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annotate gsm-fw/lldbg/README @ 992:a7b0b426f9ca

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target-utils: boot ROM UART autodetection revamped The new implementation should work with both the familiar Calypso C035 boot ROM version found in our regular targets as well as the older Calypso F741979B version found on the vintage D-Sample board.
author Mychaela Falconia <falcon@ivan.Harhan.ORG>
date Wed, 30 Dec 2015 21:28:41 +0000
parents f5affe83ba2d
children
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865
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1 The hack implemented in this directory is a poor girl's substitute for JTAG,
f5affe83ba2d lldbg hack (poor girl's substitute for JTAG) implemented
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2 concocted for the purpose of debugging hard crashes in the firmware. The
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3 intended usage is that the developer seeking to troubleshoot perplexing
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4 misbehavior in the firmware inserts a call to lldbg_entry() as a form of
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5 breakpoint at the earliest place in the main fw where it is seen that something
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6 has gone astray, and when the thread of code execution hits this lldbg_entry()
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7 function, the regular Nucleus environment gets completely frozen. Lldbg entry
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8 code saves all registers and the return address (LR) on whatever stack it is
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9 was called on, then disables all interrupts and switches to its own stack which
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10 is not used by any other part of the fw.
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11
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12 This lldbg code is linked together with the main fw when feature lldbg is
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13 enabled in build.conf, but it stands as its own separate body. All functions
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14 and variables within lldbg are prefixed with lldbg_ to avoid any clashes, and
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15 once this lldbg code gains control, it does everything on its own without making
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16 use of *any* part of the regular fw. The code that forms lldbg is based on
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17 FreeCalypso target-utils (loadagent) and is fully independent of the regular fw.
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18
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19 Lldbg uses its own UART input and output code as well: the entry code waits for
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20 any previous output (presumably RVTMUX) to go out, then sends out an STX-wrapped
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21 message to make the lldbg entry visible in rvtdump/rvinterf, preceded by 3 STX
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22 characters to terminate any RVTMUX packet in progress. From this point onward
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23 all further communication is done in an ASCII terminal fashion: upon seeing the
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24 lldbg entry message, the user needs to kill rvtdump/rvinterf and switch to
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25 fc-serterm. All lldbg code runs with interrupts disabled, thus UART input and
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26 output are polled.
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27
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28 Once in lldbg mode, the user (developer) can execute various memory dump
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29 commands to see the frozen state of the fw upon lldbg entry, and hopefully
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30 figure out what went wrong earlier.