一、概述
嵌入式linux系统启动过程一般包含以下几个过程：
- 上电
- 执行IROM(芯片内部只读存储器)中固化的代码。
- 执行uboot
- 加载内核到内存，启动内核。
- 挂载根文件系统rootfs
- 执行其中的脚本和应用程序。
二、linux操作系统的几个组成部分介绍
1、uboot介绍
（1）概述
- 是一个通用的bootloader
- Boot:完成硬件的初始化
-
Loader:加载操作系统内核，并启动内核.
- uboot支持多种处理器架构：ARM、PowerPC 、X86 、 Mips等等。
- 支持加载启动多种操作系统：linux、wince、VXworks等
- uboot的源码下载地址：https://ftp.denx.de/pub/u-boot/
（2）作用
- CPU的初始化（芯片级）主要采用ARM汇编语言，包含初始化缓存、MMC、Watchdog、终端、clock等。
- 板级初始化（采用C语言编写）：主要初始化串口、网卡、USB、LCD等设备.
-
为操作系统的启动准备参数，加载操作系统镜像到内存，并启动.
(3)u-boot的开机打印信息分析
U-Boot 2014.07 (Apr 10 2021 - 02:16:01)---U-boot版本号
//时钟系统中各个总线上的时钟频率
PLL : [0] = 800000000, [1] = 800000000, [2] = 780000000, [3] = 800000000
(0) PLL1: CPU FCLK = 800000000, HCLK = 200000000 (G0)
(7) PLL1: CPU FCLK = 800000000, HCLK = 200000000 (G1)
(2) PLL3: MEM FCLK = 800000000, DCLK = 800000000, BCLK = 400000000, PCLK = 200000000
(1) PLL0: BUS BCLK = 400000000, PCLK = 200000000
(8) PLL0: CCI4 BCLK = 400000000, PCLK = 200000000
(3) PLL0: G3D BCLK = 400000000
(4) PLL0: CODA BCLK = 400000000, PCLK = 200000000
(5) PLL0: DISP BCLK = 400000000, PCLK = 200000000
(6) PLL0: HDMI PCLK = 133333333
I2C: ready
DRAM: 1 GiB------------内存1GB
Heap = 0x44000000~0x46000000
Code = 0x43c00000~0x43c83e48
GLD = 0x43bffeb8
GLBD = 0x43bffe68
SP = 0x43bffe68,0x43bffe48(CURR)
PC = 0x43c06640
TAGS = 0x40000100
PAGE = 0x43c90000~0x43c9c000
MACH = [4330]
VER = 0
BOARD= [GEC6818]
MMC: NXP DWMMC: 0, NXP DWMMC: 1, NXP DWMMC: 2
In: serial
Out: serial
Err: serial
## DCDC_MODE(0x80): DCDC1[PFM], DCDC2[PFM], DCDC3[PFM], DCDC4[PWM], DCDC5[PWM]
## STATUS(0x00) : 0xe4 0x10
## IRQ(0x48) : 0x00 0x00 0x00 0x00 0x00
## CHG_TYPE : ADP
## BAT_VOL : 0mV
## BAT_CAP : 100%
DONE: Logo bmp 300 by 300 (3bpp), len=270056
DRAW: 0x47000000 -> 0x46000000
DONE: Logo bmp 300 by 300 (3bpp), len=270056
DRAW: 0x47000000 -> 0x46000000
RGB: display.0
MIPI: display.0
DSIM_ESCMODE 1 : 0xc0
DSIM_STATUS : 0x10010f
MIPI clk: 420MHz
DSIM_ESCMODE 2 : 0x0
DSIM_STATUS : 0x10010f
## Skip BAT Animation.
## IRQ(0x48) : 0x00 0x00 0x00 0x00 0x00
## chg_type : ADP
## battery_vol : 0mV
## battery_cap : 100%
## Booting
Card did not respond to voltage select!
