Booting FreeBSD on BeagleBone Black from microSD by default

[Ian Lepore]

On Wed, 2016-06-29 at 16:18 -0400, Ed Maste wrote:
> On 29 June 2016 at 00:14, Tim Kientzle <tim at kientzle.com> wrote:
> > 
> > If I remember correctly, the uboot provided on the stock eMMC image
> > was configured to see if the SD card had a Linux kernel and load
> > the kernel from that if it was there.  If the SD did not have a
> > Linux kernel, it would fall back to eMMC.
> 
> Yeah. After looking a little more I think the eMMC uboot will read
> uEnv.txt from the SD card and so it should somehow be possible to
> chain to a next stage that way, but it does not support UFS and does
> not have a 'bootelf' command.
> 
> It is possible to have the eMMC uboot load something from a FAT file
> system on the SD card and execute it (with something like fatload mmc
> 1 <addr> <file>; go <addr>) but I don't yet know enough about the
> boot
> path here to build a suitable 3rd stage uboot.
> 
> 1. eMMC SPL
> 2. eMMC u-boot, loads uEnv.txt from SD
> 3. SD u-boot
> 4. SD ubldr
> 5. kernel
> 
> > Of course, if you’re willing to hold the boot switch, you can boot
> > from SD without changing the eMMC.
> 
> Right. My goal here would be to be able to produce a SD card image
> that works out of the box, without using the boot switch or modifying
> the eMMC in any way.
> 
> My interest in being able to boot that way comes from a BSDCan
> talk[1]
> by Edicarla Andrade and Vincius Zavam. They have FreeBSD on a BBB
> controlling a hexapod robot, but they weren't allowed to modify the
> BBB (provided by their lab) in any way. So they have to press the
> boot
> switch every time they power it up. It's a shame that they're doing
> something interesting with FreeBSD, yet have this minor but annoying
> boot issue.
> 
> [1] https://www.bsdcan.org/2016/schedule/events/658.en.html
> 

Building a removable jumper wire for connecting pin 43 on connector P8
to ground (pin 1) with a 1k resistor will have the effect of
continuously holding down the sd-boot button without modifying anything
permanantly on the board.  People who build robots should be able to
handle that. :)

-- Ian