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、内核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