annotate doc/Roadmap @ 560:85562a6b6356

gsm-fw/L1/dsp/leadboot.c: import from the LoCosto source
author Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
date Mon, 04 Aug 2014 21:43:47 +0000
parents 6500e1817d9b
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1 The ultimate goal of the FreeCalypso project is to produce a GSM firmware
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2 version that:
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3
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4 a) runs on the Calypso targets of interest to us: not only the FIC/Openmoko
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5 GTA0x GSM modem, but also some complete dumbphones like Mot C139 and/or
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6 Pirelli DP-L10;
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7
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8 b) operates each target device it runs on in a way that is practically usable:
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9 to me (lead developer Space Falcon) practical usability means providing a
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10 standard AT command interface on modems like GTA0x and operating as a self-
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11 contained standard cellphone with a UI on complete "dumbphone" targets;
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12
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13 c) is rebuildable from source in a way that would allow us to exercise the
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14 traditional Four Freedoms of free software as defined by the FSF.
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15
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16 Our starting point is TI's Leonardo reference firmware version from Sotovik, a
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17 mixture of source and linkable binary object modules. It can be built into a
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18 complete and functional firmware image with TI's TMS470 compiler+linker+etc
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19 toolchain, and can run on two hardware targets:
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20
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21 a) TI's Leonardo development board for the Calypso/Iota/Rita chipset -
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22 legendary unobtainium hardware;
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23
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24 b) FIC/Openmoko GTA0x GSM modem - while this device is *not* what I wish to use
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25 with the "end user" hat on, it is an available and working hw platform for
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26 development.
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27
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28 This Leonardo starting point does kinda-sorta satisfy the "rebuildable from
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29 source" requirement, as I was able to port it from the Leonardo board to the
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30 GTA0x modem, but it is still quite far from what the FreeCalypso project
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31 ultimately seeks to achieve.
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32
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33 What I, the lead developer, ultimately seek is not an AT-controlled modem
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34 embedded in a "Linux" smartphone, but a complete and standalone "dumbphone",
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35 with the UI driven by the Calypso. Transforming our starting and working
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36 reference version from something that runs on the GTA0x modem and presents an
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37 AT command interface into something that can run on a complete "dumbphone" and
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38 present a self-contained UI can be done in several possible ways:
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39
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40 1. One could proceed further with the same approach that was used to produce
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41 leo2moko: keep making small incremental changes to the Leonardo semi-src,
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42 while keeping most of the binary object blobs intact at least initially, and
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43 sticking with the original proprietary TMS470 compiler toolchain (running
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44 under wine, as it's a bunch of M$ Windows binaries) throughout the journey.
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45
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46 Advantage: no massive "forklift" step required at the very beginning of the
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47 journey, one can do small incremental steps instead. One possible roadmap
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48 with this approach might be (just thinking out loud here) to start with
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49 finding a way to redirect the AT command channel to something wrapped inside
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50 RVTMUX, then port the fw to run on a target like Pirelli DP-L10, still
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51 controlled via AT commands but passed via RVTMUX, and then try to add the
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52 "dumbphone" UI layers. Some of the binary object blobs would certainly need
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53 to be "cracked" and modified in this process, most sensibly by replacing
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54 each blob in need of modification with a reconstructed source piece, but
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55 this work would be done incrementally, rather than in a massive "forklift"
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56 step upfront.
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57
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58 Disadvantage: this approach requires sticking with the original proprietary
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59 compiler toolchain throughout at least most of the journey, which means
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60 keeping the Weendoze poison that toolchain comes with. This aspect was
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61 enough to turn me personally away from this approach.
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62
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63 2. One could move away from the proprietary compiler toolchain and replace it
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64 with gcc, while still retaining those parts of the Leonardo firmware for
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65 which we only have linkable binary objects but no C source, by teaching the
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66 GNU toolchain (gcc+binutils) to emulate the TMS470 compiler's ABI: COFF
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67 objects, a very different way of doing ARM/Thumb interworking, and a host of
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68 other differences from how the GNU toolchain for ARM normally does things.
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69 (I've done similar work on GCC and GNU Binutils in the past; let us not
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70 forget that GNU started out by emulating various inter-component interfaces,
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71 including ABIs, of various proprietary Unix versions.)
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72
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73 The advantage of this approach should be obvious. While in the long run we
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74 would certainly like to get rid of all binary object blobs and replace them
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75 with reconstructed source pieces, life would be much easier if we could do
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76 this work incrementally, one blob at a time in a regression-tested working
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77 fw version, rather than all upfront.
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78
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79 Disadvantage: teaching gcc and binutils to emulate TI's TMS470 ABI well
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80 enough to where one could compile some sources with gcc, link the resulting
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81 .o's with TI's blobs and get a working fw image at the end would be no easy
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82 matter. In fact, it would be a massive amount of work, and considering that
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83 this work would serve absolutely no purpose but to allow linking with some
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84 proprietary binary blobs, finding the justification for the effort would be
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85 difficult.
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86
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87 3. The approach I am currently following is the most ambitious one: start by
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88 deblobbing the firmware, i.e., replacing all binary object blobs with
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89 reconstructed source pieces, and then simply compile the all-C source with
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90 the GNU toolchain for ARM7 in its default ABI.
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91
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92 Advantage: having the firmware in the form of full C source that builds with
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93 a free compiler (a compiler that is itself bona fide Free Software) is
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94 obviously the ultimate best. No blobs means no need to waste creative energy
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95 on emulating proprietary compiler ABIs.
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96
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97 Disadvantage: all the hardest work is upfront, and needs to be done in a
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98 "forklift" manner.
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99
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100 It should be obvious that incrementality, or division into manageable, bite-
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101 sized pieces, is an essential requirement for successful execution of any large
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102 scale project, and FreeCalypso is no different. Because our chosen approach
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103 unfortunately does not allow us to make small incremental modifications to a
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104 fully functional fw version, I currently achieve the needed incrementality in a
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105 different way: by starting out with a firmware "skeleton" that compiles into an
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106 image and runs on the hardware, but does nothing useful yet, and then slowly
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107 adding chunks of "meat" to this skeleton in incremental steps aiming in the
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108 direction of the complete & working fw version that is our reference. As I add
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109 each "chunk of meat" to our gcc-built gsm-fw, I test it on the hardware and
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110 compare its operation against the leo2moko reference version - that is our form
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111 of pseudo-regression testing.
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112
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113 Where the missing source bits come from
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114 =======================================
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115
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116 How can we replace a binary object blob with a corresponding source piece if we
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117 lack the original source? In two ways:
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118
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119 1. In many cases we can lift the corresponding source piece from another TI
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120 source leak, e.g., the LoCosto one. Calypso and LoCosto chipsets are
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121 different, so code specific to LoCosto hardware won't run on Calypso targets,
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122 but much of the code is chipset-independent. The LoCosto source leak is
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123 important because it contains real C source for many of the chipset-
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124 independent components for which our Leonardo reference version has only
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125 binary blobs.
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126
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127 2. When no original or suitable substitute source can be found anywhere, we
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128 have to bite the bullet, pass the blob through a suitable disassembler (I
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129 wrote one that is specifically taylored for reversing code built with the
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130 TMS470 compiler), and then write a piece of new C code that replicates the
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131 logic found in the disassembled blob.