[yocto] Location of U-Boot environment for BeagleBone Black

[Craig McQueen]

I'm building Yocto for BeagleBone using the meta-ti layer and the kernel and U-Boot provided by that layer.

I'm having some trouble understanding the location where U-Boot saves its environment for BeagleBone Black.

After doing some reading, it looks as though it might be saving environment on a "special" eMMC boot partition (which is not the same as the FAT16 partition 1 I created to store MLO and u-boot.img).

However, this is only when booting from the on-board eMMC I think. When booting from SD card, where is environment stored? I can do "saveenv" command in U-Boot, and it says it's saving it to MMC.

U-Boot# saveenv
Saving Environment to MMC...
Writing to MMC(1)... done

But then after a reboot, it says:

MMC: block number 0x100 exceeds max(0x0)
MMC: block number 0x200 exceeds max(0x0)
*** Error - No Valid Environment Area found
Using default environment

So does that mean that U-Boot can't use a saved environment when booting from SD card?

Actually, based on some simple testing, it seems that it _is_ writing to the boot partition of the on-board eMMC, and loading it when it reboots from SD card, despite the error message. This is confusing.

What about the case of a board with no eMMC, such as BeagleBone White? Where would it save environment then?

How can I erase the saved environment when booting from eMMC, to ensure that U-Boot defaults will be used? I see that when Linux boots, there are block devices /dev/mmcblk0boot0 and /dev/mmcblk0boot1. But it seems I can't write to them:

# dd if=/dev/zero of=/dev/mmcblk0boot1 bs=1k count=1k
dd: writing '/dev/mmcblk0boot1': Operation not permitted
1+0 records in
0+0 records out

-- 
Craig McQueen