android adb源码分析(2)

本篇分析usb_linux_client.c中的usb_init()，它的代码如下：void usb_init()
{
if (access(USB_FFS_ADB_EP0, F_OK) == 0)
{
usb_ffs_init();

}
else{
usb_adb_init();

}
}
调用usb_adb_init():

static void usb_adb_init()
{
usb_handle *h;
adb_thread_t tid;
int fd;

h = calloc(1, sizeof(usb_handle));

h->write = usb_adb_write;
h->read = usb_adb_read;
h->kick = usb_adb_kick;
h->fd = -1;

adb_cond_init(&h->notify, 0);
adb_mutex_init(&h->lock, 0);

// Open the file /dev/android_adb_enable to trigger
// the enabling of the adb USB function in the kernel.
// We never touch this file again - just leave it open
// indefinitely so the kernel will know when we are running
// and when we are not.
fd = unix_open("/dev/android_adb_enable", O_RDWR);
if (fd < 0) {
D("failed to open /dev/android_adb_enable\n");
} else {
close_on_exec(fd);
}

D("[ usb_init - starting thread ]\n");
if(adb_thread_create(&tid, usb_adb_open_thread, h)){
fatal_errno("cannot create usb thread");
}
}

它初始化了usb_handle，并把它作为参数创建usb_adb_open_thread()线程。这里不能打开/dev/android_adb_enable。h->fd的值会在线程usb_adb_open_thread中赋值，并把它做为h->write(), h->read(), h->kick()的文件句柄，h->kick()函数功能是把h->fd置为-1。usb_adb_open_thread的代码如下：static void *usb_adb_open_thread(void *x)
{
struct usb_handle *usb = (struct usb_handle *)x;
int fd;

while (1) {
// wait until the USB device needs opening
adb_mutex_lock(&usb->lock);
while (usb->fd != -1)
adb_cond_wait(&usb->notify, &usb->lock);
adb_mutex_unlock(&usb->lock);

D("[ usb_thread - opening device ]\n");
do {
/* XXX use inotify? */
fd = unix_open("/dev/android_adb", O_RDWR);
if (fd < 0) {
// to support older kernels
fd = unix_open("/dev/android", O_RDWR);
}
if (fd < 0) {
adb_sleep_ms(1000);
}
} while (fd < 0);
D("[ opening device succeeded ]\n");

close_on_exec(fd);
usb->fd = fd;

D("[ usb_thread - registering device ]\n");
register_usb_transport(usb, 0, 0, 1);
}

// never gets here
return 0;
}
这个线程的作用是一进入立即打开/dev/android_adb或/dev/android，如果成功，则调用register_usb_transport()后再次循环，并阻塞在以下代码处
while(usb->fd != -1)
       adb_cond_wait(&usb->notify, &usb->lock);
当usb->kick()调用后fd的值被赋为-1，并发送cond唤醒上面的代码。