字符驱动 poll方法小记（by liukun321咕唧咕唧）

     被poll方法卡了一天，只因对内核源码了解太少啊。

      看LDD3的poll的时候，就被书上所讲的搞得云里雾里，确实把握不了她的脉络，，当时想：不就是个poll吗，无非就是调个poll_wait嘛。。。SO,我就那么草草的从那章飘过了。

      可是poll一用起来，我汗了。搞了一天，进程只要睡过去就醒不过来。应用代码实现的功能很简单，怎么就醒不过来呢？显然，驱动有问题。

可是poll方法里没休眠啊，怎么睡得？

  我应用里调用了select函数，可能跟他的实现有关吧，于是找到下面的代码。  

int do_select(int n, fd_set_bits *fds, struct timespec *end_time)
{
	ktime_t expire, *to = NULL;
	struct poll_wqueues table;
	poll_table *wait;
	int retval, i, timed_out = 0;
	unsigned long slack = 0;

	rcu_read_lock();
	retval = max_select_fd(n, fds);
	rcu_read_unlock();

	if (retval < 0)
		return retval;
	n = retval;

	poll_initwait(&table);
	wait = &table.pt;
	if (end_time && !end_time->tv_sec && !end_time->tv_nsec) {
		wait = NULL;
		timed_out = 1;
	}

	if (end_time && !timed_out)
		slack = estimate_accuracy(end_time);

	retval = 0;
	for (;;) {
		unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;

		inp = fds->in; outp = fds->out; exp = fds->ex;
		rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;

		for (i = 0; i < n; ++rinp, ++routp, ++rexp) {
			unsigned long in, out, ex, all_bits, bit = 1, mask, j;
			unsigned long res_in = 0, res_out = 0, res_ex = 0;
			const struct file_operations *f_op = NULL;
			struct file *file = NULL;

			in = *inp++; out = *outp++; ex = *exp++;
			all_bits = in | out | ex;
			if (all_bits == 0) {
				i += __NFDBITS;
				continue;
			}

			for (j = 0; j < __NFDBITS; ++j, ++i, bit <<= 1) {
				int fput_needed;
				if (i >= n)
					break;
				if (!(bit & all_bits))
					continue;
				file = fget_light(i, &fput_needed);
				if (file) {
					f_op = file->f_op;
					mask = DEFAULT_POLLMASK;
					if (f_op && f_op->poll)
						mask = (*f_op->poll)(file, retval ? NULL : wait);
					fput_light(file, fput_needed);
					if ((mask & POLLIN_SET) && (in & bit)) {
						res_in |= bit;
						retval++;
					}
					if ((mask & POLLOUT_SET) && (out & bit)) {
						res_out |= bit;
						retval++;
					}
					if ((mask & POLLEX_SET) && (ex & bit)) {
						res_ex |= bit;
						retval++;
					}
				}
			}
			if (res_in)
				*rinp = res_in;
			if (res_out)
				*routp = res_out;
			if (res_ex)
				*rexp = res_ex;
			cond_resched();
		}
		wait = NULL;
		if (retval || timed_out || signal_pending(current))
			break;
		if (table.error) {
			retval = table.error;
			break;
		}

		/*
		 * If this is the first loop and we have a timeout
		 * given, then we convert to ktime_t and set the to
		 * pointer to the expiry value.
		 */
		if (end_time && !to) {
			expire = timespec_to_ktime(*end_time);
			to = &expire;
		}

		if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
					   to, slack))
			timed_out = 1;
	}

	poll_freewait(&table);

	return retval;
}

果然，在if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE, to, slack))timed_out = 1;因为这句进程睡过去了。kernel重新调度其他进程。

找到进程休眠，没找到在哪被唤醒啊。看源码可以知道它的休眠和wait_event_interrupt的实现类似，也是将进程，休眠在等待队列wait_queue_head_t上，那么唤醒应该也是类似的（目前只能这么猜了，对内核了解的太少了）。

难道这就是问题所在了？少了wake_up函数？

在应该激活该休眠进程的函数加上wake_up函数，跑一下，

哈，果然醒来了。