pdf
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276
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277 All of these calibration values are stored in a bunch of files under the
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278 /gsm/rf subtree, and these files seem to be "owned" by the L1 code. The latter
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279 has RAM data structures which correspond to these files; upon normal boot the
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280 initialization code looks in FFS, and if it finds any of the RF calibration
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281 files, it reads each present file into the corresponding RAM data structure,
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282 overwriting the compiled-in defaults. It appears (slightly uncertain because I
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283 have not yet reintegrated the code in question into our own gsm-fw) that the RF
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284 calibration files in FFS come into being as follows:
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285
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286 * The RF calibration code in L1 (i.e., part of the main GSM fw) performs the
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287 measurements and stores results in its RAM data structures as commanded by
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288 the production test station through the "test mode" interface;
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289
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290 * A final test mode command directs the above L1 code to write its RAM data
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291 structures into FFS.
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292
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293 Once I actually learn this RF calibration process properly in connection with
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294 building my own Calypso-based GSM "dumbphone", I'll be able to say exactly what
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295 it would take to recreate these RF calibration values if they are lost. But
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296 until then the only advice I can give is to make a backup copy of your modem
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297 FFS with fc-loadtool, and to save it securely.
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298
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299 FreeCalypso support for TIFFS
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300 =============================
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301
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302 Aside from implementing and using it in our own gsm-fw, FreeCalypso will offer
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303 the following support for TIFFS:
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304
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305 1. A tiffs host utility is being written which will allow a user to list and
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306 extract content from saved FFS images (read out of flash with fc-loadtool)
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307 "in vitro". It will be a restructured and (hopefully) improved version of
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308 the mpffs-* tools released back in the summer of SE52 (A.D. 2013); the latter
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309 already perform the advertised function, but I seek to integrate some other
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310 functionality which I developed in an ad hoc side project ("pirollback"),
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311 and I'm taking the opportunity to make the MPFFS->TIFFS renaming.
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312
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313 (The mpffs-* tools mentioned above have been written based on reverse eng only,
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314 before I found any source code for TI's FFS firmware implementation! Now that
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315 we have the source, some terminological and other inevitable misunderstandings
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316 can be corrected.)
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317
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318 2. A number of FC tools may be strung together into a kit for editing the FFS
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319 content of a GSM device, e.g., for changing the IMEI. The following pieces
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320 will be involved:
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321
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322 * What is destined to eventually become our totally free GSM fw (the gsm-fw
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323 source subtree at the top of freecalypso-sw) does not contain any of the
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324 actual GSM protocol stack (or even L1) functionality yet, but it already
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325 contains both the FFS code and those components (ETM and TMFFS[12]) which
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326 are needed for interfacing an external "test mode shell" to this FFS
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327 implementation through the RVTMUX interface. And when our gsm-fw does gain
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328 the actual GSM functionality, the ability to build a minimal FFS+ETM-only
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329 configuration will still be retained.
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330
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331 * The minimal FFS+ETM subset of gsm-fw can be built into a ramImage (runs
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332 entirely from RAM via fc-xram, no flashing), and run on a physical device
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333 such as the GTA0x GSM modem via the fc-xram host utility;
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334
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335 * After loading the ramImage, fc-xram will immediately exec our rvinterf host
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336 utility described in the RVTMUX write-up;
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337
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338 * Once the GSM device is running what is effectively an FFS editing agent out
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339 of RAM, accessed via rvinterf over the serial channel, the user will be able
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340 to run fc-tmsh (or perhaps the FFS operations will be implemented in some
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341 other utility, we'll see), and that "test mode shell" will provide commands
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342 for writing things to FFS exactly like one would do in the factory production
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343 line environment for which TI taylored their tools.
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344
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345 The "in vivo" method of editing the FFS content of a GSM device described above
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346 will probably sound very convoluted, and you may find yourself asking for a way
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347 to do it "in vitro" instead: read the FFS out of flash with fc-loadtool, edit
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348 that image "in vitro" with some utility on your PC, and then use fc-loadtool
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349 again to program it back into your device. But consider that an "in vitro" FFS
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350 modification would involve erasing and rewriting all sectors of your FFS,
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351 whereas an "in vivo" modification of some small file like the IMEI would be
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352 just a short flash write operation without any erasures at all, i.e., kinder
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353 on the flash.
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354
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355 In any case, the "in vivo" method will definitely be available soon because all
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356 of the components involved therein are also needed for other development uses
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357 in the FreeCalypso project, whereas developing a fully-functional "in vitro"
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358 alternative (one that can create an FFS image "de novo" from a tree of files
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359 and directories a la mkfs.jffs2, or add new files to an existing TIFFS image
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360 etc) would be a good amount of extra work which we otherwise don't need - hence
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361 the latter is not very likely to be written any time soon.
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362
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363 However, if the "in vitro" modification you seek is something trivial like
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364 changing the byte content of a file such as /pcm/IMEI or /gsm/com/rfcap without
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365 changing its length, you will be able to use the "in vitro, read-only" tiffs
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366 host utility to find the exact byte location of the file data within the TIFFS
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367 image, and use your favourite hex editor to whack whatever new byte content you
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368 like at that offset.