AM335X的SD卡更新系统学习记录

一般利用一张SD卡就能进行系统的更新，以前一直不知是什么原理，最近了解了下，对了解到的内容做个记录。使用的是AM335X平台，系统是Linux，文件系统是EXT3：

1、首先需要一张分好分区的SD卡（分区方法这里不做详细介绍，一般可使用TI官方提供的create-sdcard.sh脚本可以进行分区）。其中分区1（FAT32）存放MLO、u-boot.img、uImage三个文件；分区2（EXT3）存放需要从SD卡启动的Linux文件系统。

2、AM335X上电后，根据Boot Sequence启动配置（LCD0-LCD15引脚，具体可参考TI官方的335X参考手册），从相应的存储设备启动，这里配置的是从SD卡启动。

3、AM335X上电后首先执行固化在芯片内部ROM中的代码，我们称之为BLO，BLO的功能很强大，能读取SD卡中的第一个分区（必须是FAT格式）的MLO文件，复制到AM335X内部的RAM中吞下，复制的起始地址为0x402f0400，所以编译MLO时，连接地址必须设置为0x402F0400。

4、一般的，MLO的作用是：禁用中断、关看门狗、设置栈、初始化时钟、初始化DDR3、初始化串口、最后复制S卡中boot（FAT32）分区的u-boot.img到DDR3的0x80000000（内存）处运行。MLO的最大体积可以达到109K，对于简单的逻辑程序绰绰有余了。

5、u-boot的功能主要是启动内核：u-boot从SD卡的boot(FAT32)分区复制uImage到内存运行。

6、启动Linux后挂载SD卡的rootfs分区（EXT3）的文件系统

7、文件系统是利用Busybox工具集创建的，首先内核会启动BusyBox里的Init进程。init进程首先主要对/etc/inittab文件进行解析，然后按照它的指示创建各种子进程。

8、inittab文件的内容主要以下几部分。其中系统运行后最先执行的是/etc/rc.d/rcS进程，并且它只执行一次，其它进程只有等它执行完毕后才能运行，u-boot、内核、文件系统的烧写在这里执行。

 ::sysinit:/etc/rc.d/rcS
 ::respawn:/sbin/getty  tty1
 ::ctrlaltdel:/sbin/reboot
 ::shutdown:/etc/rc.d/rcS stop
 ::restart:/sbin/init

9、接着继续看到/etc/rc.d/rcS文件，这是一个脚本文件，下面一一分析它做的工作

#!/bin/sh

# minimal startup script, will work with msh (this is best available in
# MMUless format).

mode=${:-start}                           #若第一个参数存在，则mode的值为第一个参数；否则mode等于start

# load the configuration information
if [ -x /etc/rc.d/rc.conf ]
then
    . /etc/rc.d/rc.conf                    #不管/etc/rc.d/rc.conf是否可执行，都去执行它，等价于sh /etc/rc.d/rc.conf
fi

if [ $mode = "start" ]                     #若mode的值为start，
then
    services=$cfg_services                 #那么services等于$cfg_services（这个值在/etc/rc.d/rc.conf中定义："crond mount-proc-sys hostname depmod modules udev filesystems inetd"）
else
    services=$cfg_services_r               #否则services等于$cfg_services_r （这个值在/etc/rc.d/rc.conf中定义："inetd filesystems udev modules depmod hostname mount-proc-sys crond"）
fi
cfg_services=${:-$services}               #若第二个参数存在，则cfg_services的值为第二个参数；否则cfg_services等于services

# run the configured sequence              #按配置的顺序，启动各个进程
for i in $cfg_services
do
    if [ -x /etc/rc.d/init.d/$i ]             #如果进程是可执行的，那么
    then
        echo "/etc/rc.d/init.d/"$i $mode      #打印进程信息
        /etc/rc.d/init.d/$i $mode             #挨个执行
    fi
done

if [ $# -ge  ]                              #如果参数个数大于等于2
then
    exit                                    #退出
fi
if [ $mode = "start" ]                       #若mode的值为start，
then
    # show all kernel log messages
    if [ -x /proc/sys/kernel/printk ]        #若/proc/sys/kernel/printk可执行
    then
        #echo  >  /proc/sys/kernel/printk
        echo  >  /proc/sys/kernel/printk    #清空/proc/sys/kernel/printk
    fi

    # echo set kernel halt time
    echo -e "\033[9;0]" > /dev/tty0

    #echo Bringing lo up
    /sbin/ifconfig lo 127.0.0.1 up >/dev/null
    /sbin/route add -net 127.0.0.0 netmask 255.0.0.0 gw 127.0.0.1 dev lo >/dev/null

    #echo eth0 setting                              #网卡0设置
    if [ -x /etc/eth0-setting ]
    then
        echo "Found /etc/eth0-setting..."
        /etc/rc.d/ifconfig-eth0 /etc/eth0-setting   #在/etc/eth0-setting文件中配置IP、MASK、GATEWAY信息
    fi

