view doc/Flash-boot-wa @ 624:7485e75d9477

srec-regions: removed unnecessary prohibition against address going backward
author Mychaela Falconia <falcon@freecalypso.org>
date Thu, 27 Feb 2020 08:12:11 +0000
parents efb93f3e4ac7
children
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There is a tiny (120 bytes SREC file) program called flash-boot-wa that was
written in the spring of 2017 for the purpose of working around a problem that
happened on only one first-batch FCDEV3B board and never happened on any other
board - but Murphy's law had it that this one troubled board just had to be the
one on which my (Mother Mychaela's) very initial development and bring-up work
was done.

The defect exhibited on that one board was as follows: it had no problem booting
serially (fc-iram, fc-loadtool, fc-xram) and it had no problem booting from
flash in mode 0 (see the Flash-boot-modes article in the freecalypso-docs
repository), but booting from flash in mode 1 (the flash boot mode used by
FC Magnetite, which is our primary firmware) was troubled.  The exact failure
mode and the root cause were never solved, but the issue most likely involved
the watchdog reset in some way (it occurs as part of flash boot mode 1 but not
in mode 0 and not in serial downloading), and because Calypso's FDP output goes
low during watchdog reset (or at least TI's CAL000 document says so), it is
plausible that the root cause involved the Spansion flash chip getting unhappy
as a result of being jerked with extra resets which don't meet its reset timing
requirements.

Our new FCDEV3B V2 boards no longer use Calypso's FDP output (it is left
unconnected) and feature a new flash reset circuit of our own design that meets
the reset timing requirements of our Spansion flash chip, hence the flash boot
problem seen on that one FCDEV3B S/N 001 board is not expected to recur on any
of our current or future boards.  However, our little flash-boot-wa program is
kept around: removing a previously-released 120-byte program for no good reason
is not the way of FOSS.

This flash-boot-wa program is loaded serially via fc-iram; it disables the boot
ROM and jumps to address 0 (the opposite of what we do in compalstage for Mot
C1xx phones), thereby indirectly booting the made-for-boot-mode-1 firmware image
in the flash.  The intended usage was as follows:

fc-iram -h fcfam /dev/ttyXXX /opt/freecalypso/target-bin/flash-boot-wa.srec rvinterf

It is also worth noting that fc-iram has been extended to support second program
invokation just like fc-xram (used in the invokation line above) just for this
peculiar use case.  The flash-boot-wa.srec helper can also be booted via
fc-xram.