hotplug為linux的一個熱拔插系統(tǒng)盒至，在很多應用都有用到椒功，如網(wǎng)口的拔插华烟，USB的拔插心墅，按鍵的觸發(fā)...非常廣泛的應用酿矢，下面舉幾個例子來進行理解這個過程

1.gpio-button-hotplug（內(nèi)核層發(fā)送hotplug到procd）

gpio-button-hotplug為kernel的一個package榨乎，位于package/kernel/gpio-button-hotplug/

要把CONFIG_PACKAGE_kmod-gpio-button-hotplug=y選項打開

1.1驅動

"gpio-keys"和"gpio-keys-polled"就是兩個platform設備，如下：

static struct platform_driver gpio_keys_driver = {
    .probe  = gpio_keys_probe,
    .remove = gpio_keys_remove,
    .driver = {
        .name   = "gpio-keys",
        .owner  = THIS_MODULE,
        .of_match_table = of_match_ptr(gpio_keys_of_match),
    },
};

static struct platform_driver gpio_keys_polled_driver = {
    .probe  = gpio_keys_polled_probe,
    .remove = gpio_keys_remove,
    .driver = {
        .name   = "gpio-keys-polled",
        .owner  = THIS_MODULE,
        .of_match_table = of_match_ptr(gpio_keys_polled_of_match),
    },
};

static int __init gpio_button_init(void)
{
    int ret;

    ret = platform_driver_register(&gpio_keys_driver);
    if (ret)
        return ret;

    ret = platform_driver_register(&gpio_keys_polled_driver);
    if (ret)
        platform_driver_unregister(&gpio_keys_driver);

    return ret;
}

static void __exit gpio_button_exit(void)
{
    platform_driver_unregister(&gpio_keys_driver);
    platform_driver_unregister(&gpio_keys_polled_driver);
}

module_init(gpio_button_init);
module_exit(gpio_button_exit);

與以前的做法一樣瘫筐，device和driver蜜暑，driver這邊有了，device要么arch里面要么dts里面策肝。

看下DTS里面的配置

gpio-keys-polled {
    compatible = "gpio-keys-polled";
    #address-cells = <1>;
    #size-cells = <0>;
    poll-interval = <20>;

    power {
        label = "power";
        gpios = <&gpio0 24 1>;  //GPIO24 line is low, key is pressed
        linux,code = <116>;     //KEY_POWER
    };
    reset {
        label = "reset";
        gpios = <&gpio1 11 1>;  //GPIO43 line is low, key is pressed
        linux,code = <0x198>;   //KEY_RESTART
    };
};

上面的這些參數(shù)数冬，在gpio_keys_get_devtree_pdata函數(shù)里面都會進行解析判斷格式是否正確钧排，錯誤在啟動注冊時就會直接提示錯誤信息。

另一種就是在arch里面注冊，
在zkernel/3.10.49/arch/mips/mtk/dev-gpio-buttons.c里面提供了注冊接口蚂子，調(diào)用即可绅这。

#define ZRMT7621_KEYS_POLL_INTERVAL         20
#define ZRMT7621_KEYS_DEBOUNCE_INTERVAL     (3 * ZRMT7621_KEYS_POLL_INTERVAL)

static struct gpio_keys_button zrmt7621_gpio_buttons[] __initdata = {
    {
        .desc       = "reset",
        .type       = EV_KEY,
        .code       = KEY_RESTART,
        .debounce_interval = ZRMT7621_KEYS_DEBOUNCE_INTERVAL,
        .gpio       = ZRMT7621_GPIO_BUTTON_RESET,
        .active_low = 1,
    },
};


ramips_register_gpio_buttons(-1, ZRMT7621_KEYS_POLL_INTERVAL,
         ARRAY_SIZE(zrmt7621_gpio_buttons),
                    zrmt7621_gpio_buttons);

