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annotate target-utils/compalstage/README @ 700:db9a8e88e63f

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target-utils lunadrv program written, compiles
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 07 May 2020 02:19:51 +0000
parents 56b1bab3e09b
children
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Mychaela Falconia <falcon@freecalypso.org>
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1 FreeCalypso loadtools have been designed from the beginning to work through
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2 the Calypso chip's own boot ROM. This approach works great for Openmoko and
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3 Pirelli targets, but Compal phones unfortunately have this Calypso boot ROM
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4 disabled at the board level. To run our own code in these phones instead of
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5 booting the regular firmware, we need to go through Compal's own boot code.
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6 The latter allows loading code into IRAM and jumping to it, but not in the
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7 same way as how we do it through the Calypso boot ROM.
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9 One could argue that the "proper" way to support these Compal phones would be
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10 to build a different version of our loadagent that is designed to be loaded
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11 through Compal's boot code instead of the Calypso boot ROM, and then redesign
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12 our fc-loadtool and fc-xram utilities to work with different loadagents loaded
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13 in different ways on different target devices. But I don't feel like doing
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14 that - too invasive to the once-clean design of loadtools.
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16 Hence I am adopting a different solution that works in the same way as
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17 OsmocomBB's "chain loading": the IRAM image that is fed to Compal's boot code
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18 is not our real loadagent, but a tiny piece of code that enables the Calypso
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19 boot ROM and jumps to it. All loadtools host programs will include this
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20 optional "Compal stage" at the beginning, enabled for targets that need it,
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21 but will then always fall into the Calypso boot ROM IRAM download path.
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23 The approach I'm adopting is doubly inefficient for Mot C139/140 phones whose
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24 bootloader effectively requires that the downloaded image be ~15 KiB long
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25 even when we only need to download 32 bytes. But hey, our ultimate goal is to
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26 produce our own Calypso phones, rather than hack those made by diabolical
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27 manufacturers who do not Respect Your Freedom; running our own firmware on the
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28 Mot C139 is only a transitional step, so making fc-loadtool/fc-xram entry
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29 slower by a second or two is probably an acceptable price for keeping the
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30 code clean for the boot-ROM-enabled free world.
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32 In this directory we build the several different versions of compalstage
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33 needed for different C1xx models.