搭建tftp服务器
TFTP（Trivial File Transfer Protocol，简单文件传输协议），是一个基于UDP协议实现的用于在客户机和服务器之间进行简单文件传输的协议，适合于开销不大、不复杂的应用场合。tftp协议专门为小文件传输而设计，只能从服务器上获取文件，或者向服务器写入文件，不能列出目录，也不能进行认证。
在嵌入式Linux开发中，TFTP协议在很多场合都被用到，甚至是必不可少的，特别是在内核调试阶段，在U-Boot下通过tftp协议从服务器上加载内核并运行，既方便又快捷。
一般的发行版默认都没有安装tftp服务以及tftp服务器。tftp服务依赖于网络守护进程服务程序，可选择安装inetd或者升级版的xinetd，然后安装tftp和tftp-server，接着修改或者创建/etc/xinetd.d/tftp配置文件，最后重启xinetd，启动tftp服务。
安装xinetd：
$ sudo apt-get install xinetd安装tftp和tftpd：
$ sudo apt-get install tftp tftpd或者安装tftp的增强版本tftp-hpa：
$ sudo apt-get install tftp-hpa tftpd-hpa建立/etc/default/tftpd-hpa文件，写入如下内容：
# /etc/default/tftpd-hpa
TFTP_USERNAME="tftp"
TFTP_DIRECTORY="/opt/tftpboot"
TFTP_ADDRESS=":69"
TFTP_OPTIONS="--secure"其中TFTP_DIRECTORY设置的/var/tftpboot目录是tftp服务器的目录，设置其访问权限为777：
$ sudo mkdir -pv /opt/tftpboot/
$ sudo chmod 777 /opt/tftpboot/启动或者重启tftpd-hpa服务：
$ sudo service tftpd-hpa start // 或者restart在tftpboot目录下创建一个test.txt文件，启动另外一个终端，输入：
$ tftp 127.0.0.1
tftp> get test.txt # 获取test.txt文件
tftp> q # 退出tftp在终端输入ls命令，可以看到当前目录下获取到的test.txt文件。
开发板烧录程序
启动板子，系统打印如下日志：
U-Boot 1.1.3 (May 31 2017 - 12:06:08)
Board: Ralink APSoC DRAM: 128 MB
relocate_code Pointer at: 87f60000
******************************
Software System Reset Occurred
******************************
flash manufacture id: c2, device id 20 18
find flash: MX25L12805D
============================================
Ralink UBoot Version: 4.3.0.0
--------------------------------------------
ASIC 7628_MP (Port5<->None)
DRAM component: 1024 Mbits DDR, width 16
DRAM bus: 16 bit
Total memory: 128 MBytes
Flash component: SPI Flash
Date:May 31 2017 Time:12:06:08
============================================
icache: sets:512, ways:4, linesz:32 ,total:65536
dcache: sets:256, ways:4, linesz:32 ,total:32768
##### The CPU freq = 575 MHZ ####
estimate memory size =128 Mbytes
RESET MT7628 PHY!!!!!!
Please choose the operation:
1: Load system code to SDRAM via TFTP.
2: Load system code then write to Flash via TFTP.
3: Boot system code via Flash (default).
4: Entr boot command line interface.
7: Load Boot Loader code then write to Flash via Serial.
9: Load Boot Loader code then write to Flash via TFTP. 出现如上界面后，按下1。如果我们不能保证编译出来的固件是没有问题的，那么这个是最好的选项，因为它只会下载到内存运行而不会影响flash里的代码。
设置相应的IP地址和文件名，按下回车健就可以下载了：
1: System Load Linux to SDRAM via TFTP.
Please Input new ones /or Ctrl-C to discard
Input device IP (192.168.1.1) ==:192.168.0.100
Input server IP (192.168.1.100) ==:192.168.0.103
Input Linux Kernel filename () ==:uImage
netboot_common, argc= 3
NetTxPacket = 0x87FB3380
KSEG1ADDR(NetTxPacket) = 0xA7FB3380
NetLoop,call eth_halt !
NetLoop,call eth_init !
Trying Eth0 (10/100-M)
Waitting for RX_DMA_BUSY status Start... done
ETH_STATE_ACTIVE!!