    #echo eth1 setting                              #网卡1设置
    if [ -x /etc/eth1-setting ]
    then
        echo "Found /etc/eth1-setting..."
        /etc/rc.d/ifconfig-eth1 /etc/eth1-setting   #在/etc/eth1-setting文件中配置IP、MASK、GATEWAY信息
    fi

    #echo wlan0 setting                              #wlan0设置
    if [ -x /etc/wlan0-setting ]
    then
        echo "Found /etc/wlan0-setting..."
        wpa_supplicant -B -D wext -c /etc/wpa.conf -i wlan0
        wpa_cli add_network
        wpa_cli enable_network
        /etc/rc.d/ifconfig-wlan0 /etc/wlan0-setting
    fi

fi

# run rc.local if present and executable
if [ -x /etc/rc.d/rc.local ]                       #如果/etc/rc.d/rc.local可执行
then

    echo "Found /etc/rc.d/rc.local..."
    /etc/rc.d/rc.local $mode                       #那么，直接执行它。整个启动脚本的重点在这里，后面分析它
fi

# run startx if present and executable
if [ -x /usr/bin/startx ]
then
    if [ $mode = "start" ]
then
        rm /tmp/.X0-lock
        startx
    fi
fi

/etc/rc.d/rcS脚本可以总结为如下的功能：
a、配置脚本运行的环境变量，文件为/etc/rc.d/rc.conf
b、运行系统服务进程：crond mount-proc-sys hostname depmod modules udev filesystems inetd
c、设置网络IP/etc/rc.d/ifconfig-eth0 /etc/eth0-setting、/etc/rc.d/ifconfig-eth1 /etc/eth1-setting
d、运行/etc/rc.d/rc.local脚本程序

10、分析/etc/rc.d/rc.local start脚本程序

#!/bin/sh
mode=${:-start}                              #mode等于传入的参数1为start
if [ $mode = "start" ]                        #如果mode的值等于start，那么运行以下程序
then

#set sgtl5000 volume ~
amixer cset numid=                         #猜测是设置蜂鸣器音量，还不确定

echo "start format board mmc..."
echo  > /sys/class/leds/user_buzzer/brightness   #打开蜂鸣器，说明开始运行此脚本更新系统

/etc/rc.d/spi.sh                                  #运行/etc/rc.d/spi.sh脚本，把/boot/u-boot.spi.512拷贝到Flash中，以后程序从此处取出u-boot运行
    以下为/etc/rc.d/spi.sh脚本内容：
        #!/bin/sh

        echo "****************************************************"
        echo "****************************************************"
        echo ""
        echo " Hua Ming spi Flashing Script - 01/01/2015"
        echo ""

        echo "erase spi flash..."
        flash_erase /dev/mtd0  

        echo "write boot/u-boot.spi.512 ..."
        flashcp /boot/u-boot.spi. /dev/mtd0
        echo "done"    

/etc/rc.d/mmc.sh /dev/mmcblk1               #运行/etc/rc.d/mmc.sh脚本对mmc以ext3文件系统格式进行分区，分为4个区分别为sys、opt、user、Reserved。

mount /dev/mmcblk1p1 /mnt                   #挂接sys分区，将rfs_335x.tar.gz的文件进行解压缩，里面存放了最小根文件系统
cd /mnt
tar -xzvf ../rfs_335x.tar.gz

mount /dev/mmcblk1p2 /mnt/opt               #挂接opt分区，将HmReader.tar.gz解压缩，里面存放了应用程序
tar -xzvf ../../HmReader.tar.gz

cd boot
ln -s boot.scr.mmc1 boot.scr

sync

echo  > /sys/class/leds/user_buzzer/brightness    #蜂鸣器关闭
echo "format finished ..."                         #格式化结束，更新完成

fi

/etc/rc.d/rc.local start脚本程序功能总结如下：
1、将/boot/u-boot.spi.512拷贝到Flash中，在从spi启动时启动u-boot。
2、对MMC1进行分区，分为四个区分别为sys、opt、user、Reserved
3、将最小根文件系统解压缩到sys分区
4、将应用程序解压缩到opt分区

11、对/etc/rc.d/mmc.sh脚本的分析：

#!/bin/sh

echo "****************************************************"
echo "****************************************************"
echo ""
echo " Hua Ming Example Flashing Script - 01/01/2015"
echo ""

if [[ -z $ ]]                #如果传入脚本的参数1不存在的话
then
        echo "mmc.sh Usage:"
        echo "  mmc.sh <Dev>"
        echo "  Example: mmc.sh /dev/mmcblk0"
        exit                    #退出脚本
fi

