How to update U-Boot on pico-imx7d board
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Required software on the host PC:

- imx_usb_loader: https://github.com/boundarydevices/imx_usb_loader

Build U-Boot for pico:

$ make mrproper
$ make pico-imx7d_defconfig
$ make

This generates the SPL and u-boot-dtb.img binaries.

1. Loading U-Boot via USB Serial Download Protocol

Note: This method is convenient for development purposes.
If the eMMC has already a U-Boot flashed with DFU support then
the user can go to step 2 below in order to update U-Boot.

Put pico board in USB download mode (refer to the PICO-iMX7D Quick Start Guide
page 3)

Connect a USB to serial adapter between the host PC and pico.

Connect a USB cable between the OTG pico port and the host PC.

Note: Some computers may be a bit strict with USB current draw and will
shut down their ports if the draw is too high. The solution for that is
to use an externally powered USB hub between the board and the host computer.

Open a terminal program such as minicom.

Copy SPL and u-boot-dtb.img to the imx_usb_loader folder.

Load the SPL binary via USB:

$ sudo ./imx_usb SPL

Load the u-boot-dtb.img binary via USB:

$ sudo ./imx_usb u-boot-dtb.img

Then U-Boot starts and its messages appear in the console program.

Use the default environment variables:

=> env default -f -a
=> saveenv

2. Flashing U-Boot into the eMMC

Run the DFU agent so we can flash the new images using dfu-util tool:

=> dfu 0 mmc 0

Flash SPL and u-boot-dtb.img into the eMMC running the following commands on a PC:

$ sudo dfu-util -D SPL -a spl

$ sudo dfu-util -D u-boot-dtb.img -a u-boot

Remove power from the pico board.

Put pico board into normal boot mode.

Power up the board and the new updated U-Boot should boot from eMMC.

Booting in Falcon mode
======================

Generate a uImage kernel:

$ make imx_v6_v7_defconfig (Using the default imx_v6_v7_defconfig configuration
just for an example. In order to boot faster the user should customize the
defconfig by only enabling the minimal required drivers).

$ make -j4 uImage LOADADDR=0x80008000

$ cp arch/arm/boot/uImage /tftpboot
$ cp arch/arm/boot/dts/imx7d-pico-pi.dtb /tftpboot

In the U-Boot prompt:

Setup the server and board IP addresses:
=> setenv serverip 192.168.0.10
=> setenv ipaddr 192.168.0.11

Get the dtb file:
=> tftp ${fdt_addr} imx7d-pico-pi.dtb

Get the kernel:
=> tftp ${loadaddr} uImage

Write the kernel at 2MB offset:
=> mmc write ${loadaddr} 0x1000 0x5000

Setup the bootargs:
=> setenv bootargs 'console=ttymxc4,115200 root=/dev/mmcblk2p1 rootfstype=ext4 rootwait rw'

Prepare args:
=> spl export fdt ${loadaddr} - ${fdt_addr}
## Booting kernel from Legacy Image at 80800000 ...
   Image Name:   Linux-5.2.14
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    9077544 Bytes = 8.7 MiB
   Load Address: 80008000
   Entry Point:  80008000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 83000000
   Booting using the fdt blob at 0x83000000
   Loading Kernel Image
   Using Device Tree in place at 83000000, end 8300b615
subcommand not supported
subcommand not supported
   Using Device Tree in place at 83000000, end 8300e615
Argument image is now in RAM: 0x83000000
=>

Write 1MB of args data (0x800 sectors) to 1MB offset (0x800 sectors):

=> mmc write ${fdt_addr} 0x800 0x800

In order to boot with Falcon mode, activate the CONFIG_SPL_OS_BOOT
option in the defconfig

--- a/configs/pico-imx7d_defconfig
+++ b/configs/pico-imx7d_defconfig
@@ -67,3 +67,4 @@ CONFIG_USB_GADGET_VENDOR_NUM=0x0525
 CONFIG_USB_GADGET_PRODUCT_NUM=0xa4a5
 CONFIG_CI_UDC=y
+CONFIG_SPL_OS_BOOT=y

Then rebuild U-Boot:

$ make pico-imx7d_defconfig
$ make -j4

Launch UMS:
=> ums 0 mmc 0

Flash the new binaries:

$ sudo dd if=SPL of=/dev/sdX bs=1k seek=1; sync
$ sudo dd if=u-boot-dtb.img  of=/dev/sdX bs=1k seek=69; sync

And then SPL binary will load and jump directly to the kernel:

U-Boot SPL 2019.10-rc3-00284-g001c8ea94a-dirty (Sep 10 2019 - 12:46:01 -0300)
Trying to boot from MMC1
[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 5.2.14 (fabio@fabio-OptiPlex-7010) (gcc version 7.4.0 (Ubuntu/Linaro 7.4.0-1ubuntu1~18.04.1)) #30 SMP Wed Sep 10 12:36:27 -03 2019
[    0.000000] CPU: ARMv7 Processor [410fc075] revision 5 (ARMv7), cr=10c5387d
[    0.000000] CPU: div instructions available: patching division code
[    0.000000] CPU: PIPT / VIPT nonaliasing data cache, VIPT aliasing instruction cache
[    0.000000] OF: fdt: Machine model: TechNexion PICO-IMX7D Board and PI baseboard
...