upgrading u-boot--what version? where to get it?

[Pavlos1]

If you want to have multiple OSes installed on one boot device, I would install Tow-Boot to the SPI because it has a built-in boot selector.

Hah... yeah, it's Tow-Boot that started me down this path. The Tow-Boot install instructions (the ones I read) say it should be installed by creating a bootable-SDCard installer, and then booting from it. Guess what wouldn't boot with my 4-year-old u-boot?

If you're willing to take your PBP apart, you will find an eMMC-disable switch. If you flick that then it should boot normally from a Tow-Boot SD card. (i.e. It will use the bootloader on the SD card instead of trying to use the bootloader on your eMMC to boot the SD card.)

If not, a potential software workaround is to use the rk2aw installer from within your stock (i.e. installed to eMMC) OS, then power off the PBP and insert a Tow-Boot SD card. rk2aw will then automatically boot using the bootloader on the SD card, at which point you can use the Tow-Boot installer to overwrite the existing rk2aw install with Tow-Boot.

Also, now that I finally have serial console set up, I see that u-boot (at least the version on my PBP) has a built-in boot selector as well.

Tow-Boot will give you a graphical selector (IIRC), so you don't have to plug in the serial adapter for every boot....

[wdt]

If you know how to use dd , AND NOT SCREW-UP, is it not simpler to blank (dev/zero)
the uboot area on emmc? If uboot is not there, then it can't be loaded
DO make a backup of MBR and first 16M so you can restore as needed
----
It is a bit tedious to type (in uboot terminal) boot_targets=mmc1 or whatever
every time you want to change defaults