STARTTIME=$(date +%s)            #获得当前时间

##---------Start of variables---------------------##

## Declare eMMC device name here
#DRIVE="/dev/mmcblk0"
DRIVE=$                    #DRIVE=/dev/mmcblk1

##----------End of variables-----------------------##

## Kill any partition info that might be there
dd if=/dev/zero of=$DRIVE bs=4k count=    #猜测是清除设备原有的分区
sync
sync                                       #马上写入设备

## Figure out how big the eMMC is in bytes
SIZE=`fdisk -l $DRIVE | grep Disk | awk '{print $5}'`    #算出块设备的大小（以字节为单位）

## Translate size into segments, which traditional tools call Cylinders. eMMC is not a spinning disk.
## We are basically ignoring what FDISK and SFDISK are reporting because it is not really accurate.
## we are translating this info to something that makes more sense for eMMC.
CYLINDERS=`echo $SIZE/// | bc`    #作数学运算算出一个柱面的大小

## Check to see if the eMMC partitions have automatically loaded from the old MBR.
## This might have occured during the boot process if the kernel detected a filesystem
## before we killed the MBR. We will need to unmount and kill them by writing 4k zeros to the
## partitions that were found.
#检查是否已经被挂载，并且已经分区。若有，则清除

check_mounted(){
  is_mounted=$(grep ${DRIVE}p /proc/mounts | awk '{print $2}')

  if grep -q ${DRIVE}p /proc/mounts; then
      echo "Found mounted partition(s) on " ${DRIVE}": " $is_mounted
      umount $is_mounted
      counter=
      for i in $is_mounted; do \
          echo "4k erase on ${DRIVE}p${counter}";
          dd if=/dev/zero of=${DRIVE}p${counter} bs=4k count=;
          counter=$((counter+));
      done
  else
      echo "No partition found. Continuing."
  fi
}

check_mounted;

## Partitioning the eMMC using information gathered.
## Here is where you can add/remove partitions.
## We are building  partitions:
##  . FAT, size =  cylinders *  heads *  sectors *  bytes/sec = ~70MB
##  . EXT3, size =  ($CYLINDERS-[ for fat]) cylinders *  heads *  sectors *  bytes/sec = ~1l.7GB
##
## You will need to change the lines ",9,0c0C,*", "10,,,-" to suit your needs.  Adding is similar,
## but you will need to be aware of partition sizes and boundaries.  Use the man page for sfdisk.
echo "Partitioning the eMMC..."
sfdisk -D -H  -S  -C $CYLINDERS $DRIVE << EOF
,,0x0c,*
,,,-
,,,-
,,,-
EOF

## This sleep is necessary as there is a service which attempts
## to automount any filesystems that it finds as soon as sfdisk
## finishes partitioning.  We sleep to let it run.  May need to
## be lengthened if you have more partitions.
sleep 

## Check here if there has been a partition that automounted.
##  This will eliminate the old partition that gets
##  automatically found after the sfdisk command.  It ONLY
##  gets found if there was a previous file system on the same
##  partition boundary.  Happens when running this script more than once.
##  To fix, we just unmount and write some zeros to it.
check_mounted;

## Clean up the dos (FAT) partition as recommended by SFDISK
#dd if=/dev/zero of=${DRIVE}p1 bs= count=

## Make sure posted writes are cleaned up
sync
sync

## Format the eMMC into  partitions
echo "Formatting the eMMC into 4 partitions..."

mkfs.ext3 -L "sys" ${DRIVE}p1
mkfs.ext3 -L "opt" ${DRIVE}p2
mkfs.ext3 -L "user" ${DRIVE}p3
mkfs.ext3 -L "Reserved" ${DRIVE}p4

## Make sure posted writes are cleaned up
sync
sync
echo "Formatting done."            #格式化完成

## The block below is only necessary if using 'dd'.
## Force check the filesystem for consistency and fix errors if any.
## Resize partition to the length specified by the MBR.
## /sbin/e2fsck -fy ${DRIVE}p2
## /sbin/resize2fs ${DRIVE}p2

ENDTIME=$(date +%s)
echo "It took $(($ENDTIME - $STARTTIME)) seconds to complete this task..."    #计算格式化EMMC花了多长时间
## Reboot
echo ""
echo "********************************************"
echo " Hua Ming  Example Flash Script is complete."
echo ""

总结为以ext3文件系统格式对MMC进行分区，分为4个区分别为sys、opt、user、Reserved。其中sys存放最小根文件系统，opt存放应用程序，后续可以对它单独进行更新。

12、到这里所有系统更新完成，拔掉SD卡，重新上电后根据Boot Sequence启动配置（LCD0-LCD15引脚，具体可参考TI官方的335X编程手册），从相应的存储设备启动，这里配置的是SPI接口的Flash启动。说启动可能不准确，其实是内部的ROM从SPI的flash中拷贝镜像到RAM中运行。然后启动u-boot，u-boot启动后会从MMC的/sys分区的/boot目录下取出uImage镜像再到RAM中运行，接着Linux系统挂接到/sys系统。然后整个系统正常运行。