usb-only rpi 3b+ (+UART foo)

[John Kennedy]

On Sun, Aug 05, 2018 at 09:49:29PM -0700, bob prohaska wrote:
> My information is now very old, but I _thought_ one had to have u-boot on the
> SD card at a minimum. There was an option within u-boot to "boot usb" which 
> implied the boot process could be transferred to a USB device, but I think
> an SD card is required to get things started.

The RPI3B+ will boot into USB (without SD card), or at least it will using the
same SD card plugged into a Dynex DX-CR112 memory card reader.  That gets you
past the rainbow screen and into U-Boot land.  U-Boot (?) seems to be choking
at that point.

It goes through the partitions (setting currdev to disk0p#), but it is printing
efipart_readwrite messages (rw=1, blk=2591 size=272 status=7).  Varies by
partition, but rw=1 and status=7 for them and long delays.  Putting the same
SD card into the SD slot and it boots up fine.

As far as booting...

	https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/host.md

I want to say that is a fairly recent change, ~April 2018.

Hmm.  My Samsung disk is USB3.