im8mn: Loading U-Boot via serial download mode

[Fabio Estevam]

Hi Adam,

Looking at your imx8mn beacon patches, it seems you managed to boot
the imx8mn beacon board via serial download mode.

I am trying to achieve the same on an imx8mn-evk running top of tree
U-Boot and added the following changes:

--- a/configs/imx8mn_evk_defconfig
+++ b/configs/imx8mn_evk_defconfig
@@ -50,11 +50,22 @@ CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_CACHE=y
+CONFIG_USB=y
+# CONFIG_SPL_DM_USB is not set
+CONFIG_USB_EHCI_HCD=y
+CONFIG_USB_GADGET=y
+CONFIG_USB_GADGET_MANUFACTURER="FSL"
+CONFIG_USB_GADGET_VENDOR_NUM=0x0525
+CONFIG_USB_GADGET_PRODUCT_NUM=0xa4a5
+CONFIG_CI_UDC=y
+CONFIG_SDP_LOADADDR=0x0
+CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_CMD_REGULATOR=y
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_OF_CONTROL=y
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_ENV_OVERWRITE=y
+CONFIG_ENV_IS_NOWHERE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y

Then I put the board into serial download mode and I run:

$ sudo uuu flash.bin

The following output appears on the console:

U-Boot SPL 2022.04-00856-g246e03476ba3-dirty (Apr 20 2022 - 16:24:48 -0300)
SEC0:  RNG instantiated
Normal Boot
WDT:   Started watchdog at 30280000 with servicing (60s timeout)
Trying to boot from BOOTROM
Find img info 0x&48020a00, size 872
Need continue download 1024
Download 694696, Total size 696232
NOTICE:  BL31: v2.2(release):rel_imx_5.4.47_2.2.0-0-gc949a888e909
NOTICE:  BL31: Built : 16:07:45, Apr 20 2022

What would be the other uuu command that needs to be run so that
U-Boot proper can be loaded?

Thanks,

Fabio Estevam