在上面的button結構體里面要定義好對應的type和code赔桌，

這邊故意將linux,code里面的數(shù)值寫成數(shù)值瓢剿，是為了讓我們看到更底層的定義，在gpio-button-hotplug.c里面有如下定義帘睦，

static struct bh_map button_map[] = {
    BH_MAP(BTN_0,           "BTN_0"),
    BH_MAP(BTN_1,           "BTN_1"),
    BH_MAP(BTN_2,           "BTN_2"),
    BH_MAP(BTN_3,           "BTN_3"),
    BH_MAP(BTN_4,           "BTN_4"),
    BH_MAP(BTN_5,           "BTN_5"),
    BH_MAP(BTN_6,           "BTN_6"),
    BH_MAP(BTN_7,           "BTN_7"),
    BH_MAP(BTN_8,           "BTN_8"),
    BH_MAP(BTN_9,           "BTN_9"),
    BH_MAP(KEY_BRIGHTNESS_ZERO, "brightness_zero"),
    BH_MAP(KEY_CONFIG,      "config"),
    BH_MAP(KEY_COPY,        "copy"),
    BH_MAP(KEY_EJECTCD,     "eject"),
    BH_MAP(KEY_HELP,        "help"),
    BH_MAP(KEY_LIGHTS_TOGGLE,   "lights_toggle"),
    BH_MAP(KEY_PHONE,       "phone"),
    BH_MAP(KEY_POWER,       "power"),
    BH_MAP(KEY_RESTART,     "reset"),
    BH_MAP(KEY_RFKILL,      "rfkill"),
    BH_MAP(KEY_VIDEO,       "video"),
    BH_MAP(KEY_WIMAX,       "wwan"),
    BH_MAP(KEY_WLAN,        "wlan"),
    BH_MAP(KEY_WPS_BUTTON,      "wps"),
};

在include/dt-bindings/input/linux-event-codes.h里面有如下定義袍患，所以就知道最終的數(shù)值了。

#define KEY_RESTART     0x198

#define KEY_INSERT      110
#define KEY_DELETE      111
#define KEY_MACRO       112
#define KEY_MUTE        113
#define KEY_VOLUMEDOWN  114
#define KEY_VOLUMEUP    115
#define KEY_POWER       116 /* SC System Power Down */
#define KEY_KPEQUAL     117
#define KEY_KPPLUSMINUS 118
#define KEY_PAUSE       119
#define KEY_SCALE       120 /* AL Compiz Scale (Expose) */

驅動加載成功則會有如下信息：

root@zihome:/sys/devices/platform/gpio-keys-polled# ls
driver     modalias   subsystem  uevent

1.2 key的應用層處理

當按鍵時竣付，則觸發(fā)button_hotplug_event函數(shù)(gpio-button-hotplug.c)：調(diào)用button_hotplug_create_event產(chǎn)生uevent事件诡延，調(diào)用button_hotplug_fill_even填充事件(JSON格式)，并最終調(diào)用broadcast_uevent發(fā)出uevent廣播信息古胆，后由內(nèi)核netlink_broadcast函數(shù)(linux-3.10.49/net/netlink/af_netlink.c)

netlink的實現(xiàn)原理可以看下面這篇文件的介紹肆良，為socket通信，內(nèi)核發(fā)出socket廣播逸绎，上層應用(procd)只需要監(jiān)聽這個socket事件即可惹恃。
https://blog.csdn.net/Swallow_he/article/details/84073545

上述廣播，被procd進程中的hotplug_handler (procd/plug/hotplug.c) 收到棺牧，并根據(jù)etc/hotplug.json中預先定義的JSON內(nèi)容匹配條件巫糙，定位到對應的執(zhí)行函數(shù)，具體為：

[ "if",
        [ "and",
                [ "has", "BUTTON" ],
                [ "eq", "SUBSYSTEM", "button" ],
        ],
        [ "exec", "/etc/rc.button/%BUTTON%" ]
],

