Day 3 — Linux Boot Process¶

Goal¶

Understand the boot process of an embedded Linux system.

Linux Boot Flow¶

Power On
 ↓
Boot ROM
 ↓
Bootloader
 ↓
Kernel
 ↓
Device Tree
 ↓
Root File System
 ↓
init
 ↓
User Space

Boot Components¶

Boot ROM¶

Code stored inside the SoC.

Responsibilities:

Locate boot device
Load bootloader

Bootloader¶

Example:

U-Boot

Responsibilities:

Initialize hardware
Load kernel
Load device tree
Start kernel

Kernel¶

Responsibilities:

Initialize memory
Start scheduler
Load drivers

Device Tree¶

Describes hardware configuration of the board.

Examples:

UART
SPI
GPIO
interrupts

Root Filesystem¶

Provides the user-space environment.

Contains:

/bin
/etc
/usr
/lib

init¶

First user-space process started by the kernel.

Example:

systemd

Lab¶

Kernel version¶

uname -a

Kernel command line¶

cat /proc/cmdline

Output:

reboot=w coherent_pool=1M 8250.nr_uarts=1 pci=pcie_bus_safe cgroup_disable=memory numa_policy=interleave nvme.max_host_mem_size_mb=0  numa=fake=8 system_heap.max_order=0 iommu_dma_numa_policy=interleave smsc95xx.macaddr=88:A2:9E:BF:5C:4C vc_mem.mem_base=0x3fc00000 vc_mem.mem_size=0x40000000  console=ttyAMA10,115200 console=tty1 root=PARTUUID=be722d08-02 rootfstype=ext4 fsck.repair=yes rootwait cfg80211.ieee80211_regdom=TW

Boot log¶

dmesg | less

Root filesystem¶

mount | grep " / "