Net: GEC6818 mac init...
dwmac.c0060000
Hit any key to stop autoboot: 0---倒计时结束加载启动内核，倒计时结束前按下键盘上的任意按键，进入U-BOOT命令行
（4）uboot的命令
=> help //在uboot下执行help看uboot下的帮助文档
? - alias for 'help'
base - print or set address offset
bdinfo - print Board Info structure
blkcache - block cache diagnostics and control
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootefi - Boots an EFI payload from memory
bootelf - Boot from an ELF image in memory
bootm - boot application image from memory
bootp - boot image via network using BOOTP/TFTP protocol
bootvx - Boot vxWorks from an ELF image
bootz - boot Linux zImage image from memory
cmp - memory compare
coninfo - print console devices and information
cp - memory copy
crc32 - checksum calculation
dhcp - boot image via network using DHCP/TFTP protocol
dm - Driver model low level access
echo - echo args to console
editenv - edit environment variable
env - environment handling commands
erase - erase FLASH memory
exit - exit script
ext2load - load binary file from a Ext2 filesystem
ext2ls - list files in a directory (default /)
ext4load - load binary file from a Ext4 filesystem
ext4ls - list files in a directory (default /)
ext4size - determine a file's size
false - do nothing, unsuccessfully
fatinfo - print information about filesystem
fatload - load binary file from a dos filesystem
fatls - list files in a directory (default /)
fatmkdir - create a directory
fatrm - delete a file
fatsize - determine a file's size
fatwrite - write file into a dos filesystem
fdt - flattened device tree utility commands
flinfo - print FLASH memory information
fstype - Look up a filesystem type
go - start application at address 'addr'
gpio - query and control gpio pins
gpt - GUID Partition Table
help - print command description/usage
iminfo - print header information for application image
imxtract - extract a part of a multi-image
itest - return true/false on integer compare
ln - Create a symbolic link
load - load binary file from a filesystem
loadb - load binary file over serial line (kermit mode)
loads - load S-Record file over serial line
loadx - load binary file over serial line (xmodem mode)
loady - load binary file over serial line (ymodem mode)
loop - infinite loop on address range
ls - list files in a directory (default /)
md - memory display
mdio - MDIO utility commands
mii - MII utility commands
mm - memory modify (auto-incrementing address)
mmc - MMC sub system
mmcinfo - display MMC info
mtd - MTD utils
mw - memory write (fill)
nfs - boot image via network using NFS protocol
nm - memory modify (constant address)
part - disk partition related commands
ping - send ICMP ECHO_REQUEST to network host
printenv - print environment variables
protect - enable or disable FLASH write protection
pxe - commands to get and boot from pxe files
random - fill memory with random pattern
reset - Perform RESET of the CPU
run - run commands in an environment variable
save - save file to a filesystem
saveenv - save environment variables to persistent storage
setenv - set environment variables
setexpr - set environment variable as the result of eval expression
sf - SPI flash sub-system
showvar - print local hushshell variables
size - determine a file's size
sleep - delay execution for some time
source - run script from memory
sysboot - command to get and boot from syslinux files
test - minimal test like /bin/sh
tftpboot - boot image via network using TFTP protocol
true - do nothing, successfully
usb - USB sub-system
usbboot - boot from USB device
version - print monitor, compiler and linker version
- bdinfo:打印出开发板的信息
GEC6818# bdinfo
arch_number = 0x000010EA-------开发板的编号，要与内核源码中开发板编号一致
boot_params = 0x40000100-------启动内核所需参数在内存的0x40000100处
DRAM bank = 0x00000000
-> start = 0x40000000-----------内存的起始地址，结束地址0x7fffffff
-> size = 0x40000000-----------内存的大小 1GB
eth0name = dwmac.c0060000---网卡的名字
ethaddr = 00:e2:1c:ba:e8:60 --网卡的MAC地址
current eth = dwmac.c0060000
ip_addr = 192.168.51.5----------U-BOOT设置的网卡IP
baudrate = 115200 bps-----------串口的波特率
TLB addr = 0x7FFF0000
relocaddr = 0x46000000
reloc off = 0x00000000
irq_sp = 0x7DF6BF00
sp start = 0x43BFFE6
- 启动内核的命令
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootm - boot application image from memory
bootp - boot image via network using BOOTP/TFTP protocol
- printenv：打印环境变量
GEC6818# printenv
baudrate=115200
bootargs=lcd=at070tn92 tp=gslx680-linux root=/dev/mmcblk0p2 rw rootfstype=ext4
bootcmd=ext4load mmc 2:1 0x48000000 uImage;bootm 0x48000000
两个重要的环境变量：
bootargs启动参数：告诉内核一些重要启动信息
lcd=at070tn92-------------LCD屏驱动芯片的型号
tp=gslx680-linux-----------触摸屏的型号
root=/dev/mmcblk0p2----根文件系统在emmc的第2个分区
rw rootfstype=ext4--------根文件系统可读可写，类型是ext4
bootcmd--------------------启动命令环境变量,告诉U-boot一些信息
bootcmd=ext4load mmc 2:1 0x48000000 uImage;bootm 0x48000000
以ext4格式把内核镜像uImage从emmc的第1分区加载到内存的0x48000000处，然后去到内存0x48000000处，启动内核
bootdelay=3---------------倒计时3S
bootfile=uImage
ethact=dwmac.c0060000
ethaddr=00:e2:1c:ba:e8:60
ethprime=RTL8211------网卡芯片的型号
gatewayip=192.168.51.1-----------------默认网关
ipaddr=192.168.51.5---------------------开发板IP
netmask=255.255.255.0----------------子网掩码
qtcrc=-1489582083
serverip=192.168.51.2-------------------服务器IP
stderr=serial
stdin=serial
stdout=serial
- setenv ：设置（修改）环境变量的值
GEC6818# setenv bootdelay 5 -----------把环境变量bootdelay的值修改为5
GEC6818# saveenv-----------------------------保存环境变量的修改
Saving Environment to MMC...
