SWNote/FAQ.md

1. What about WAN , and is it WAN can inside bridge or not ( display bridge info access brctl )?

WAN(Wide Area Network) , difference between LAN (Local Area Network) used to connect with Internet. Whatever WAN device and LAN device can work within Network or Link layer.

(Note : WAN interface 可以在 bridge 内，也可以不在 bridge 内， 在 bridge 内则是属于桥接模式，对应于网络的二层交换； 如果不在 bridge 内则属于路由模式对应于网络模型的三层路由交换。可以通过以下方式查看。)

$brctl show 

2. switch bewteen with ePon and gPon within MLD

Type	Working Mode	OLT Server
ePon	0 or 4	interface epon 0/12
gPon	1	interface gpon 0/2
xgPon	6	interface epon 0/10

display registed info with-in OLT server

telnet login root@172.25.17.250 , root/admin , xgpon password 0000000049 for mine.

>enable
#config
#interface epon 0/12  			(Note: interface gpon 0/2  ， interface xgpon 0/10)
#display ont info 0 all 		(Note: 查看所有的ONT状态)
#display ont info 0 1

ePon Register:

# atbp show
Number of MAC Addresses (1-32)    :  19  
Base MAC Address                  :  00:19:cb:0a:05:47  
Serial Number                     :  0019cb000001  
Password for Useradmin            :  0019cb000001  
SSID for first wireless (10-31)   :  Huatian  
Password for first wireless (8-31):  0019cb000001  
WAN working mode [0|1|2|3|4|5|6|7]:  4  
GPON Serial Number                :  MSTC12345678  
GPON Password                     :  1234567890  
Semi-manufactured SN              :  0Y0000000000  

more loid for register:

LOID	atbp Mac	Real Mac
mstcwxsw3loid26	00:19:CB:0A:05:57	0019-CB0A-0560
mstcwxsw3loid29	00:19:CB:0A:05:87	0019-CB0A-0590
mstcwxsw3loid27	00:19:CB:0A:05:67	0019-CB0A-0570

xgPon Register

# atbp show
Number of MAC Addresses (1-32)    :  19  
Base MAC Address                  :  00:19:cb:0a:05:47  
Serial Number                     :  0019cb000001  
Password for Useradmin            :  0019cb000001  
SSID for first wireless (10-31)   :  Huatian  
Password for first wireless (8-31):  0019cb000001  
WAN working mode [0|1|2|3|4|5|6|7]:  6  
GPON Serial Number                :  MSTC0B000047  
GPON Password                     :  1234567890  
Semi-manufactured SN              :  0Y0000000000 

(Note: should be upper case for gPon SN.)

3. maven problem when first clone and building MT7526G_CMCC

When first building project for "sysapps/public/gpl/felix_framework_cuc_cmcc" with maven while should downloads many manve dependence.

when download failed, could verify and copy the '/home/{user_name}/.m2' to the current user home path such like :

$ cp -R /home/don/.m2 /home/chear/

or update maven setting.xml while add Aliyun to maven repo , in this by '/usr/share/apache-maven-3.2.5/conf/setting.xml'

<mirrors>
 <!-- mirror
  | Specifies a repository mirror site to use instead of a given repository. The repository that
  | this mirror serves has an ID that matches the mirrorOf element of this mirror. IDs are used
  | for inheritance and direct lookup purposes, and must be unique across the set of mirrors.
  |
 <mirror>
   <id>mirrorId</id>
   <mirrorOf>repositoryId</mirrorOf>
   <name>Human Readable Name for this Mirror.</name>
   <url>http://my.repository.com/repo/path</url>
 </mirror>
  -->
  <mirror>
    <id>nexus-aliyun</id>
    <mirrorOf>*</mirrorOf>
    <name>Nexus aliyun</name>
    <url>http://maven.aliyun.com/nexus/content/groups/public</url>
  </mirror>
</mirrors>

3. different bewteen "aclocal" and "autoconf"

