一、概述

嵌入式linux系統啟動過程一般包含以下幾個過程：

• 上電
• 執行IROM(晶片內部只讀儲存器)中固化的程式碼。
• 執行uboot
• 載入核心到記憶體，啟動核心。
• 掛載根檔案系統rootfs
• 執行其中的指令碼和應用程式。

二、核心的幾個組成部分介紹

1、uboot介紹

（1）概述

• 是一個通用的bootloader

• Boot:完成硬體的初始化

• Loader:載入作業系統核心，並啟動核心.

• uboot支援多種處理器架構：ARM、PowerPC 、X86 、 Mips等等。

• 支援載入啟動多種作業系統：linux、wince、VXworks等

• uboot的原始碼下載地址：
  https://ftp.denx.de/pub/u-boot/

（2）作用

• CPU的初始化（晶片級）主要採用ARM組合語言，包含初始化快取、MMC、Watchdog、終端、clock等。
• 板級初始化（採用C語言編寫）：主要初始化串列埠、網絡卡、USB、LCD等裝置.

• 為作業系統的啟動準備引數，載入作業系統映象到記憶體，並啟動.

(3)u-boot的開機列印資訊分析

U-Boot 2014.07 (Apr 10 2021 - 02:16:01)---U-boot版本號

//時鐘系統中各個總線上的時鐘頻率
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------------記憶體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---倒計時結束載入啟動核心，倒計時結束前按下鍵盤上的任意按鍵，進入U-BOOT命令列
 

（4）uboot的命令

=> help   //在uboot下執行help看uboot下的幫助文件
?         - 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:打印出開發板的資訊

GEC6818# bdinfo
arch_number = 0x000010EA-------開發板的編號，要與核心原始碼中開發板編號一致
boot_params = 0x40000100-------啟動核心所需引數在記憶體的0x40000100處
DRAM bank   = 0x00000000
-> start    = 0x40000000-----------記憶體的起始地址，結束地址0x7fffffff
-> size     = 0x40000000-----------記憶體的大小  1GB
eth0name    = dwmac.c0060000---網絡卡的名字
ethaddr     = 00:e2:1c:ba:e8:60 --網絡卡的MAC地址
current eth = dwmac.c0060000
ip_addr     = 192.168.51.5----------U-BOOT設定的網絡卡IP
baudrate    = 115200 bps-----------串列埠的波特率
TLB addr    = 0x7FFF0000
relocaddr   = 0x46000000
reloc off   = 0x00000000
irq_sp      = 0x7DF6BF00
sp start    = 0x43BFFE6　

• 啟動核心的命令

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：列印環境變數

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

兩個重要的環境變數：

bootargs啟動引數：告訴核心一些重要啟動資訊
lcd=at070tn92-------------LCD屏驅動晶片的型號
tp=gslx680-linux-----------觸控式螢幕的型號
root=/dev/mmcblk0p2----根檔案系統在emmc的第2個分割槽
rw rootfstype=ext4--------根檔案系統可讀可寫，型別是ext4

bootcmd--------------------啟動命令環境變數,告訴U-boot一些資訊
bootcmd=ext4load mmc 2:1 0x48000000 uImage;bootm 0x48000000
以ext4格式把核心映象uImage從emmc的第1分割槽載入到記憶體的0x48000000處，然後去到記憶體0x48000000處，啟動核心
bootdelay=3---------------倒計時3S
bootfile=uImage
ethact=dwmac.c0060000
ethaddr=00:e2:1c:ba:e8:60
ethprime=RTL8211------網絡卡晶片的型號

gatewayip=192.168.51.1-----------------預設閘道器
ipaddr=192.168.51.5---------------------開發板IP
netmask=255.255.255.0----------------子網掩碼
qtcrc=-1489582083
serverip=192.168.51.2-------------------伺服器IP
stderr=serial
stdin=serial
stdout=serial