mmc rescan简析

mmc rescan是mmc host的detect work的功能函数，该函数用于探测目标卡的类型并且根据mmc/sd/sdio协议进行comm的初始化。

void mmc_rescan(struct work_struct *work)
{
	static const unsigned freqs[] = { 400000, 300000, 200000, 100000 };
	struct mmc_host *host =
		container_of(work, struct mmc_host, detect.work);
	int i;
	bool extend_wakelock = false;

	if (host->rescan_disable)
		return;

	mmc_bus_get(host);

	/*
	 * if there is a _removable_ card registered, check whether it is
	 * still present
	 */
	if (host->bus_ops && host->bus_ops->detect && !host->bus_dead
	    && !(host->caps & MMC_CAP_NONREMOVABLE))
		host->bus_ops->detect(host);//这里所做的事情：当我们进入delay work时，我们首先要检查一下,目标卡是否依然存在，如果仍然存在而且目标卡是可插拔的，那么我们需要先将该卡在内核留下的一些痕迹清除，重新探测该卡。所以一般如果是目标卡复位的话，一般是调用内核提供的专用的复位接口(如sdio_reset_comm等)，而不是调度host的这个delay work,

	host->detect_change = 0;

	/* If the card was removed the bus will be marked
	 * as dead - extend the wakelock so userspace
	 * can respond */
	if (host->bus_dead)
		extend_wakelock = 1;

	/*
	 * Let mmc_bus_put() free the bus/bus_ops if we've found that
	 * the card is no longer present.
	 */
	mmc_bus_put(host);
	mmc_bus_get(host);

	/* if there still is a card present, stop here */
	if (host->bus_ops != NULL) { //判断是否bus的引用计数是否为0,如果不为0 ，说明bus仍然被引用，不能再继续下面的探测了。
		mmc_bus_put(host);
		goto out;
	}

	/*
	 * Only we can add a new handler, so it's safe to
	 * release the lock here.
	 */
	mmc_bus_put(host);

	if (host->ops->get_cd && host->ops->get_cd(host) == 0)
		goto out;

	mmc_claim_host(host);
	for (i = 0; i < ARRAY_SIZE(freqs); i++) {
		if (!mmc_rescan_try_freq(host, max(freqs[i], host->f_min))) {
			extend_wakelock = true;
			break;
		}
		if (freqs[i] <= host->f_min)
			break;
	}
	mmc_release_host(host);

 out:
	if (extend_wakelock)
		wake_lock_timeout(&host->detect_wake_lock, HZ / 2);
	else
		wake_unlock(&host->detect_wake_lock);
	if (host->caps & MMC_CAP_NEEDS_POLL) {
		wake_lock(&host->detect_wake_lock);
		mmc_schedule_delayed_work(&host->detect, HZ);
	}
}