1. autoscan (autoconf): 扫描源代码以搜寻普通的可移植性问题，比如检查编译器，库，头文件等，生成文件configure.scan,它是configure.ac的一个雏形。
2. aclocal (automake):根据已经安装的宏，用户定义宏和acinclude.m4文件中的宏将configure.ac文件所需要的宏集中定义到文件 aclocal.m4中。aclocal是一个perl 脚本程序，它的定义是：“aclocal - create aclocal.m4 by scanning configure.ac”

user input files   optional input     process          output files
================   ==============     =======          ============

                    acinclude.m4 - - - - -.
                                          V
                                      .-------,
configure.ac ------------------------>|aclocal|
                 {user macro files} ->|       |------> aclocal.m4
                                      `-------'

3. autoheader(autoconf): 根据configure.ac中的某些宏，比如cpp宏定义，运行m4，声称config.h.in

user input files    optional input     process          output files
================    ==============     =======          ============

                    aclocal.m4 - - - - - - - .
                                             |
                                             V
                                     .----------,
configure.ac ----------------------->|autoheader|----> autoconfig.h.in
                                     `----------'

4. automake: automake将Makefile.am中定义的结构建立Makefile.in，然后configure脚本将生成的Makefile.in文件转换为Makefile。如果在configure.ac中定义了一些特殊的宏，比如AC_PROG_LIBTOOL，它会调用libtoolize，否则它会自己产生config.guess和config.sub

user input files   optional input   processes          output files
================   ==============   =========          ============

                                     .--------,
                                     |        | - - -> COPYING
                                     |        | - - -> INSTALL
                                     |        |------> install-sh
                                     |        |------> missing
                                     |automake|------> mkinstalldirs
configure.ac ----------------------->|        |
Makefile.am  ----------------------->|        |------> Makefile.in
                                     |        |------> stamp-h.in
                                 .---+        | - - -> config.guess
                                 |   |        | - - -> config.sub
                                 |   `------+-'
                                 |          | - - - -> config.guess
                                 |libtoolize| - - - -> config.sub
                                 |          |--------> ltmain.sh
                                 |          |--------> ltconfig
                                 `----------'

5. autoconf:将configure.ac中的宏展开，生成configure脚本。这个过程可能要用到aclocal.m4中定义的宏。

user input files   optional input   processes          output files
================   ==============   =========          ============

                   aclocal.m4 - - - - - -.
                                         V
                                     .--------,
configure.ac ----------------------->|autoconf|------> configure ----->autoconfig.h,Makefile

4. Print sal debug info for Hisilicon platfrom

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF0001000 dbg 0xff print 0xff sys 0

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF6003000 dbg 0xff print 0xff sys 0
(to debug wan.)

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF0004000 dbg 0xff print 0xff sys 1
(to debug sal emu service , such like hi_emu_speed.)

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF7003000 dbg 0xff print 0xff sys 1
(to debug hal )

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF600c000 dbg 0xff print 0xff sys 1
(to debug wlan )

root@OpenWrt:~# cli /home/cli/log_cmd/log/cfg_set -v module 0xF600d000 dbg 0xff print 0xff sys 1
(to debug traffic )

(Note: all debug macro defined at drivers/include/common/hi_sysdef.h)

5. Transfer data with in Hisilicon platform

cli /home/cli/hal/port/port_mirror_set -v igr 0x200 egr 0x200 dport 0

6. Linux Tools diagrams

Linux Static Performance Tools

Linux bcc /Berkeley Packet Filter Trancing Tools

Linux Tuning Tools

Linux Benchmark Tools

Linux Observability Tools

7. Switch the province

root@OpenWrt:~# hi_cfm set sysinfo.province jt
root@OpenWrt:~# rm -f /usr/local/factory/factory.txt
root@OpenWrt:~# hi_cfm config
root@OpenWrt:~# sync
root@OpenWrt:~# reboot
root@OpenWrt:~# hi_cfm test restore

8. ebtables & iptables

ebtables

ebtables is an application program used to set up and maintain the tables of rules (inside the Linux kernel) that inspect Ethernet frames. It is analogous to the iptables application, but less complicated, due to the fact that the Ethernet protocol is much simpler than the IP protocol. more detail in ebtables/iptables interaction on a Linux-based bridge

• prerouting：数据进来还未查询路由表之前的规则。
• input：由外部发往用户空间内部的规则。(说直白点就是负责过滤目标地址是本机的数据包)
• forward：不进入用户空间，进行路由转发的规则。(负责转发流经主机但不进入本机的数据包)
• postrouting：查询完路由表后，将要转发的规则。
• output：由用户空间内部发往外部的规则。(负责处理本机发出的数据包)