最終會執(zhí)行/etc/rc.button/里面的對應的腳本颊乘，如reset/power参淹，腳本的名字要跟button_map結構里面的一致。

root@LEDE:/# cat etc/rc.button/power 
#!/bin/sh

[ "${ACTION}" = "released" ] || exit 0

exec /sbin/poweroff

return 0

root@LEDE:/# cat etc/rc.button/reset 
#!/bin/sh

. /lib/functions.sh

OVERLAY="$( grep ' /overlay ' /proc/mounts )"

case "$ACTION" in
pressed)
        [ -z "$OVERLAY" ] && return 0

        return 5
;;
timeout)
        . /etc/diag.sh
        set_state failsafe
;;
released)
        if [ "$SEEN" -lt 1 ]
        then
                echo "REBOOT" > /dev/console
                sync
                reboot
        elif [ "$SEEN" -gt 5 -a -n "$OVERLAY" ]
        then
                echo "FACTORY RESET" > /dev/console
                jffs2reset -y && reboot &
        fi
;;
esac

return 0

2.WAN口網(wǎng)線是否插入檢測（phy內(nèi)核發(fā)出）

內(nèi)核檢測到WAN口變化后會創(chuàng)建hotplug消息（broadcast_uevent）乏悄，發(fā)送給procd浙值，再轉發(fā)到對應的模塊

static void phy_hotplug_work(struct work_struct *work)
{
    struct bh_event *event = container_of(work, struct bh_event, work);
    int ret = 0;

    event->skb = alloc_skb(BH_SKB_SIZE, GFP_KERNEL);
    if (!event->skb)
        goto out_free_event;

    ret = bh_event_add_var(event, 0, "%s@", event->action);
    if (ret)
        goto out_free_skb;

    ret = phy_hotplug_fill_event(event);
    if (ret)
        goto out_free_skb;

    if (event->type) {
        printk(KERN_NOTICE "phy: port%u %s(irq)\n", event->port_num, event->action);
    } else {
        printk(KERN_NOTICE "phy: port%u %s(dev)\n", event->port_num, event->action);
    }

    NETLINK_CB(event->skb).dst_group = 1;
    broadcast_uevent(event->skb, 0, 1, GFP_KERNEL);

 out_free_skb:
    if (ret) {
        kfree_skb(event->skb);
    }
 out_free_event:
    kfree(event);
}

發(fā)出后就會觸發(fā)以下腳本，在腳本里面添加我們需要的內(nèi)容

vim /etc/hotplug.d/phy/00-wan

case "$wan_ifname" in                                                
"eth"*)                                                              
        if [ "$wan_port" = "$PORTNUM" ]; then                        
                logger -t "phy" "$PORTNUM $ACTION"                   
                mkdir -p /tmp/status >/dev/null 2>&1                 
                case "$ACTION" in                                    
                "linkup")                                            
                        echo "1" >/tmp/status/wan_port_status        
                        ubus call zboard set_wan "{\"status\":1,\"port\":$wan_port}"
                        dhcp_handle_up "wan" "$wan_ifname"                          
                        ;;                                                          
                "linkdown")                                                         
                        echo "0" >/tmp/status/wan_port_status                       
                        ubus call zboard set_wan "{\"status\":0,\"port\":$wan_port}"
                        dhcp_handle_down "wan" "$wan_ifname"                        
                        ;;                                                          
                esac                                                                
                                                                                    
                phy_hotplug "wan" $ACTION                                           
        fi                                                                          
        ;;                                                                          
*)                                                                                  
        if [ "$wan_port" = "$PORTNUM" ]; then                                       
                logger -t "phy" "$PORTNUM $ACTION for wisp"                         
                mkdir -p /tmp/status >/dev/null 2>&1                                
                case "$ACTION" in                                                   
                "linkup")                                                           
                        echo "1" >/tmp/status/wan_port_status                       
                        ubus call zboard set_wan "{\"status\":1,\"port\":$wan_port}"
                        ;;                                                          
                "linkdown")                                                         
                        echo "0" >/tmp/status/wan_port_status                       
                        ubus call zboard set_wan "{\"status\":0,\"port\":$wan_port}"
                        ;;                                                          
                esac                                                                
        fi                                                                          
        ;;                                                                          
esac                

