Booting FreeBSD on BeagleBone Black from microSD by default

[Ian Lepore]

On Mon, 2016-04-11 at 06:33 -0700, Tim Kientzle wrote:
> > On Apr 10, 2016, at 9:06 PM, Hal Murray <hmurray at megapathdsl.net>
> > wrote:
> > 
> > 
> > Does anybody have the recipe for setting things up so that power on
> > boots 
> > from the microSD card without holding down the button?
> 
> If the ROM cannot boot from eMMC, it will try to boot from microSD.
> 
> The easiest way to make the eMMC not bootable is to zero out the
> boot sectors using dd:
> 
>      dd if=/dev/zero of=<eMMC device> count=100
> 
> Of course, this makes the eMMC entirely unusable.  I think you
> can then reformat the eMMC (make sure it does not have a FAT
> partition and does not have boot blocks) so it can be used without
> the ROM trying to boot from it.
> 
> This way, the ROM will load U-Boot from microSD.
> 
> > 
> > I'm pretty sure it will be simple after I know how to do it. 
> >  Google found info on the idea, but I couldn't find the file that
> > needs editing.  It's off in uBoot land.
> 
> Even if the ROM loads U-Boot from eMMC, the default Linux
> U-Boot setup on the eMMC can be configured to then load
> the Linux kernel from microSD.
> 
> If you install the FreeBSD version of U-Boot to eMMC, you might be
> able to
> do the same thing.  Personally, I find the recipe above much
> simpler.
> 
> Cheers,
> 
> Tim

No need to zero out the onboard emmc or mess with its too-complicated
linux-centric env vars...  The AM335x ROM boot code loads u-boot from
the first FAT partition it finds with the active/bootable flag set. 
 Normally that's the emmc, but you can turn it off and then it'll look
for a partition on the external sdcard.  Once you've booted to freebsd
normally, do

  gpart unset -a active -i 1 mmcsd1

And now it will boot from the sdcard unless you turn the active flag
back on on the emmc.

- Ian