(Note: ebtables每个阶段的的过滤时机都比iptables要早。（这个不是绝对的从本机上层下来的报文经过postrouting是先 iptables 再 ebtables）

表 表是内置且固定的，共有三种: filter, nat, broute，用-t选项指定。最常用的就是filter了，所以不设-t时默认就是这个表。 filter过滤本机流入流出的数据包是默认使用的表，nat用于地址转换-mac地址，broute用于以太网桥-产生一个桥路器。 链（chains） 链有内置和自定义两种 。不同的表内置的链不同，这个从数据包的流程图中就可以看出来。所谓自定义的链也是挂接在对应的内置链内的，使用-j让其跳转到新的链中。如果以太网帧没有匹配到当前的规则，就会去检查下一个规则。(实例见使用场景介绍) 规则也叫目标（targets） 每个链中有一系列规则，每个规则定义了一些过滤选项。每个数据包都会匹配这些项，一但匹配成功就会执行对应的动作。 所谓动作，就是过滤的行为了。有四种，ACCEPT，DROP，RETURN和CONTINUE。 详解一下： 当帧匹配一个规则(rule)时，下一个动作(action)由target指定。

Usage:

• chains: There are three ebtables tables with built-in chains in the Linux kernel. These tables are used to divide functionality into different sets of rules. Each set of rules is called a chain. Each chain is an ordered list of rules that can match Ethernet frames. If a rule matches an Ethernet frame, then a processing specification tells what to do with that matching frame. The processing specification is called a 'target'. However,if the frame does not match the current rule in the chain, then the next rule in the chain is examined and so forth. The user can create new (user-defined) chains that can be used as the 'target' of a rule. User-defined chains are very useful to get better performance over the linear traversal of the rules and are also essential for structuring the filtering rules into well-organized and maintainable sets of rules.
• target: A firewall rule specifies criteria for an Ethernet frame and a frame processing specification called a target. When a frame matches a rule, then the next action performed by the kernel is specified by the target. The target can be one of these values: ACCEPT, DROP, CONTINUE, RETURN, an 'extension' (see below) or a jump to a user-defined chain.
  • ACCEPT means to let the frame through. DROP means the frame has to be dropped. In the BROUTING chain however,
  • the ACCEPT and DROP target have different meanings (see the info provided for the -t option).
  • CONTINUE means the next rule has to be checked. This can be handy, f.e., to know how many frames pass a certain point in the chain, to log those frames or to apply multiple targets on a frame. RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. For the extension targets please refer to the TARGET EXTENSIONS section of this man page.
• tables: As stated earlier, there are three ebtables tables in the Linux kernel. The table names are filter, nat and broute. Of these three tables, the filter table is the default table that the command operates on. If you are working with the filter table, then you can drop the '-t filter' argument to the ebtables command. However, you will need to provide the -t argument for the other two tables. Moreover, the -t argument must be the first argument on the ebtables command line, if used.

9. Linux File System

1. Romfs

   romfs is a space-efficient, small, read-only filesystem originally for Linux and used by some Linux based projects. It is a block-based filesystem, that means it makes use of block (or sector) accessible storage driver (like disks, CDs, ROM drives). It is part of stock Linux kernels since about version 2.1.21 (about January, 1997). All current (2.4--2.6) kernel sources contain support for romfs, but depending on the distributor, it might not have been compiled in.

   romfs makes two shortcuts: first, it's read-only, you can't simply use your disk if it's a romfs disk, you must build its image beforehand. Second, it stores only the absolute minimum required from a filing system. No modification dates, no unix permissions. These all might sound as large drawbacks, but they are usually not.

2. squashfs

   It is a read-only file system that lets you compress whole file systems or single directories, write them to other devices/partitions or to ordinary files, and then mount them directly (if a device) or using a loopback device (if it is a file). It compressed file systems used for tiny-sized and embedded Linux systems.

   The mksquashfs tool, which creates squashed file systems (in a file or on a block device) and the unsquashfs tool, which extracts multiple files from an existing squashed file system.