Writing to MMC(2)... doner
- reset:重启主板
- go:去到内存某个地址执行程序。
GEC6818# go 0x40000000------------去到内存0x40000000处，执行程序
## Starting application at 0x40000000 ..
- ping ：测试网络连通性
Environment size: 843/32764 bytes
GEC6818# setenv ipaddr 192.168.1.21 -----设置开发板自己的IP
GEC6818# setenv netmask 255.255.255.0
GEC6818# setenv gatewayip 192.168.1.1
GEC6818# setenv serverip 192.168.1.20
GEC6818# saveenv
Saving Environment to MMC...
Writing to MMC(2)... done
Writing to MMC(2)... done
GEC6818# ping 192.168.1.20
Speed: 100, full duplex
Using dwmac.c0060000 device
host 192.168.1.20 is alive----------------网络连通
- tftp:运行tftpd32.exe，相当于在windows下搭建了tftp服务器
GEC6818# setenv serverip 192.168.1.20---在开发板U-BOOT中设置服务器IP为windows IP
GEC6818# save
Saving Environment to MMC...
Writing to MMC(2)... done
GEC6818# tftp 0x40000000 tftpd32.ini-----从tftp服务器上下载一个名字为tftpd32.ini的文件到内存0x40000000处
Speed: 100, full duplex
Using dwmac.c0060000 device
TFTP from server 192.168.1.20; our IP address is 192.168.1.21
Filename 'tftpd32.ini'.
Load address: 0x40000000
Loading: #
7.8 KiB/s
done
Bytes transferred = 530 (212 hex)
2、内核
（1）内核的作用
-
进程调度与进程通信
- 内存管理
- 文件管理
- 设备驱动管理
- 网络通信---TCP/IP
kernel源码下载地址：https://mirrors.edge.kernel.org/pub/linux/kernel/
（2）linux内核启动时的输出信息
## Booting kernel from Legacy Image at 48000000 ...
Image Name: Linux-3.4.39-gec -----------内核版本
3 主版本号
4 次版本号 偶数：稳定版 奇数：开发版
39 修订版本号
Gec 扩展版本号
Image Type: ARM Linux Kernel Image (uncompressed)
Data Size: 5575624 Bytes = 5.3 MiB
Load Address: 40008000
Entry Point: 40008000
Verifying Checksum ... OK
Loading Kernel Image ... OK
Starting kernel ... --------------------开始启动内核
[ 0.000000] Booting Linux on physical CPU 0 -------------在CPU0 上启动Linux
[ 0.000000] Memory: 1024MB = 1024MB total
[ 0.000000] Memory: 810728k/810728k available, 237848k reserved, 272384K highmem
虚拟内存的划分
[ 0.000000] Virtual kernel memory layout:
[ 0.000000] vector : 0xffff0000 - 0xffff1000 ( 4 kB)
[ 0.000000] fixmap : 0xfff00000 - 0xfffe0000 ( 896 kB)
[ 0.000000] vmalloc : 0xef800000 - 0xfee00000 ( 246 MB)
[ 0.000000] lowmem : 0xc0000000 - 0xef600000 ( 758 MB)
[ 0.000000] pkmap : 0xbfe00000 - 0xc0000000 ( 2 MB)
[ 0.000000] modules : 0xbf000000 - 0xbfe00000 ( 14 MB)
[ 0.000000] .text : 0xc0008000 - 0xc0a65214 (10613 kB)
[ 0.000000] .init : 0xc0a66000 - 0xc0aa3100 ( 245 kB)
[ 0.000000] .data : 0xc0aa4000 - 0xc0b3ec88 ( 620 kB)
[ 0.000000] .bss : 0xc0b3ecac - 0xc0d203c8 (1926 kB)
外设的初始化
[ 0.173000] usbcore: registered new interface driver usbfs
[ 0.174000] usbcore: registered new interface driver hub
[ 0.174000] usbcore: registered new device driver usb
[ 0.174000] i2c-gpio i2c-gpio.0: using pins 99 (SDA) and 98 (SCL)
[ 0.175000] i2c-gpio i2c-gpio.3: using pins 80 (SDA) and 79 (SCL)
[ 0.176000] s3c-i2c s3c2440-i2c.1: i2c-1: S3C I2C adapter
