--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/Flash-boot-defect	Tue May 02 03:24:30 2017 +0000
@@ -0,0 +1,49 @@
+As of this writing (2017-05-01), there is an unexplained hardware problem on
+some of our FCDEV3B boards in that flashed firmware images which use flash boot
+mode 1 (see the Flash-boot-modes article) fail to boot.  It is not currently
+known how many boards are affected by this problem; it is possible that the
+Mother's S/N 001 board is the only one that exhibits this oddity.
+
+This problem particularly affects our FC Magnetite firmware, as the latter uses
+flash boot mode 1 just like TI's TCS211 fw from which it originates.  OTOH, our
+FC Citrine firmware, which uses flash boot mode 0, boots just fine.  A minimal
+test case has been created under target-utils/flash-boot-test in this
+repository: it is a simple loadagent-like standalone application that is built
+to be booted from flash instead of expecting to be loaded serially, and it is
+built in two versions, one for mode 0 and another for mode 1.  Both versions
+work on an Openmoko-made GTA02 (the mode 1 version continuously reboots every
+few seconds because it doesn't disable the watchdog timer, but it is still very
+clearly alive) as well as on those FCDEV3B boards which aren't affected, but on
+FCDEV3B S/N 001 the mode 1 version fails to boot just like the full Magnetite
+firmware.
+
+The Mother of FreeCalypso does not currently have any prognosis as to when or
+even if the mysterious hardware problem that causes flash boot mode 1 to fail
+can be fixed.  Logical reasoning tells us that it must be a hardware problem,
+as the flash boot mode in question works without a hitch on every pre-existing
+Calypso device known to us, and similar logical reasoning tells us that the
+watchdog timer probably has to be involved in some way, as it is the mechanism
+underlying flash boot mode 1 (again, see the Flash-boot-modes article), but I
+am at a total loss when it comes to what kind of board-level problem could
+possibly produce such behaviour.  But then if only the S/N 001 board is affected
+and no others, it could just be a defective chip.  More data points need to be
+gathered before we shall know whether or not we have a real problem.
+
+For those who do have a flash-boot-challenged FCDEV3B board, two workarounds
+have been developed, in this chronological order:
+
+1. One can boot the board in the serial download mode, and download a
+   teensy-tiny piece of code that disables the boot ROM and jumps to address 0.
+   If the flash contains a firmware image meant to be booted in mode 1, this
+   image will get indirectly booted in this manner.  Run a command like this:
+
+fc-iram -h fcfam /dev/ttyXXX /opt/freecalypso/target-bin/flash-boot-wa.srec rvinterf
+
+(fc-iram has been extended to support second program invokation just like
+ fc-xram, just for this peculiar use case.  The flash-boot-wa.srec helper can
+ also be booted via fc-xram.)
+
+2. After doing the above, I found a way to patch the Magnetite firmware image
+   to boot in mode 0 - see the Flash-boot-mode-hack write-up in the Magnetite
+   source tree.  With this patch applied, FC Magnetite happily boots directly
+   from flash on my board without needing fc-iram or fc-xram assistance.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/doc/Flash-boot-modes	Tue May 02 03:24:30 2017 +0000
@@ -0,0 +1,52 @@
+The Calypso chip includes an on-die boot ROM that allows the boot process to be
+interrupted and diverted by an external host sending some special characters
+into either of the two UARTs; this mechanism is what allows us to load code into
+RAM and to reload the flash on Calypso GSM devices without having to resort to
+JTAG or chip desoldering or other extreme measures.  In normal operation, when
+the boot path is NOT being diverted by an external serial download, the boot ROM
+transfers control to the regular firmware in the flash - but there are two
+different modes in which the flash fw image may be booted.
+
+In order for the flash fw image to be considered bootable by the Calypso boot
+ROM, the 32-bit word at flash address 0x2000 must equal either 0 or 1; if it
+equals any other value, the boot ROM will consider the flash fw image to be
+invalid (e.g., blank flash) and will wait forever for a serial download instead
+of proceeding with flash boot.  Depending on whether this word at 0x2000 equals
+0 or 1, the flash fw image will be booted in one of two very different ways;
+we shall call them flash boot mode 0 and flash boot mode 1, respectively.
+
+In flash boot mode 0 the following 32-bit word at flash address 0x2004 must
+contain the address of the flash fw image entry point (ARM/Thumb selection in
+the least-significant bit); the boot ROM will simply jump to this address with
+a BX instruction.  When the flash fw image is booted in this manner, the boot
+ROM is still mapped at address 0 and the first 8 KiB of flash are inaccessible
+except via the 0x03000000 alternate mapping, unless the firmware later changes
+the 0xFFFFFB10 register.  This boot mode is intended for flash fw images that
+use the interrupt and exception vectors in the ROM (branching to IRAM addresses
+0x80001C-0x800034) for their interrupt and exception handling.
+
+Flash boot mode 1 is different: instead of jumping directly to the flash fw
+image, the boot ROM copies a small piece of its code into IRAM and jumps to that
+code; the copied code disables the boot ROM via the 0xFFFFFB10 register (puts
+the external flash at address 0) and induces a processor reset through the
+watchdog timer.  It is not clear to us exactly what blocks are affected by the
+watchdog reset, but bits 9:8 of the 0xFFFFFB10 register are not reset, hence
+the ARM processor now boots from the reset vector in the flash as if the boot
+ROM weren't there - and the latter really is not there after having disabled
+itself.
+
+Flash boot mode 0 is only usable on Calypso C035 silicon (the "new" kind);
+while all commercial Calypso GSM devices targeted by FreeCalypso feature Calypso
+chips of the correct "new" kind, the people at TI who wrote and maintained their
+official firmware also had to work with older Calypso C05 chips featured on the
+early D-Sample and Leonardo boards.  The earlier boot ROM code version in those
+early Calypso chips also implements the two boot modes which we call mode 0 and
+mode 1, but its implementation of mode 0 is broken and unusable, therefore TI's
+firmware people only used flash boot mode 1.  On the other hand, newer firmware
+designs made for current rather than historical hardware will probably find
+mode 0 to be cleaner, more intuitive and more convenient.
+
+All TI official firmwares use flash boot mode 1, our FreeCalypso Magnetite
+firmware does likewise, being a direct derivative of TI's TCS211 fw, but our
+FC Citrine firmware uses flash boot mode 0, as that part of the Citrine fw is
+our own original design.