I followed online instructions from other threads I found, which stated I needed to mount the emmc device as a usb mass storage gadget in u-boot:
ums 0 mmc 2
I determined mmc 2 from using "mmc list" in uboot and seeing the only options were mmc 0, 1, and 2, and 0 shows as a "bad mmc device" when I try to query it, and mmc 2 shows as unreadable ext4 partition, so i assumed that would be the correct device, also since mmc2 is connected to sd 4, and the sd card is sd 3 on the schematic, so that all seemed to check out.
i was able to see that the emmc device enumerated in my host computer as /dev/sdb, so i used the dd command just as i would when copying the sd card image to a regular sd card, except this time with sdb instead of mmcblk0:
sudo dd if=card.sdcard of=/dev/sdb bs=1M && sync
that exact command works perfectly fine for /dev/mmcblk0 when i have an sd card inserted in my host computer, and i've booted from that sd card on my sabre board, so i know the image is good.
I then set the switch 6 boot-cfg to 01100011 which, per the schematic, corresponds to MMC/eMMC Boot, 1-bit mode, SD4 boot. I can't see any reason why that setting wouldn't work, and I verified the eMMC IC is definitely populated on the board. However, when I attempt to boot, I see no serial console output nor any other indication that the device is booting from eMMC. If I switch back to the SD card, and reset the boot-cfg switches, it boots fine.
I'm guessing someone will tell me to use the MFG tool, but I really don't like that app, and it's really confusing especially to try to teach to a manufacturing tech who will be flashing my custom board after it's fabricated. In my mind this should be as simple as doing the exact steps I mentioned above, but as I said it's not working for some reason.
One other thing: if I list the contents (after using the above dd command) with
ls mmc 2:1 and 2:2
in uboot I get the boot partition and file system partition just as I see on the SD card (i.e., i see the same output as ls mmc 1:1 and ls mmc 1:2). This tells me the dd command worked just fine over usb.
