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Loading Linux Images via Ethernet and TFTP Print


This note explains how to load images via Ethernet from U-Boot. With an Ethernet connection available, U-Boot can load Linux images from a TFTP host quickly and easily. This is the development and software manufacturing option that is preferable with U-Boot and Linux.

The download procedure is based on the tftpboot command provided by the U-Boot command interface. tftboot implements a download capability over Ethernet using the TFTP protocol and has the following synopsis:

tftpboot <file> [<load_addr>]

If you do not specify a load address, then the value will be taken from the loadaddr environment variable. On the Vybrid SOM, loadaddr is set as follows, placing the download buffer into the on-module DDR:

Vybrid U-Boot > print loadaddr
loadaddr=0x80007fc0
Vybrid U-Boot >

The MAC address of the Ethernet interface is defined by the ethaddr environment variable.The IP address of the board is defined by the ipaddr U-Boot environment variable. The TFTP server IP address is defined by the serverip U-Boot environment variable. Make sure you define these environment variables to values that make sense for your LAN and save them in the on-module NAND Flash:

Vybrid U-Boot > setenv ipaddr 192.168.1.132
Vybrid U-Boot > setenv serverip 192.168.1.65
Vybrid U-Boot > saveenv
Saving Environment to NAND...
Erasing NAND...
Erasing at 0xa0000 -- 100% complete.
Writing to NAND... done
Vybrid U-Boot >

Note that the ethaddr variable is preset by Emcraft to a unique MAC address at the factory. To avoid possible conflicts in a LAN, we do not recommend updating ethaddr although you have this option in case you require it for some reason:

Vybrid U-Boot > printenv ethaddr
ethaddr=C0:B1:3C:88:88:85
Vybrid U-Boot > setenv ethaddr 3C:FB:96:77:88:AB
Vybrid U-Boot > saveenvSaving Environment to NAND...
Erasing NAND...
Erasing at 0xa0000 -- 100% complete.
Writing to NAND... done.
Vybrid U-Boot>

Once the transmission using tftpboot finishes, the file will be in memory at the specified load address. The loadaddr environment variable will automatically be set to the address the tftpboot command used. The filesize environment variable will automatically be set to the number of bytes transferred during the load operation.

Then you are free to do whatever you like with the loaded image. You can boot Linux from the image (assuming it is a bootable Linux file), copy it to some other place (for instance, NAND Flash), display the memory, etc.

One typical command sequence involving tftpboot is defined in the netboot environment variable, which by default is set in U-Boot as follows:

Vybrid U-Boot > print netboot
netboot=tftp ${image} && run args addip && run boot_dtb
Vybrid U-Boot >

What netboot does is load from a TFTP host a file defined by image (the tftp command), then add the TCP/IP related parameters to the kernel command string (the args and addip commands), and finally boot Linux from the just loaded image (the boot_dtb macro).

Let's use netboot to boot Linux via TFTP from the sample Linux image (rootfs.uImage) included in the Emcraft software distribution. Copy rootfs.uImage to the TFTP directory on the host and then from U-Boot on the target set the image environment variable to point to the Linux file:

Vybrid U-Boot > set image vf6/rootfs.uImage
Vybrid U-Boot > saveenv
Saving Environment to NAND...
...
Vybrid U-Boot> run netboot
Vybrid U-Boot > run netboot
Using FEC0 device
TFTP from server 172.17.0.1; our IP address is 172.17.80.3
Filename 'psl/vy/lcdkit/vf6-2.2.1/rootfs.uImage'.
Load address: 0x80007fc0
Loading: #################################################################
################################################################
#################################################################
#################################################################
#################################################################
#################################################################
done
Bytes transferred = 7019816 (6b1d28 hex)

NAND read: device 0 offset 0x2a0000, size 0x7000
28672 bytes read: OK
## Booting kernel from Legacy Image at 80007fc0 ...
Image Name:   Linux-4.5.0-vf6-2.2.1
Image Type:   ARM Linux Kernel Image (uncompressed)
Data Size:    7019752 Bytes = 6.7 MiB
Load Address: 80008000
Entry Point:  80008000
## Flattened Device Tree blob at 80000100
Booting using the fdt blob at 0x80000100
XIP Kernel Image ... OK
OK
Loading Device Tree to 9fdee000, end 9fdf79b8 ... OK

Starting kernel ...

init started: BusyBox v1.17.0 (2016-09-19 11:07:14 MSK)
...
~# ls
Settings           init              sample.ko
app                lib               sbin
bin                media             start_qtquick.touch
boot               mnt               sys
dev                proc              tmp
etc                qtdemo            usr
home               root              var
httpd              run
~ #

You must set up a TFTP server on the development host to allow downloading images to the target over Ethernet. The procedure is extensively documented in the Internet. Just google for "how to set up a tftp server" and follow the advice from some top articles.

Here are some troubleshooting tips, in case tftpboot does not work for you from U-Boot:

  1. As trivial as it sounds make sure that the board is connected to the LAN with an Ethernet cable.
  2. Make sure you have copied a file you are trying to download to the TFTP server directory on the host.
  3. Disable the firewall on the host. If a firewall is enabled, it will block TFTP requests from the target.
  4. On the target, make sure that you have set ipaddr and serverip correctly. Check ethaddr and make sure that you don't have another embedded board (eg. another VF6 SOM) configured for the same MAC address (this shouldn't be the case unless you have changes the ethaddr variable on some of your VF6 modules).