3.網(wǎng)絡檢測添加LED閃爍(/sbin/hotplug-call)

3.1 zdetect網(wǎng)絡檢測模塊

在zrouter/zpackages/zihome/utils/zdetect/src/zdetect.c里面會發(fā)送hotplug event當網(wǎng)絡變化的時候：

static void inet_hotplug(const char* action)
{
    char *argv[3];
    int pid;

    pid = fork();
    if (pid < 0) {
        dbg_printf(MSG_INFO, "hotplug_event fork failed!");
        return;
    } else if (pid == 0) {
        setenv("ACTION", action, 1); 

        argv[0] = HOTPLUG_PATH;
        argv[1] = "inet";
        argv[2] = NULL;
        execvp(argv[0], argv);
        exit(127);
    }   
}

3.2 收到inet模塊hotplug消息后

14.07/package/base-files/files/etc/hotplug.d/inet/00-inet

#!/bin/sh

case "$ACTION" in
        "online")
        logger -t "inet" "detect online"
        zihome_led yellow on
        ;;
        "offline")
        logger -t "inet" "detect offline"
        zihome_led yellow 1000 1000
        ;;
        *) ;;
esac

3.3 調(diào)用led執(zhí)行腳本

14.07/package/base-files/files/sbin/zihome_led

#!/bin/sh

. /lib/functions/leds.sh

led_path="zihome:""$1"

led

4.iface（netifd）

每次網(wǎng)絡接口啟動（up）或者關閉（down）的時候檩小，所有在/etc/hotplug.d/iface/目錄中的腳本都會以字母順序執(zhí)行开呐。根據(jù)一個不成文的規(guī)則，會在每個腳本的前面加上一個數(shù)字前綴來設置正確的運行順序。這就是為什么腳本名稱都像：/etc/hotplug.d/iface/<nn>-<scriptname>的原因负蚊。

https://blog.csdn.net/vivianliulu/article/details/79629836

static void call_hotplug(void)
{
    const char *device = NULL;
    if (list_empty(&pending))
        return;

    current = list_first_entry(&pending, struct interface, hotplug_list);
    current_ev = current->hotplug_ev;
    list_del_init(&current->hotplug_list);

    if ((current_ev == IFEV_UP || current_ev == IFEV_UPDATE) && current->l3_dev.dev)
        device = current->l3_dev.dev->ifname;

    D(SYSTEM, "Call hotplug handler for interface '%s', event '%s' (%s)\n",
    current->name, eventnames[current_ev], device ? device : "none");
    run_cmd(current->name, device, current_ev, current->updated);
}

static void run_cmd(const char *ifname, const char *device, enum interface_event event,
        enum interface_update_flags updated)
{
    char *argv[3];
    int pid;

    pid = fork();
    if (pid < 0)
        return task_complete(NULL, -1);

    if (pid > 0) {
        task.pid = pid;
        uloop_process_add(&task);
        return;
    }

    setenv("ACTION", eventnames[event], 1);
    setenv("INTERFACE", ifname, 1);
    if (device)
        setenv("DEVICE", device, 1);

    if (event == IFEV_UPDATE) {
        if (updated & IUF_ADDRESS)
            setenv("IFUPDATE_ADDRESSES", "1", 1);
        if (updated & IUF_ROUTE)
            setenv("IFUPDATE_ROUTES", "1", 1);
        if (updated & IUF_PREFIX)
            setenv("IFUPDATE_PREFIXES", "1", 1);
        if (updated & IUF_DATA)
            setenv("IFUPDATE_DATA", "1", 1);
    }

    argv[0] = hotplug_cmd_path;
    argv[1] = "iface";
    argv[2] = NULL;
    execvp(argv[0], argv);
    exit(127);
}