This method is used to flash a factory fresh board.
When the BootRom code fails to verify bootloader, it will boot into MaskRom mode, which waits for host to send bootloader code through USB, then we can use upgrade_tool(provided by rockchip) to sned a bootloader and make it take control.
When bootloader(provided by rockchip) take control, we could transfer image to it and flash the eMMC.
This method is used to update kernel or rootfs.
Enter "ums mmc 1" and uboot will be acted as a USB Mass Storage.
You can move your kernel,dtb and rootfs to it.
This method can help you update system via sd-card.
Insert your sd-card and reset the device.As we said in boot-sequence, device will detect sd-card and boot in it.
So we can use this way to boot in a ramdisk and flash firmware.
This method is like what you do in android device. Press button and reset the device， make the device in fastboot mode.Then you can transfer your firmware via otg port.
To make update safely, you can use a backup for kernel.
You can save two images(zimage_a zimage_b) in boot partition, and decide which one to use by extlinux.conf.
If you are worried that fatfs may be easily broken，you can use a,b partion and use bootable flag to determind which one to use.