Linux提供兩種非同步的hotplug機制通知user-space裝置狀態的改變，一是usermode helper，另一個則是透過netlink。
usermode helper
每當kernel收到hotplug event便會執行"CONFIG_UEVENT_HELPER_PATH"（預設值是/sbin/hotplug，可以透過修改/proc/sys/kernel/hotplug修改預設值），在embedded system中，常用的是busybox，所以，通常會把UEVENT HELPER換成/sbin/mdev（找時間來寫寫busybox），執行UEVENT HELPER會攜帶一些環境變數，如ACTION/DEVPATH/SUBSYSTEM/HOME/PATH等等，透過這些環境變數，可以取得Device Name，MAJOR/MINOR number等等，我們先來看部份的程式碼，往後有機會在作深度研究。
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
char *envp_ext[])
{
... 略...
/* call uevent_helper, usually only enabled during early boot */
if (uevent_helper[0]) {
char *argv [3];
argv [0] = uevent_helper;
argv [1] = (char *)subsystem;
argv [2] = NULL;
retval = add_uevent_var(env, "HOME=/");
if (retval)
goto exit;
retval = add_uevent_var(env,
"PATH=/sbin:/bin:/usr/sbin:/usr/bin");
if (retval)
goto exit;
retval = call_usermodehelper(argv[0], argv,
env->envp, UMH_WAIT_EXEC);
}
... 略...
}
我利用一個shell script來取代UEVENT HELPER，主要目的是要印出有哪些環境變數，和傳遞哪些參數給UEVENT HELPER。
#!/bin/sh env >> /helper.log echo "CMD:" $@ >> /helper.log echo "---------- end ----------" >> /helper.log
netlink
另外一條路徑就是netlink了，基本上也是傳遞相同的資訊。
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
char *envp_ext[])
{
... 略...
#if defined(CONFIG_NET)
/* send netlink message */
if (uevent_sock) {
struct sk_buff *skb;
size_t len;
/* allocate message with the maximum possible size */
len = strlen(action_string) + strlen(devpath) + 2;
skb = alloc_skb(len + env->buflen, GFP_KERNEL);
if (skb) {
char *scratch;
/* add header */
scratch = skb_put(skb, len);
sprintf(scratch, "%s@%s", action_string, devpath);
/* copy keys to our continuous event payload buffer */
for (i = 0; i < env->envp_idx; i++) {
len = strlen(env->envp[i]) + 1;
scratch = skb_put(skb, len);
strcpy(scratch, env->envp[i]);
}
NETLINK_CB(skb).dst_group = 1;
retval = netlink_broadcast(uevent_sock, skb, 0, 1,
GFP_KERNEL);
/* ENOBUFS should be handled in userspace */
if (retval == -ENOBUFS) {
retval = 0;
}
} else {
retval = -ENOMEM;
}
}
#endif
... 略...
}
user-space的code如下：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/poll.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <linux/types.h>
#include <linux/netlink.h>
int main(int argc, char *argv[])
{
struct sockaddr_nl nls;
struct pollfd pfd;
char buf[512];
memset(&nls, 0, sizeof(nls));
nls.nl_family = AF_NETLINK;
nls.nl_pid = getpid();
nls.nl_groups = -1;
pfd.events = POLLIN;
pfd.fd = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT);
if (pfd.fd == -1) {
printf("Not root\n");
exit(1);
}
if (bind(pfd.fd, (void*)&nls, sizeof(nls))) {
printf("bind failed\n");
exit(1);
}
while (-1 != poll(&pfd, 1, -1)) {
int i, len = recv(pfd.fd, buf, sizeof(buf), MSG_DONTWAIT);
if (len == -1) {
printf("recv\n");
exit(1);
}
i = 0;
while (i < len) {
printf("%s\n", buf + i);
i += strlen(buf+i) + 1;
}
}
printf("\n");
return 0;
}
