This page explains how to boot the Linux® kernel from U-Boot through a TFTP server installed on a host PC, based on the pxe U-Boot command.
1. Documentation[edit source]
The documentation is available in the U-Boot source:
2. TFTP server installation on host PC[edit source]
The procedure, to install a TFTP server on your PC host, is explained here [1].
3. TFTP server configuration[edit source]
U-Boot follows pxelinux's rules to download from the TFTP server a PXE configuration file located in the pxelinux.cfg directory.
See more details here[2]
U-Boot pxe command loads a PXE configuration file in pxelinux.cfg directory of the TFTP server and parses it.
3.1. PXE configuration file[edit source]
Each PXE configuration file has same format as the extlinux.conf file, generated by a standard Yocto distribution.
The format of the PXE configuration text file is explained in this chapter. More explanations on the name of the file loaded by U-Boot are given in next chapter.
For each element of the menu :
- the LABEL is a string, for example the boot device such as an e•MMC, SD card, NAND, and so on.
- the kernel and the device tree loaded by U-Boot are defined via KERNEL and FDT options.
- the Linux® kernel boot arguments are defined via the APPEND option, including the rootfs to use;
For example, for STM32MP157C-EV1 Evaluation board 
, with the rootfs on the 6th partition of the SD card (/dev/mmcblk0p6), the PXE configuration file is:
menu title Select the boot mode DEFAULT sdcard LABEL sdcard KERNEL uImage FDT stm32mp157c-ev1.dtb APPEND root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200
To fully boot on a network (kernel and rootfs), the rootfs to use can be defined :
- as initramfs with INITRD command in configuration file (the file uInitrd need to be generated, it is a generic feature not explained in this WIKI).
- or as NFS mounting point with Linux® kernel boot arguments nfsroot[3] added in APPEND command (root=/dev/nfs nfsroot=... ) and NFS server running on the PC[4].
In the configuration file, the menu can have several options (see LABEL, DEFAULT and TIMEOUT commands in README.pxe for details).
For example
menu title Select the boot mode DEFAULT initramfs TIMEOUT 20 LABEL initramfs KERNEL uImage FDT stm32mp157c-ev1.dtb INITRD uInitrd APPEND rootwait rw earlyprintk console=ttySTM0,115200 LABEL nfs KERNEL uImage FDT stm32mp157c-ev1.dtb APPEND root=/dev/nfs nfsroot=192.168.1.1:/nfsroot ip=dhcp rootwait rw earlyprintk console=ttySTM0,115200 LABEL sdcard KERNEL uImage FDT stm32mp157c-ev1.dtb APPEND root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200 LABEL eMMC KERNEL uImage FDT stm32mp157c-ev1.dtb APPEND root=/dev/mmcblk1p6 rootwait rw earlyprintk console=ttySTM0,115200
3.2. pxelinux.cfg directory[edit source]
Because more than one board may be booted from the same server, the "PXE configuration file" name depends on U-boot parameters (hardware address of the board/IP address).
U-Boot searches for a PXE configuration file in the pxelinux.cfg directory in this order:
- UUID comes from "pxeuuid" variable
- hardware type and address: ARP type "1" for Ethernet and MAC address
- IP address of the board and each subnet mask
- at the end, the file named "default-$CONFIG_SYS_ARCH-$CONFIG_SYS_SOC"
You need to provide at least one configuration file on the TFTP server per board.
Each file of the pxelinux.cfg directory can have the same content (or can be a symbolic link to a default one).
For example with STM32MP1 Series, when:
- UUID is not defined
- ethaddr=00:80:e1:01:2d:6f
- IP =10.48.0.141 (hexa coding=0A30008D)
- CONFIG_SYS_ARCH=arm and CONFIG_SYS_SOC="stm32mp"
U-Boot searches the configuration file is this order:
- "01-00-80-e1-01-2d-6f"
- 0A30008D then subnet 0A30008, 0A3000, 0A300 ...
