Booting process of Linux: Difference between revisions

Revision as of 03:09, 10 January 2025

Revision as of 22:00, 11 January 2025

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In [[UEFI]] systems, the Linux kernel can be executed directly by UEFI firmware via the EFI boot stub,<ref>{{Cite web |title=EFI stub kernel - Gentoo Wiki |url=https://wiki.gentoo.org/wiki/EFI_stub_kernel |access-date=2020-11-02 |website=wiki.gentoo.org}}</ref> but usually uses [[GRUB 2]] or [[Gummiboot (software)|systemd-boot]] as a bootloader.<ref name="Intel2000">{{cite web |last=Kinney |first=Michael |date=1 September 2000 |title=Solving BIOS Boot Issues with EFI |url=http://systems.cs.colorado.edu/Documentation/IntelDataSheets/xscalemagazine.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070123141151/http://systems.cs.colorado.edu/Documentation/IntelDataSheets/xscalemagazine.pdf |archive-date=23 January 2007 |access-date=14 September 2010 |pages=47–50}}</ref><ref name="ElReg1">{{cite news |date=23 September 2011 |title=MS denies secure boot will exclude Linux |url=https://www.theregister.co.uk/2011/09/23/ms_denies_uefi_lock_in/ |access-date=24 September 2011 |publisher=The Register}}</ref>

If [[UEFI#Secure_Boot|UEFI Secure Boot]] is supported, a "shim" or "Preloader" is often booted by the UEFI before the bootloader or EFI-stub-bearing kernel.<ref>{{Cite Webweb |title=Using a Signed Bootloader - Arch Wiki |url=https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot#Using_a_signed_boot_loader |access-date=2024-12-05 |website=wiki.archlinux.org}}</ref> Even if [[UEFI#Secure_Boot|UEFI Secure Boot]] is disabled this may be present and booted in case it is later enabled. It merely acts to add an extra signing key database providing keys for signature verification of subsequent boot stages without modifying the UEFI key database, and chains to the subsequent boot step the same as the UEFI would have.

The system startup stage on embedded Linux system starts by executing the firmware / program on the on-chip [[boot ROM]], which then load bootloader / operating system from the storage device like eMMC, eUFS, NAND flash, etc.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} The sequences of system startup are varies by processors{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} but all include hardware initialization and system hardware testing steps.{{Sfn|M. Tim Jones|2006|loc=, "System startup"}} For example in a system with an i.MX7D processor and a bootable device which stores the OS (including U-Boot), the on-chip boot ROM sets up the [[DDR SDRAM|DDR memory]] controller at first which allows the boot ROM's program to obtain the SoC configuration data from the external bootloader on the bootable device.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=28}} The on-chip boot ROM then loads the U-Boot into DRAM for the bootloader stage.{{Sfn|Alberto Liberal De Los Ríos|2017|loc=, "Linux Boot Process"|p=29}}