TFTP from server 192.168.0.103; our IP address is 192.168.0.100
Filename 'uImage'.
TIMEOUT_COUNT=10,Load address: 0x80a00000
Loading: Got ARP REPLY, set server/gtwy eth addr (58:00:e3:4b:48:01)
Got it
#################################################################
#################################################################
###############################################################Got ARP REQUEST, return our IP
##
############################
done
Bytes transferred = 1139143 (1161c7 hex)
NetBootFileXferSize= 001161c7
Erasing SPI Flash...
.
Writing to SPI Flash...
.
done下载完毕后，系统就会自行运行起来，打印如下信息：
Automatic boot of image at addr 0x80A00000 ...
## Booting image at 80a00000 ...
Image Name: MIPS OpenWrt Linux-3.18.75
Image Type: MIPS Linux Kernel Image (lzma compressed)
Data Size: 1139079 Bytes = 1.1 MB
Load Address: 80000000
Entry Point: 80000000
Verifying Checksum ... OK
Uncompressing Kernel Image ... OK
No initrd
## Transferring control to Linux (at address 80000000) ...
## Giving linux memsize in MB, 128
Starting kernel ...
[ 0.000000] Linux version 3.18.75 (txgcwm@E470) (gcc version 4.8.3 (OpenWrt/Linaro GCC 4.8-2014.04 r49389) ) #5 Sun Oct 22 17:39:20 CST 2017
[ 0.000000] Board has DDR2
[ 0.000000] Analog PMU set to hw control
[ 0.000000] Digital PMU set to hw control
[ 0.000000] SoC Type: MediaTek MT7628AN ver:1 eco:2
[ 0.000000] bootconsole [early0] enabled
[ 0.000000] CPU0 revision is: 00019655 (MIPS 24KEc)
[ 0.000000] Determined physical RAM map:
[ 0.000000] memory: 08000000 @ 00000000 (usable)
[ 0.000000] Initrd not found or empty - disabling initrd
[ 0.000000] Zone ranges:
[ 0.000000] Normal [mem 0x00000000-0x07ffffff]
[ 0.000000] Movable zone start for each node
[ 0.000000] Early memory node ranges
[ 0.000000] node 0: [mem 0x00000000-0x07ffffff]
[ 0.000000] Initmem setup node 0 [mem 0x00000000-0x07ffffff]
[ 0.000000] Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes.
[ 0.000000] Primary data cache 32kB, 4-way, PIPT, no aliases, linesize 32 bytes
[ 0.000000] No valid device tree found, continuing without
[ 0.000000] Built 1 zonelists in Zone order, mobility grouping on. Total pages: 32512
[ 0.000000] Kernel command line: rootfstype=squashfs,jffs2
[ 0.000000] PID hash table entries: 512 (order: -1, 2048 bytes)
[ 0.000000] Dentry cache hash table entries: 16384 (order: 4, 65536 bytes)
[ 0.000000] Inode-cache hash table entries: 8192 (order: 3, 32768 bytes)
[ 0.000000] Writing ErrCtl register=0000aa00
[ 0.000000] Readback ErrCtl register=0000aa00
[ 0.000000] Memory: 126372K/131072K available (2520K kernel code, 127K rwdata, 516K rodata, 156K init, 186K bss, 4700K reserved, 0K cma-reserved)
[ 0.000000] SLUB: HWalign=32, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
[ 0.000000] NR_IRQS:256
[ 0.000000] Kernel panic - not syncing: Failed to find ralink,mt7620a-sysc node
[ 0.000000] ---[ end Kernel panic - not syncing: Failed to find ralink,mt7620a-sysc node可惜了，这次编译的系统并没有正常运行起来，但tftp功能正确运行起来了。
再次重启的时候，按下4，进入到uboot环境，观察一下环境变量，我们发现第一次的设置它已保存下来了，信息如下：
MT7628 # printenv
bootcmd=tftp
bootdelay=1
baudrate=115200
ethaddr="00:AA:BB:CC:DD:10"
tested=1
filesize=1161c7
fileaddr=80A00000
ipaddr=192.168.0.100
serverip=192.168.0.103
autostart=no
bootfile=uImage
stdin=serial
stdout=serial
stderr=serial
Environment size: 226/4092 bytes参考文章
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