mdev 實現 SD card 或 USB mass storage 自動掛載(automount)

Embedded system中實現 SD card 或 USB mass storage 自動掛載(automount) ,是透過
busybox mdev來達成.

我分成幾個部份來討論實做：1.編譯組態  2.設定檔  3.程式流程

[1.編譯組態]

    組態部份需要編譯linux kernel與busybox.
==========
linux kernel:
==========
1. 檢查kernel路徑下 .config當案;確定選取了底下組態:
CONFIG_HOTPLUG = y
CONFIG_PROC_FS=y
CONFIG_SYSFS=y
CONFIG_NET=y
CONFIG_PROC_SYSCTL=y
<note1>如果選項未選取,請經由make menuconfig選取;請不要編輯.config!
<note2> Kernel 選項:
>>> Generic Driver Options
() path to uevent helper    /* 可以不用現在更改;由設定檔來更改;下面說明.*/
[*] Maintain a devtmpfs filesystem to mount at /dev
[*] Automount devtmpfs at /dev, after the kernel mounted the rootfs


==========
busybox-1.19.4
==========
>>> Build Options (我將busybox編譯為靜態連結)
[*] Build BusyBox as a static binary (no shared libs)

>>> Linux System Utilities
[*] mdev
[*] Support /etc/mdev.conf

[*] Support command execution at device addition/removal

[2.設定檔]

在設定檔,首先要更改初始化的script;然後更改/etc/mdev.conf
========
 init script
========
根據busybox說明文件(參考文獻1),需要在初始化過程執行script中加入底下設定!

Here's a typical code snippet from the init script:

[0] mount -t proc proc /proc
[1] mount -t sysfs sysfs /sys
[2] echo /sbin/mdev > /proc/sys/kernel/hotplug
[3] mdev -s

Alternatively, without procfs the above becomes:
[1] mount -t sysfs sysfs /sys
[2] sysctl -w kernel.hotplug=/sbin/mdev
[3] mdev -s


Of course, a more "full" setup would entail executing this before the previous
code snippet:
[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev
[5] mkdir /dev/pts
[6] mount -t devpts devpts /dev/pts

我的範例：/squashfs_init (我的embedded system根目錄下的init)

#!/bin/busybox ash

echo "run squashfs_init"

/bin/busybox mount -t tmpfs -o mode=0755 tmpfs /dev

/bin/busybox mount -t tmpfs -o mode=0777 tmpfs /tmp

/bin/busybox mount -t tmpfs -o mode=0755 tmpfs /var

/bin/busybox mount -t tmpfs -o mode=0755 tmpfs /bin

/bin/busybox mount -t tmpfs -o mode=0755 tmpfs /usr

/bin/busybox mount -t tmpfs -o mode=0755 tmpfs /sbin

/bin/busybox mkdir -p /var/run

/bin/busybox mkdir -p /var/locks

/bin/busybox mkdir -p /dev/sys

/bin/busybox mkdir -p /dev/pts

/bin/busybox mkdir -p /dev/shm

/bin/busybox mkdir -p /usr/bin

/bin/busybox mkdir -p /usr/sbin

/bin/busybox mount -t proc   proc   /proc

/bin/busybox mount -t devpts devpts /dev/pts

/bin/busybox mount -t sysfs sysfs /sys

# Populate /dev according to /sys

#echo /etc/dt_sd.sh  > /proc/sys/kernel/hotplug

/bin/busybox --install -s

/bin/echo /sbin/mdev  > /proc/sys/kernel/hotplug

/sbin/mdev -s

ln -sf /bin/busybox /bin/linuxrc

ln -s /bin/wpa_supplicant  /bin/wpa_supplicant

ln -s /bin/wpa_cli /bin/wpa_cli

/bin/linuxrc

exec /sbin/init "$@" </dev/console >/dev/console 2>&1

<note1> /bin/echo /sbin/mdev > /proc/sys/kernel/hotplug

mdev使用event來實作熱插拔動作(hotplug),上述這個設定會改變Linux kernel中的
event_helper 變數;當熱插拔動作發生時,Linux kernel會呼叫event_helper所指的user mode程式;
如此mdev便會被執行並判斷哪一個裝置發生熱插拔,並且mdev會根據mdev.conf執行內含的command.所以我們只要將對應的動作告訴mdev.conf,它就會完成我們想要完成的熱插拔動作.