[ 0.176000] s3c-i2c s3c2440-i2c.1: slave address 0x10
[ 0.176000] s3c-i2c s3c2440-i2c.1: bus frequency set to 97 KHz
[ 0.178000] s3c-i2c s3c2440-i2c.2: i2c-2: S3C I2C adapter
[ 0.178000] s3c-i2c s3c2440-i2c.2: slave address 0x10
[ 0.178000] s3c-i2c s3c2440-i2c.2: bus frequency set to 97 KHz
网络协议
[ 0.331000] NET: Registered protocol family 2
[ 0.331000] IP route cache hash table entries: 32768 (order: 5, 131072 bytes)
[ 0.332000] TCP established hash table entries: 131072 (order: 8, 1048576 bytes)
[ 0.334000] TCP bind hash table entries: 65536 (order: 8, 1572864 bytes)
[ 0.336000] TCP: Hash tables configured (established 131072 bind 65536)
[ 0.336000] TCP: reno registered
[ 0.336000] UDP hash table entries: 512 (order: 3, 32768 bytes)
[ 0.336000] UDP-Lite hash table entries: 512 (order: 3, 32768 bytes)
[ 0.337000] NET: Registered protocol family 1
[ 0.337000] RPC: Registered named UNIX socket transport module.
[ 0.337000] RPC: Registered udp transport module.
[ 0.337000] RPC: Registered tcp transport module.
[ 0.337000] RPC: Registered tcp NFSv4.1 backchannel transport module.
挂载根文件系统
[ 5.996000] VFS: Mounted root (ext4 filesystem) on device 179:2.
[ 6.003000] devtmpfs: mounted
[ 6.005000] Freeing init memory: 244K
[ 6.009000] Write protecting the kernel text section c0008000 - c0a2f000
[ 6.016000] rodata_test: attempting to write to read-only section:
[ 6.022000] write to read-only section trapped, success
[ 6.166000] EXT4-fs (mmcblk0p2): re-mounted. Opts: data=ordered
[ 6.539000] eth0: device MAC address 6e:3a:61:c8:01:f4
[ 6.546000] stmmac_open: failed PTP initialization
可以用mount命令查看所有的挂载信息和文件系统格式
[root@GEC6818 /]#mount
rootfs on / type rootfs (rw)
/dev/root on / type ext4 (rw,sync,relatime,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=405364k,nr_inodes=101341,mode=755)
proc on /proc type proc (rw,relatime)
devpts on /dev/pts type devpts (rw,relatime,gid=5,mode=620)
tmpfs on /dev/shm type tmpfs (rw,relatime,mode=777)
tmpfs on /tmp type tmpfs (rw,relatime)
tmpfs on /run type tmpfs (rw,nosuid,nodev,relatime,mode=755)
sysfs on /sys type sysfs (rw,relatime）
3、根文件系统
（1）概述
根文件系统是一个“包”，里面包含很多内容
- shell命令和可执行程序：/bin /sbin /usr/bin
- 设备文件： /dev
- 配置文件或脚本/etc 例如：/etc/profile就是一个脚本
- 库文件（动态库） *.so /lib /usr/lib
- /proc 该目录下的文件，存放了系统运行的实时状态信息
[root@GEC6818 /]#ls /proc/
1 22 49 buddyinfo meminfo
10 23 5 bus misc
100 24 50 cgroups modules
101 25 51 cmdline mounts
102 26 52 config.gz net
103 27 53 consoles pagetypeinfo
104 28 6 cpu partitions
105 29 7 cpuinfo sched_debug
以数字命名的目录文件,数字是进程的PID，目录下文件中存储的是进程的状态信息
- /sys 该目录下的文件存储了驱动相关信息
- /mnt ------挂载点
- /usr-----用户程序或库文件
- /opt ------第三方的大软件
- /val--------------系统日志目录
根文件系统挂载后：
- 执行脚本/etc/init.d/rcS
- 执行/etc/profile
- 装载驱动模块
- 执行iot应用程序。