- default-arm-stm32mp
With ethaddr=00:80:e1:01:2d:6f, the tftp directory (for example /tftpboot) may look like:
├── pxelinux.cfg directory │ ├── 01-00-80-e1-01-2d-6f configuration file for MAC address 00:80:e1:01:2d:6f ├── stm32mp157c-ev1.dtb device tree ├── uImage kernel
An other example with
- different boards (identified by a MAC address)
each pxe file selects a different device tree (ed1, ev1, dk1, dk2) or kernel file - fallback with the IP address submask (symbolic link to default-arm-stm32mp)
- default-arm-stm32mp (final fallback file, name based on CONFIG_SYS_SOC)
├── pxelinux.cfg │ ├── 01-00-80-e1-01-2d-6f configuration file for MAC address 00:80:e1:01:2d:6f │ ├── 01-00-80-e1-42-42-8c configuration file for MAC address 00:80:e1:42:42:8c │ ├── 01-00-80-e1-42-42-cd configuration file for MAC address 00:80:e1:42:42:cd │ ├── 01-00-80-e1-42-46-76 configuration file for MAC address 00:80:e1:42:46:6f │ ├── 0A300 -> default-arm-stm32mp configuration file for IP address 10.48.0*.** │ ├── 0A30001 -> default-arm-stm32mp configuration file for IP address 10.48.00.1* │ ├── 0A30004 -> default-arm-stm32mp configuration file for IP address 10.48.00.4* │ ├── 0A30008 -> default-arm-stm32mp configuration file for IP address 10.48.00.8* │ └── default-arm-stm32mp default configuration file ├── stm32mp157c-ed1.dtb ├── stm32mp157c-ev1.dtb ├── stm32mp157a-dk1.dtb ├── stm32mp157c-dk2.dtb ├── uImage ├── uInitrd
4. Board command in U-boot console[edit source]
You can execute the DISTRO bootcmd for PXE:
env set serverip <the PC address>
run bootcmd_pxe
5. Step by step example[edit source]
- STM32MP157C-EV1 board with MAC address: ethaddr=00:80:e1:01:60:da
- Install and configure a TFTP server:
- IP address is 10.201.21.107
- directory is :
├── pxelinux.cfg
│ ├── 01-00-80-e1-01-60-da
├── stm32mp157c-ev1.dtb
├── uImage
With the text file 01-00-80-e1-01-60-da ( #PXE configuration file in #pxelinux.cfg directory) :
menu title Select the boot mode
DEFAULT sdcard
LABEL sdcard
KERNEL uImage
FDT stm32mp157c-ev1.dtb
APPEND root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200
In the U-Boot console:
STM32MP> env set serverip 10.201.21.107 set the server address STM32MP> run bootcmd_pxe execute the DISTRO command BOOTP broadcast 1 DHCP client bound to address 10.48.1.229 (19 ms) missing environment variable: pxeuuid missing environment variable: bootfile Retrieving file: pxelinux.cfg/01-00-80-e1-01-60-da Using ethernet@5800a000 device TFTP from server 10.201.21.107; our IP address is 10.48.1.229; sending through gateway 10.48.3.254 Filename 'pxelinux.cfg/01-00-80-e1-01-60-da'. load configuration file Load address: 0xc4200000 Loading: # 105.5 KiB/s done Bytes transferred = 435 (1b3 hex) Config file found Select the boot mode display menu 1: sdcard Enter choice: 1: sdcard timeout: default choice missing environment variable: bootfile Retrieving file: uImage load kernel Using ethernet@5800a000 device TFTP from server 10.201.21.107; our IP address is 10.48.1.229; sending through gateway 10.48.3.254 Filename 'uImage'. Load address: 0xc2000000 Loading: ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################################################################# ################ 785.2 KiB/s done Bytes transferred = 5734976 (578240 hex) append: root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200 missing environment variable: bootfile Retrieving file: stm32mp157c-ev1.dtb load device tree Using ethernet@5800a000 device TFTP from server 10.201.21.107; our IP address is 10.48.1.229; sending through gateway 10.48.3.254 Filename 'stm32mp157c-ev1.dtb'. Load address: 0xc4000000 Loading: ############################ 646.5 KiB/s done Bytes transferred = 138450 (21cd2 hex) ## Booting kernel from Legacy Image at c2000000 ... booting Image Name: Linux-4.19.49 Created: 2019-09-04 11:20:19 UTC Image Type: ARM Linux Kernel Image (uncompressed) Data Size: 5734912 Bytes = 5.5 MiB Load Address: c2000040 Entry Point: c2000040 Verifying Checksum ... OK ## Flattened Device Tree blob at c4000000 Booting using the fdt blob at 0xc4000000 XIP Kernel Image ... OK Using Device Tree in place at c4000000, end c4024cd1 SF: Detected mx66l51235l with page size 256 Bytes, erase size 64 KiB, total 64 MiB Starting kernel ... starting kernel [ 0.000000] Booting Linux on physical CPU 0x0 .... [ 0.000000] Kernel command line: root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200 ....
6. References[edit source]