sunxi学习：[0]全志SoC启动过程分析
启动流程：Boot ROM→SPL→U-boot(→Kernel→RootFileSystem)
一、BROM阶段
全志SoC有一个非常具体的启动过程。首先，它执行一个微小的片上 Boot ROM（BROM），然后检查按钮的FEL模式，然后开始检查各种存储选项的有效的引导签名在正确的位置。
上电后，SoC开始从BROM所在的地址0xffff0000获取指令。BROM分为两部分：第一部分（0xffff0000）是FEL模式，第二部分是eGON.BRM（位于0xffff4000）。
FEL:FEL是包含在Allwinner设备上的BootROM中的低级子例程。它用于使用USB的设备的初始编程和恢复。所以，全志平台一般不会砖，就是有这个FEL。
一般来说，BROM首先检查SD卡(SD/MMC)启动可用性，其次再检查Nand Flash是否可以启动，然后再检查SPI Nor Flash的启动可用性，如果都无法启动则进入 FEL 模式。当程序初始化启动介质成功后，就从固定位置读入bootloader的Boot0到SRAM，然后跳到SRAM执行。
将u-boot烧录进SD卡
SD Card Layout：
|start|size|usage
|-|-|
|0 |8KB |Unused, available for partition table etc.
|8 |24KB |Initial SPL loader
|32 |512KB |U-Boot
|544 |128KB |environment
|672 |352KB |reserved
|1024 |- |Free for partitions
根据SD Card Layout表，使用dd将u-boot烧录进SD卡：
sudo dd if=u-boot-sunxi-with-spl.bin of=${card} bs=1024 seek=8
二、U-boot SPL阶段
SPL(Secondary Program Loader)程序流程如下：
- 初始化ARM处理器
- 初始化串口控制台
- 配置时钟和最基础的分频
- 初始化SDRAM
- 配置引脚多路复用功能
- 启动设备初始化（即上面选择的启动设备）
- 加载完整的u-boot/kernel程序并转交控制权
使用FEL/USBBoot启动spl
sudo ./sunxi-fel uboot u-boot-sunxi-with-spl.bin
U-Boot SPL 2017.03-rc2-gc7a25b3-dirty (Mar 11 2017 - 17:09:43)
DRAM: 256 MiB
Trying to boot from FEL
三、U-boot
由SPL引导进入的U-boot：
U-Boot 2017.03-rc2-gc7a25b3-dirty (Mar 11 2017 - 17:09:43 +0800) Allwinner Technology
CPU: Allwinner H3 (SUN8I 1680)
Model: Xunlong Orange Pi Zero
DRAM: 256 MiB
MMC: SUNXI SD/MMC: 0
MMC: no card present
mmc_init: -123, time 1
*** Warning - MMC init failed, using default environment
In: serial
Out: serial
Err: serial
Net: phy interface0
eth0: ethernet@1c30000
starting USB...
USB0: USB EHCI 1.00
USB1: USB OHCI 1.0
scanning bus 0 for devices... 1 USB Device(s) found
scanning usb for storage devices... 0 Storage Device(s) found
Hit any key to stop autoboot: 0
u-boot@sun8i $ ?
? - alias for 'help'
base - print or set address offset
bdinfo - print Board Info structure
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
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 /)
fatsize - determine a file's size
fatwrite- write file into a dos filesystem
fdt - flattened device tree utility commands
fstype - Look up a filesystem type
go - start application at address 'addr'
gpio - query and control gpio pins
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
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
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
pxe - commands to get and boot from pxe files
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
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
u-boot@sun8i $
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