3. jffs2

   The Journaling Flash File System version 2 (JFFS2) was created by Red Hat from JFFS started . JFFS2 provides a filesystem directly on the flash, rather than emulating a block device, is a log-structured writable file system.

   JFFS2 has a log based structure which works like a Journal. Log based structures perform sequential writes from beginning to end to a log file, or a buffer. Once the space is filled, it performs similar to circular logging. If a file is updated, the old file remains and a new one is placed at the end of the log. When needed, the older file can be overwritten.

4. ubifs

   UBIFS is a new flash file system developed by Nokia engineers.In a way, UBIFS may be considered as the next generation of the JFFS2 file-system.

   JFFS2 file system works on top of MTD devices, but UBIFS works on top of UBI volumes and cannot operate on top of MTD devices. In other words, there are 3 subsystems involved:

   (Note: One thing people have to understand when dealing with UBIFS is that UBIFS is very different to any traditional file system - it does not work on top of block devices (like hard drives, MMC/SD cards, USB flash drives, SSDs, etc). UBIFS was designed to work on top of raw flash, which has nothing to do with block devices. This is why UBIFS does not work on MMC cards and the like - they look like block devices to the outside world because they implement FTL (Flash Translation Layer) support in hardware, which simply speaking emulates a block device on top of the built-in raw flash. Please, make sure you understand the differences between raw flash and, say, MMC flash before dealing with UBIFS.)

   • MTD subsystem, which provides uniform interface to access flash chips. MTD provides an notion of MTD devices (e.g., /dev/mtd0) which basically represents raw flash;

   • UBI subsystem, which is a wear-leveling and volume management system for flash devices; UBI works on top of MTD devices and provides a notion of UBI volumes , UBI volumes are higher level entities than MTD devices and they are devoid of many unpleasant issues MTD devices have (e.g., wearing and bad blocks); see here for more information;

   • UBIFS file system, which works on top of UBI volumes

5. cfamfs

   A compressed read-only file system for Linux. The maximum size of CRAMFS is 256MB.

10. LA to measurement SPI nand flash .

MOSI (Master in , Slave out) , MISO (Master out , Slave in) the master means for MCU , slave means for SPI nand flash diagram as below.

(Note: when encounter data missing for in MISO , check the its that noise ,connect VSS for the connected wired should be avoid noise.)

SPI Setting for LA as below, with in this test , the '0x0F 0xC0 0x00' its get feature command send to nand chip , and then nand response 0x00 0x00 0x01 this means the nand its busy ,can not operate programing or eraseing.

11. svn revert & svn upload -r [VERSION]

both svn revert and svn upload -r are reload version

( Note: when uploading files to svn trunk with Linux was better than Windows. svn add --no-ignore )

# svn up -r 10237 hisilicon/gateway/sercice/cm/odl/source/igdCmTransferServices.c

12. reset board environment for Hisilicon platform

After Recover default environment within hi_boot ,board environment value console will change to off , this value can cause console stop output message stream, then should change this value to on

hi # setenv console on
hi # saveenv

13. 'eabi' and 'oabi' for the gun

eabi (embedded application binary interface ) oabi (old application binary interface)

14. CPU IRQ Affinity

At first stop the IRQ binding service , then bind the IRQ for eth0 to CPU1 , step as following

# echo "2" > /proc/irq/90/smp_affinity
# cat /proc/interrupts 
           CPU0       CPU1       
  0:  918926335          0    IO-APIC-edge  timer
  1:          2          0    IO-APIC-edge  i8042
  8:          0          0    IO-APIC-edge  rtc
  9:          0          0   IO-APIC-level  acpi
 12:          4          0    IO-APIC-edge  i8042
 14:    8248017          0    IO-APIC-edge  ide0
 50:        194          0   IO-APIC-level  ohci_hcd:usb2
 58:      31673          0   IO-APIC-level  sata_nv
 90:    1070374          0         PCI-MSI  eth0
233:         10          0   IO-APIC-level  ehci_hcd:usb1
NMI:       5077       2032 
LOC:  918809969  918809894 
ERR:          0
MIS:          0

15. Regular Expression

$1,$2 ... is match content in the "()", $1 means match the first "()" , $2 means match the second "( )"

