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annotate doc/IMEI @ 619:f82551c77e58

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libserial-newlnx: ASYNC_LOW_LATENCY patch reverted Reports from Das Signal indicate that loadtools performance on Debian is about the same as on Slackware, and that including or omitting the ASYNC_LOW_LATENCY patch from Serg makes no difference. Because the patch in question does not appear to be necessary, it is being reverted until and unless someone other than Serg reports an actual real-world system on which loadtools operation times are slowed compared to the Mother's Slackware reference and on which Slackware-like performance can be restored by setting the ASYNC_LOW_LATENCY flag.
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 27 Feb 2020 01:09:48 +0000
parents 4644799cb515
children 232e36a227dd
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17
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1 IMEI vs. IMEISV
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2 ===============
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3
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4 There is a subtle distinction between an IMEI and an IMEISV. The first 14
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5 digits are the same between the two: the supposedly-world-unique number of a
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6 given piece of hardware. In a traditional IMEI 15-digit number the significant
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7 14 digits are followed by a Luhn check digit, whereas an IMEISV has 16 digits:
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8 the 14 significant digits of the IMEI, *no* Luhn check digit, and two digits of
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9 "software version".
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10
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11 It is up to device manufacturers and firmware designers to decide whether or
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12 not to store the Luhn check digit in the GSM device's flash or EEPROM or
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13 whatever, but it is not sent over the air: instead the IMEISV is sent. It
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14 appears that the GSM standard authors' intent was that the IMEI part is stored
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15 immutably in each manufactured device whereas the SV digits are added by the
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16 running firmware to indicate its version, but the IMEI handling scheme
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17 implemented in TI's reference firmware and retained by many of the TI-based GSM
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18 device manufacturers (at least FIC/Openmoko and Foxconn/Pirelli) dispenses away
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19 with the IMEI vs. IMEISV distinction.
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20
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21 IMEI storage and retrieval in TI's reference firmware
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22 =====================================================
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23
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24 When running on the plain Calypso as opposed to Calypso+, TI's TCS211 reference
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25 firmware supports two ways of storing and retrieving the IMEI: obfuscated and
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26 unobfuscated. In both schemes the IMEI datum is stored as a file in the
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27 device's flash file system (FFS), and even though the FFS filename calls it the
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28 IMEI, the content of this file is really treated as the IMEISV: 16 digits are
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29 stored, the firmware function responsible for reading the IMEI datum out of FFS
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30 and passing it on to the rest of the fw is called cl_get_imeisv(), the code in
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31 this function does not transform the 16 digits in any way, and the downstream
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32 recipients of these digits treat them as the IMEISV.
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33
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34 The two specific schemes offered by TCS211 fw are as follows:
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35
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36 In the unobfuscated scheme (FF_PROTECTED_IMEI not defined), the so-called IMEI
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37 but really IMEISV is stored in an FFS file named /pcm/IMEI. The file is 8 bytes
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38 long, each byte stores two IMEISV digits, and the order of the digits within
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39 each byte is reversed relative to the natural order: first the least significant
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40 nibble is used, then the most significant nibble.
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41
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42 In the obfuscated scheme (FF_PROTECTED_IMEI is defined), the so-called IMEI but
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43 really IMEISV is stored in an FFS file named /gsm/imei.enc. The file is 16
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44 bytes long: the first 8 bytes store the 16-digit IMEISV encrypted with DES,
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45 using the Calypso die ID as the key, and the last 8 bytes store that Calypso die
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46 ID DES-encrypted with itself. Underneath the obfuscation, the 16 IMEISV digits
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47 are stored in the 8 bytes in the natural order: first the most significant
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48 nibble is used, then the least significant nibble.
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49
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50 IMEI storage and retrieval schemes implemented by device manufacturers
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51 ======================================================================
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52
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53 Openmoko devices use the unobfuscated IMEI storage method unchanged from TI's
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54 reference fw: the factory-assigned IMEI is stored in an FFS file named
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55 /pcm/IMEI, and that is where the original mokoN firmwares look for it. Further
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56 blurring the distinction between the IMEI and the IMEISV, the 16 digits stored
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57 in /pcm/IMEI (which the fw treats as the IMEISV) were factory-programmed as the
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58 15-digit IMEI (with the Luhn check digit) with an appended 0, i.e., the SV
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59 digits get set to x0 where x is the Luhn check digit.
