apparently, everything i used to think about the BB boot sequence is
not entirely true, so i'll publicly humiliate myself and ask someone
to clarify where i've gone wrong.
as i understand it, here are the first steps in the BB boot
sequence, starting from power up:
* onboard boot ROM (unchangeable)
* TI x-loader
* u-boot
first, am i correct that the very first step is the onboard boot
ROM, which is *not* the same as the x-loader? as i read it, that boot
ROM is fixed, you can't mess with that, while the x-loader is the
first step that you can, in fact, reprogram in flash if you want to
update it (same with u-boot).
so when i power up a new C3 BB with no SD card inserted, i'm not
surprised to see this:
...
Texas Instruments X-Loader 1.4.2 (Feb 19 2009 - 12:01:24)
Loading u-boot.bin from nand
U-Boot 2009.01-dirty (Feb 19 2009 - 12:22:31)
...
the above clearly represents the x-loader and u-boot that are shipped
with new BBs.
now here's where i got confused this morning. if i populate an SD
card with a much newer MLO (x-loader), u-boot.bin and uImage, insert
it, power cycle but do *not* press the USER button while this is
happening, i thought that the boot sequence would remain unchanged
(still from NAND). that's clearly not what's happening since i just
did that and here's what i got via minicom:
...
Texas Instruments X-Loader 1.4.2 (Feb 19 2009 - 12:01:24)
Reading boot sector
Loading u-boot.bin from mmc
U-Boot 2009.06-rc2 (Sep 09 2009 - 13:12:40)
...
what just happened there? at what point did the boot sequence
switch over to the SD card? as i read it, the same (NAND-based)
x-loader is running, but it clearly went to the SD card for the newer
u-boot image. why? i thought that required manual intervention on my
part, unless i've just been mistaken about that all along (a distinct
possibility).
and i just power-cycled using the USER button to get:
...
Texas Instruments X-Loader 1.4.2 (Sep 9 2009 - 09:20:15)
Reading boot sector
...
so that obviously forced the boot ROM to read the x-loader off of the
SD card.
so ... i'm still confused. if i don't intervene and power up the
BB, it seems like the onblard x-loader will be invoked, but that
x-loader will, if one exists on the SD card, invoke u-boot off of the
card, even if one exists in flash. is that correct?
rday