# Example 1
gainover123 
# /gai([\w]+?)over([\d]+)/ 
# $1 = n ; $2 = 123

# Example 2
PROVINCE_HUN
PROVINCE_TAJ
PROVINCE_HUB
# require replase 	'PROVINCE_HUN' to 'PROVINCE_HUN hun'
#					'PROVINCE_TAJ' to 'PROVINCE_TAJ taj'
# 					'PROVINCE_HUB' to 'PROVINCE_HUB hub'
# 
# Find 		"PROVINCE_([A-Z])([A-Z])([A-Z])"
# Replace 	"PROVINCE_$1$2$3 \L$1$2$3"

16 .Variable Scope for Shell script

local variable, global variable and environment variable

#!/bin/bash

function funcA(){
	# define local variable 'var', only useful with in 'funcA'
    local var=99;	
	# define global variable, working on this script.
    var2=100;
}
# define for environment variable.
export var3=200

echo $var2
echo $var3

17. kmalloc , kzmalloc , vmalloc

kmalloc to allocate serial physical memory space , has space limit, usually less than 128k.

/**
 * kmalloc - allocate memory
 * @size: how many bytes of memory are required.
 * @flags: the type of memory to allocate.
 *
 * kmalloc is the normal method of allocating memory
 * for objects smaller than page size in the kernel.
 *
 * The @flags argument may be one of:
 *
 * %GFP_USER - Allocate memory on behalf of user.  May sleep.
 *
 * %GFP_KERNEL - Allocate normal kernel ram.  May sleep.
 *
 * %GFP_ATOMIC - Allocation will not sleep.  May use emergency pools.
 *   For example, use this inside interrupt handlers.
 *
 * %GFP_HIGHUSER - Allocate pages from high memory.
 *
 * %GFP_NOIO - Do not do any I/O at all while trying to get memory.
 *
 * %GFP_NOFS - Do not make any fs calls while trying to get memory.
 *
 * %GFP_NOWAIT - Allocation will not sleep.
 *
 * %__GFP_THISNODE - Allocate node-local memory only.
 *
 * %GFP_DMA - Allocation suitable for DMA.
 *   Should only be used for kmalloc() caches. Otherwise, use a
 *   slab created with SLAB_DMA.
 *
 * Also it is possible to set different flags by OR'ing
 * in one or more of the following additional @flags:
 *
 * %__GFP_COLD - Request cache-cold pages instead of
 *   trying to return cache-warm pages.
 *
 * %__GFP_HIGH - This allocation has high priority and may use emergency pools.
 *
 * %__GFP_NOFAIL - Indicate that this allocation is in no way allowed to fail
 *   (think twice before using).
 *
 * %__GFP_NORETRY - If memory is not immediately available,
 *   then give up at once.
 *
 * %__GFP_NOWARN - If allocation fails, don't issue any warnings.
 *
 * %__GFP_REPEAT - If allocation fails initially, try once more before failing.
 *
 * There are other flags available as well, but these are not intended
 * for general use, and so are not documented here. For a full list of
 * potential flags, always refer to linux/gfp.h.
 */
static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
	if (__builtin_constant_p(size)) {
		if (size > KMALLOC_MAX_CACHE_SIZE)
			return kmalloc_large(size, flags);
#ifndef CONFIG_SLOB
		if (!(flags & GFP_DMA)) {
			int index = kmalloc_index(size);

			if (!index)
				return ZERO_SIZE_PTR;

			return kmem_cache_alloc_trace(kmalloc_caches[index],
					flags, size);
		}
#endif
	}
	return __kmalloc(size, flags);
}

vmalloc to allocate virtual memory , no space limited.

 /**
  *  vmalloc  -  allocate virtually contiguous memory
  *  @size:      allocation size
  *  Allocate enough pages to cover @size from the page level
  *  allocator and map them into contiguous kernel virtual space.
  * 
  *  For tight control over page level allocator and protection flags 
  *  use __vmalloc() instead. 
  */
 void *vmalloc(unsigned long size) 
 {
     return __vmalloc_node_flags(size, NUMA_NO_NODE,
                     GFP_KERNEL | __GFP_HIGHMEM); 
 }
 EXPORT_SYMBOL(vmalloc);

kzmalloc is for kmalloc and set zero for the space.

