Errors when using the SD card and raw mode?

[Kenny Koller]

Using these instructions I did the following:

Used dd to clear the first 64 MB of a 16 GB SD card to zeros.

Used sfdisk to create a single bootable Linux partition from 64 MB to the end of the disk.

Used dd to place MLO at 128 kB and U-Boot at 384 kB.

I formatted the partition to EXT4 and installed the root filesystem.

I'm using Yocto/Poky (Pyro branch) which is building U-Boot 2017.01.

When I boot it seems to find the SPL/MLO and load/run U-Boot:

U-Boot SPL 2017.01 (Jun 28 2017 - 23:47:37)
Trying to boot from MMC1MMC partition switch failed
*** Warning - MMC partition switch failed, using default environment

U-Boot 2017.01 (Jun 28 2017 - 23:47:37 +0000)
CPU  : AM335X-GP rev 2.1
I2C:   ready
DRAM:  512 MiB
MMC:   OMAP SD/MMC: 0, OMAP SD/MMC: 1
** First descriptor is NOT a primary desc on 1:1 **
*** Warning - bad CRC, using default environment
<ethaddr> not set. Validating first E-fuse MAC
Net:   cpsw, usb_ether
Press SPACE to abort autoboot in 2 seconds
=>

When I naively boot I get the following:

=> boot
switch to partitions #0, OK
mmc0 is current device
SD/MMC found on device 0
** File not found boot.scr **
** Unrecognized filesystem type **
switch to partitions #0, OK
mmc0 is current device
Scanning mmc 0:1...
switch to partitions #0, OK
mmc0 is current device
SD/MMC found on device 0
** Invalid partition 2 **
** First descriptor is NOT a primary desc on 1:1 **
switch to partitions #0, OK
mmc1(part 0) is current device
** No partition table - mmc 1 **
** First descriptor is NOT a primary desc on 1:1 **
switch to partitions #0, OK
mmc1(part 0) is current device
** First descriptor is NOT a primary desc on 1:1 **
SD/MMC found on device 1
** No partition table - mmc 1 **
## Error: "bootcmd_nand0" not defined
starting USB...
USB0:   Port not available.
cpsw Waiting for PHY auto negotiation to complete.....

What is U-Boot trying to do here? Is it trying to locate the uEnv? The kernel? Is it trying to mount the partition and search for /boot?

I can tell that is searching for something but can only understand about half of these messages. What is a descriptor? What is a primary desc? Why are partitions referred to as 0 rather than 1-4? What does "switch to partitions #0" mean?