Skip to content

Latest commit

 

History

History
79 lines (63 loc) · 4.07 KB

bootup.md

File metadata and controls

79 lines (63 loc) · 4.07 KB
Raw

Booting sequence in the case of FullMetalUpdate :

  1. Primary processor steps :

    1. Primary bootstrap to initialize interrupt/exception vectors, clocks and RAM
    2. U-boot is decompressed and loaded into the RAM
    3. Execution is passed to U-boot

  2. U-boot boot sequence :

    1. Flash, serial console, Ethernet MAC Address (etc.) are configured
    2. A minimal bootscript called boot.scr is executed and is used only to load a custom script called uEnv.txt, written by us. These two scripts (the minimal one and the custom one) are both defined in meta-fullmetalupdate-extra in the dynamic-layers folder. What those files contain and how they are handled is specific to the board, but they will serve the same purpose.
    3. The custom uEnv.txt u-boot script does several things :
      • It defines the proper storage interfaces and addresses (bootmmc, bootiaddr…)
      • It defines multiple boot commands (bootcmd*) which all serve different purposes :
        • bootcmd is the main boot command : it contains conditional branches used to boot either onto the current deployment or the rollback deployment. It also execute the ostree boot command bootcmd_otenv
        • bootcmd_otenv is used to load OStree environment variables, needed by OStree to boot on the proper deployment. Note that this command imports theses variables from another uEnv.txt which totally differs from our custom one, defined in Yocto. This uEnv.txt is defined in /boot/loader/uEnv.txt and written by OSTree when staging commits. Please see the OSTree documentation for more info.
        • When this script runs, it will execute these different commands and the kernel, ramdisk image, device tree blob will be loaded. The execution is then passed to the kernel.

  3. Kernel boot sequence :

    The kernel initializes the various components/drivers of the target.

    Note :

    • The typical kernel command line looks like this (on the IMX8) :

      rw rootwait console=ttymxc0,115200 ostree=/ostree/boot.0/poky/<hash>/0 ostree_root=/dev/mmcblk1p2 root=/dev/ram0 ramdisk_size=8192 rw rootfstype=ext4 consoleblank=0 vt.global_cursor_default=0 video=mxcfb0:dev=ldb,bpp=32,if=RGB32 where <hash> is the current deployment's hash.

      It contains interesting arguments such as the ostree= which is parsed by OSTree to determine the current deployment.

  4. OSTree initrd script :

    OSTree has a custom initrd script since the file system is a bit different on OSTree devices (see the ostree documentation). The script mounts the different file systems and determines the current deployment by parsing the kernel command line.

    More details this script are found in the meta-updater layer.

  5. systemd :

    systemd, the init manager, does his usual job : starting the usual target units, but also start the FullMetalUpdate client with a custom service file (which is found here).

  6. Client initialization :

    The client initializes the container ostree repo and starts the different preinstalled containers.

Useful resources :