U-boot TFTP on Zynq

Intro

Often when working with Embedded Linux devices it can be helpful to utilize TFTP from u-boot. It can be used to quickly test changes in the kernel and other components. It can also be used to program persistent storage.

Example

I would like to explain a string of commands for u-boot. Not long ago it was all black magic to me, and I would have liked someone to explain it to me.

usb start ; env set autoload no ; dhcp ; env set serverip 192.168.1.116 ; env set bitstream_addr 0x4000000 ; tftpboot ${bitstream_addr} download.bit.bin ; fpga load 0 ${bitstream_addr} ${filesize} ; pxe get ; pxe boot

Connecting to TFTP

Let’s start with connecting to the correct TFTP server.

1. usb start
2. env set autoload no
3. dhcp
4. env set serverip 192.168.1.116

In this scenario we are using a USB to Ethernet adapter (1a). We have a DHCP server (3a), but it does not contain the files we wish to download. We tell u-boot to request an address, but to not “autoload” a file from the server (2a).

After successfully performing “DHCP” (3b) there will be a new environment variable set that indicates the IP of the board. The “serverip” variable will also be set to the address of the DHCP server (3c). We must overwrite this value with the IP of the server that is hosting the files of interest (4)

u-boot addresses

In u-boot you will often be interacting with “variables”, as seen above, or directly with blocks of memory. The overall goal is to put the write things into the right memory location and then execute a command that will utilize those segments of memory (8a,10a).

To a certain degree the addresses assigned to variables can be chosen. It is important to consider the size and scope of the files you need in memory. In many cases a number of standard variables are defined reasonable spacing between.

6. env set bitstream_addr 0x4000000
7. tftpboot ${bitstream_addr} download.bit.bin
8. fpga load 0 ${bitstream_addr} ${filesize}

First we are choosing a region to place the binary bitstream (6). Next we are going to download a file named “download.bit.bin” form our TFTP server (7). It is important to note that we do not have to specify the IP of the server because the value of ${serverip} get’s used by defualt (7). We could easily override it (in this case tftpboot ${bitstream_addr} 192.168.1.116:download.bit.bin. Many commands that process files will set the “${filesize}” variable; which in this case we provide to the command that needs to use this region (8).

PXE Boot

9. pxe get
10. pxe boot

PXE boot is a extension of DHCP (and therefore TFTP) that provides more control of which files are provided to which devices (based on MAC address). The first command simply downloads the script file into a particular region of memory (9) “${scriptaddr}.

The next part (10b) will parse the “pxelinux”/“extlinux” file from the server. There can be multiple options in each config file.