=========
made.conf
=========
根據busybox說明文件(參考文獻1),檔案format如下：

<device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
    or
<device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
    or
<device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]

我的範例：/etc/mdev.conf

sd[a-z][0-9] 0:0 660  */etc/init.d/automount.sh

mmcblk[0-9]p[0-9] 0:0 660 */etc/init.d/automount.sh

<note>當熱插拔事件發生,mdev便會根據mdev.conf敘述進而執行automount.sh

我的範例：/etc/init.d/automount.sh

#!/bin/sh 

/bin/touch /var/plog

if [ "$MDEV" == "" ]; then

   echo "automount unknow error!" >> /var/plog

   exit 1

elif [ "$MDEV" == "mmcblk0p1" ]; then

    mmtype=sdcard

elif [ "$MDEV" == "sda1" ]; then 

    mmtype=usb

else

   echo "automount unknow dev:" >> /var/plog

   echo $MDEV >> /var/plog

   exit 1

fi

echo $mmtype >> /var/plog

case "$ACTION" in  

    add|"") 

       echo “Add device $MDEV” >> /var/plog

       mount -t vfat /dev/$MDEV /mnt/$mmtype

        ;;  

    remove) 

       echo “remove device $MDEV” >> /var/plog 

       umount /mnt/$mmtype

       ;;  
esac

<note1>由於event_helper將輸出轉向至/dev/null,所以我建立一個plog;將輸出全部丟往plog.

<note2> ACTION/MDEV/SUBSYSTEM ：是event_helper 參數.


ACTION: add / remove
MDEV: sda1 / mmcblk0p1 ...SUBSYSTEM: block /mmc /drivers /queues /module /ftgpio_pdp /misc /platform /i2C...

以上,熱插拔自動掛載已經完成;接下來要談談發生熱插拔uevent_helper流程！

[3.程式流程]

當熱插拔(hotplug)發生時;kernel會送一個event,是由底下kernel程式負責:
/*./lib/kobject_uevent.c*/

int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,

               char *envp_ext[])

{ ...

}

程式分析當下熱插拔是由哪一個subsystem發生;發生什麼事情;並將熱插拔事件所有參數搜集後呼叫call_usermodehelper.

   

    subsystem = uevent_ops->name(kset, kobj); 

...

    retval = add_uevent_var(env, "ACTION=%s", action_string);

    if (retval)

        goto exit;

    retval = add_uevent_var(env, "DEVPATH=%s", devpath);

    if (retval)

        goto exit;

    retval = add_uevent_var(env, "SUBSYSTEM=%s", subsystem);

    if (retail)

        goto exit;

...

    retval = call_usermodehelper(argv[0], argv,

                         env->envp, UMH_WAIT_EXEC);

...

/*./include/linux/kmod.h*/

call_usermodehelper(char *path, char **argv, char **envp, enum umh_wait wait)

{

     return call_usermodehelper_fns(path, argv, envp, wait,

                        NULL, NULL, NULL);
}

接著經過一些work queue初始化過程;程式呼叫如下：

呼叫>> ____call_usermodehelper(void *data)                // ./kernel/kmod.c

呼叫>> kernel_execve( )                                                  //./arch/arm/kernel/sys_arm.c

int kernel_execve(const char *filename,

            const char *const argv[],

            const char *const envp[])

{

...

    ret = do_execve(filename,

              (const char __user *const __user *)argv,

              (const char __user *const __user *)envp, &regs);

...

}

呼叫>> do_execve_common(  )                                        /* ./fs/exec.c */

static int do_execve_common(const char *filename,

                 struct user_arg_ptr argv,

                 struct user_arg_ptr envy,

                 struct pt_regs *regs)

{

...

}

以上!

[參考文獻]
1. made.txt