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60
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61 Foxconn, the makers of the Pirelli DP-L10, have used the obfuscated version of
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62 TI's IMEI handling mechanism instead, with an additional twist: instead of
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63 storing the 16-byte encrypted datum in /gsm/imei.enc in FFS, they have moved it
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64 into their own factory data record stored in a non-FFS sector of the flash.
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65 The content of the 16 digits treated as the IMEISV by the G23M component of the
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66 fw is the same as Openmoko's: 15-digit IMEI with the Luhn check digit followed
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67 by a 0 digit.
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68
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69 Compal, the makers of Motorola C1xx phones, have similarly moved their IMEI out
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70 of FFS into their own proprietary flash data structures, and we have never
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71 decoded the latter, hence we don't know exactly where and how their IMEI is
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72 stored. If you wish to run FreeCalypso firmware on these phones, you have to
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73 set your own IMEISV for our fw even if you are not seeking to make it different
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74 from the factory-assigned one, as we don't know how to retrieve the latter.
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75
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76 Changing the IMEI
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77 =================
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78
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79 When someone says that they wish to change the IMEI on their phone, they need
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80 to be a little clearer as to what they really mean, as there are two possible
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81 interpretations of the just-stated wish:
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82
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83 1. Transmitting a different IMEISV toward the network by running your own
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84 firmware on the device,
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85
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86 or
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87
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88 2. Changing the IMEI seen by the device's original proprietary firmware.
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89
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90 Interpretation 1 is much easier than interpretation 2: when you are writing your
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91 own firmware for an "alien" GSM device (hardware designed and made by someone
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92 other than you), it is much easier to just set your own IMEISV and be done with
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93 it than to figure out how to retrieve the factory-assigned one. Thus those
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94 device manufacturers who try to make it more difficult to change their IMEIs
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95 are actually creating the opposite effect: people will just set their own IMEISV
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96 when running their own fw on their hw.
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97
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98 Openmoko devices are a rare exception in that if you write your own IMEISV into
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99 /pcm/IMEI in FFS, your new IMEISV will take effect not only with FreeCalypso
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100 firmware, but also with the legacy mokoN fw versions, because they all look in
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101 /pcm/IMEI. The same does NOT hold with Compal/Motorola or Foxconn/Pirelli
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102 phones, however: if you wish to change their IMEI to be seen by their original
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103 proprietary firmwares, you are on your own, as we do not currently have any
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104 tools for accomplishing such a feat.
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105
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106 IMEI handling in FreeCalypso
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107 ============================
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108
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109 The FreeCalypso family of projects has adopted the following IMEI storage and
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110 retrieval scheme both for our own FreeCalypso-made hardware and for FreeCalypso
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111 firmwares running on alien hardware: all of our firmware versions regardless of
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112 target will look first in /etc/IMEISV, then in /pcm/IMEI when needing to obtain
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113 the IMEISV for GSM operation. This is the new unified convention; previously
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114 we used varying IMEISV retrieval schemes depending on the target and in
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115 different FC firmware projects. The new unified convention is backward-
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116 compatible with our previous schemes on every target.
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117
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118 The /etc/IMEISV file is a FreeCalypso invention. The file is 8 bytes long, and
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119 stores the 16 digits of the IMEISV in the natural order: first the most
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120 significant nibble is used, then the least significant nibble. This nibble
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121 order makes the IMEISV number directly readable in a hex dump of the file, and
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122 the filename /etc/IMEISV makes it clear that the last two digits are the SV and
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123 are not required to be equal to the Luhn check digit and 0.
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124
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125 Both /etc/IMEISV and /pcm/IMEI can be written with the fc-fsio utility's
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126 set-imeisv command:
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127
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128 set-imeisv fc XXXXXXXX-YYYYYY-ZZ # write /etc/IMEISV
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129 set-imeisv pcm XXXXXXXX-YYYYYY-ZZ # write /pcm/IMEI
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130
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131 When working on Openmoko devices, we recommend writing your IMEISV into
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132 /pcm/IMEI (set-imeisv pcm command) and not creating an /etc/IMEISV file: newer
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133 FC firmware versions will look in both locations, but older FC fw versions and
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134 the legacy mokoN ones look only in /pcm/IMEI. On all other targets we recommend
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135 using the new /etc/IMEISV storage format, i.e., you should use the set-imeisv fc
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136 variant.