Difference between revisions of "U-Boot"
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=== Distro Boot === | === Distro Boot === | ||
| − | Source: [https://gitlab.denx.de/u-boot/u-boot/-/blob/master/doc/README.distro README.distro] | + | Source: [https://gitlab.denx.de/u-boot/u-boot/-/blob/master/doc/README.distro README.distro] [https://developer.toradex.com/knowledge-base/distro-boot-linux] |
Standardized way of booting a [[Linux distribution]] from U-Boot, without needing to build the distribution specifically for the target device. A distribution needs to provide its own generic bootloader, e.g. [[syslinux]] (extlinux). | Standardized way of booting a [[Linux distribution]] from U-Boot, without needing to build the distribution specifically for the target device. A distribution needs to provide its own generic bootloader, e.g. [[syslinux]] (extlinux). | ||
Enabled by: <code>CONFIG_DISTRO_DEFAULTS=y</code> | Enabled by: <code>CONFIG_DISTRO_DEFAULTS=y</code> | ||
| + | |||
| + | ==== Devices order ==== | ||
| + | # Internal flash storage (e.g. eMMC) | ||
| + | # External flash storage (e.g. SD card) | ||
| + | # USB storage | ||
| + | # Network storage obtained from DHCP | ||
| + | |||
| + | Note that the first two might be swapped, depending on the configuration. Often it makes more sense to give priority to a removable media before booting from the internal storage. | ||
| + | |||
| + | For each of these devices, U-Boot will search in the first partition marked with a bootable flag (via the MBR bootable flag, or GPT legacy_bios_bootable attribute). In its absence, it will fall back to looking at the first valid partition. Disto Boot recognizes FAT and ext2/3/4 filesystems. | ||
U-Boot searches for {{code|/extlinux/extlinux.conf}} then {{code|/boot/extlinux/extlinux.conf}} on disk, or {{code|pxelinux.cfg/default}} over the network. The content is as recognized by [[syslinux]] (extlinux). It also supports its own {{code|bootcmd}} format in {{code|boot.scr}} file. | U-Boot searches for {{code|/extlinux/extlinux.conf}} then {{code|/boot/extlinux/extlinux.conf}} on disk, or {{code|pxelinux.cfg/default}} over the network. The content is as recognized by [[syslinux]] (extlinux). It also supports its own {{code|bootcmd}} format in {{code|boot.scr}} file. | ||
| − | |||
== Related == | == Related == | ||
Revision as of 19:58, 28 March 2020
Linux friendly bootloader. Used in various ARM devices like PinePhone and Orange Pi.
Contents
Boot process
Definitions
General
- SPL (Secondary Program Loader) is a small binary, generated from U-Boot source, that fits in the SRAM and loads the main U-Boot into system RAM. [1]
- TPL (Tertiary Program Loader) is a tiny SPL which loads moderate-sized middle layer called TPL and that loads the full U-Boot into RAM.
Board-specific
- ATF (ARM Trusted Firmware)
Device Tree
- DTB (Device Tree Blob) TODO: Enhance
- ITB TODO: ???
- FIT (Flat Image Tree) TODO: Enhance
Boot sequence
- Device's built-in boot code is executed (BootROM)
- It searches connected storage devices for a bootloader signature at a predefined address
- If found it loads the bootloader into SRAM and executes it
- TPL does minimal initialization, loads and executes SPL
- SPL does more extended initialization, loads and executes U-Boot
- U-Boot finishes initialization, provides a command prompt / searches for OS images / boots them
Distro Boot
Source: README.distro [2]
Standardized way of booting a Linux distribution from U-Boot, without needing to build the distribution specifically for the target device. A distribution needs to provide its own generic bootloader, e.g. syslinux (extlinux).
Enabled by: CONFIG_DISTRO_DEFAULTS=y
Devices order
- Internal flash storage (e.g. eMMC)
- External flash storage (e.g. SD card)
- USB storage
- Network storage obtained from DHCP
Note that the first two might be swapped, depending on the configuration. Often it makes more sense to give priority to a removable media before booting from the internal storage.
For each of these devices, U-Boot will search in the first partition marked with a bootable flag (via the MBR bootable flag, or GPT legacy_bios_bootable attribute). In its absence, it will fall back to looking at the first valid partition. Disto Boot recognizes FAT and ext2/3/4 filesystems.
U-Boot searches for /extlinux/extlinux.conf then /boot/extlinux/extlinux.conf on disk, or pxelinux.cfg/default over the network. The content is as recognized by syslinux (extlinux). It also supports its own bootcmd format in boot.scr file.