18. fopen & open

Both fopen and open are used to open file by specified path and file name , but fopen it is POSIX API , open its system call for Linux kernel.

Usually the fopen used to open text file , the open used to open device file.

FILE *fopen(const char *path, const char *mode);
/*
 r   Open text file for reading.  The stream is positioned at the beginning of the file.
   
 r+  Open for reading and writing.  The stream is positioned at the beginning of the file.
   
 w   Truncate file to zero length or create text file for writing.  The stream is positioned at the beginning of the file.
   
 w+  Open for reading and writing.  The file is created if it does not exist, otherwise it is truncated.  The stream is positioned at the beginning of the file.
   
 a   Open for appending (writing at end of file).  The file is created if it does not exist.  The stream is positioned at the end of the file.
   
 a+  Open  for reading and appending (writing at end of file).  The file is created if it does not exist.  The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.
 */


int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);

19 aligned for GUN C

aligned , 数组中所有元素都是紧挨着的, 为了提高内容的使用效率 ，需要把结构体大小变为 4 的整数倍，即要把结构补充成有效对齐大小的整数倍。 如果我们不把结构的大小补充为4的整数倍, 那么下一个结构很有可能不能满足结构的地址对齐了。

1. 数据类型自身的对齐值：对于char型数据，其自身对齐值为1，对于short型为2，对于int,float,double类型，其自身对齐值为4，单位字节。
2. 结构体或者类的自身对齐值是其成员中自身对齐值最大的那个值。
3. 指定对齐值：是指使用#pragma pack (value)时的指定对齐值value。
4. 数据成员、结构体和类的有效对齐值：自身对齐值和指定对齐值中小的那个值。

struct p
{
    int a;
    char b;
    short c;
}__attribute__((aligned(4))) pp;
 
struct m
{
    char a;
    int b;
    short c;
}__attribute__((aligned(4))) mm;
 
struct o
{
    int a;
    char b;
    short c;
}oo;
 
struct x
{
    int a;
    char b;
    struct p px;
    short c;
}__attribute__((aligned(8))) xx;
 
int main()
{           
    printf("sizeof(int)=%d,sizeof(short)=%d.sizeof(char)=%d\n",sizeof(int) ,sizeof(short),sizeof(char));
    printf("pp=%d,mm=%d \n", sizeof(pp),sizeof(mm));
    printf("oo=%d,xx=%d \n", sizeof(oo),sizeof(xx));
    return 0;
}

output :

sizeof(int)=4,sizeof(short)=2.sizeof(char)=1
pp=8,mm=12
oo=8,xx=24

20. Static Library & Share Library

使用动态链接生成的目标文件中凡是涉及第三方库的函数调用都是地址无关. 而在生成动态库的过程中需要将不同的目标文件 (*.o) 放在同一个代码段执行.

假如我们自己编写的程序名为Program 1，Program 1中调用了C标准库的printf()，在生成的目标文件中，不会立即确定printf()的具体地址，而是在运行时去装载这个函数，在装载阶段确定printf()的地址。这里提到的地址指的是进程在内存上的虚拟地址。动态链接库的函数地址在编译时是不确定的，在装载时，装载器根据当前地址空间情况，动态地分配一块虚拟地址空间。

而静态链接库其实是在编译时就确定了库函数地址。比如，我们使用了printf()函数，printf()函数对应有一个目标文件printf.o，静态链接时，会把printf.o链接打包到可执行文件中。在可执行文件中，printf()函数相对于文件头的偏移量是确定的，所以说它的地址在编译链接后就是确定的

21. open & fopen , read & fread & pread

open , fopen 最大的区别是 fopen 使用缓冲文件系统来打开文件，open 则是系统调用无法移植. 一般 open 用以打开你 linux 下的设备文件，而 fopen 则用来打开一般文件。最后 open 返回文件描述符，属于用户态，读写需进行用户态与内核态切换。 fopen 返回文件指针.

read 传入的是文件描述符， fread 则传入指向缓冲区保存或读取的数据。 在效率上 fread 为封装好的库函数，而read 为系统函数，一般来说，fread 效率更高。

22. Econet 紧急升级

1. GPIO_0 接地低电平