Linux内核MMC里的轮询机制

　　从这篇文章你能学到如何使用MMC框架里的轮询机制做探卡检测 ，十分简单。1 前言
　　最近遇到客户提的一个问题，大概意思是他们的SDIO Wi-Fi在卸载Wi-Fi驱动后再加载就检测不到Wi-Fi设备了。从他的问题看大概是热插拔有问题。
　　想要支持Wi-Fi复位后能重新扫描到Wi-Fi设备，需要host设置MMC_CAP_NEEDS_POLL 。#define MMC_CAP_NEEDS_POLL	(1 << 5)	/* Needs polling for card-detection */2 如何使用MMC里的轮询机制做探卡检测？
　　先说方法，后面再分析。
　　方法一：修改dts，在对应的节点增加字段broken-cd ，同时，如果有non-removable 字段，必须去掉该字段。
　　方法二：通过其他手段设置host->caps |= MMC_CAP_NEEDS_POLL 3 MMC里的轮询机制剖析3.1 在dts设置broken-cd字段，代码在哪里解析？
　　在mmc_of_parse 函数，路径是driversmmccorecore.c ，of_property_read_bool函数读broken-cd 字段，如果读到，就给host设置MMC_CAP_NEEDS_POLL 能力。int mmc_of_parse(struct mmc_host *host) {     /* 省略 */ 	/* 	 * Configure CD and WP pins. They are both by default active low to 	 * match the SDHCI spec. If GPIOs are provided for CD and / or WP, the 	 * mmc-gpio helpers are used to attach, configure and use them. If 	 * polarity inversion is specified in DT, one of MMC_CAP2_CD_ACTIVE_HIGH 	 * and MMC_CAP2_RO_ACTIVE_HIGH capability-2 flags is set. If the 	 * ＂broken-cd＂ property is provided, the MMC_CAP_NEEDS_POLL capability 	 * is set. If the ＂non-removable＂ property is found, the 	 * MMC_CAP_NONREMOVABLE capability is set and no card-detection 	 * configuration is performed. 	 */ 	/* Parse Card Detection */ 	if (of_property_read_bool(np, ＂non-removable＂)) { 		host->caps |= MMC_CAP_NONREMOVABLE; 		if (of_property_read_bool(np, ＂cd-post＂)) 			host->caps2 |= MMC_CAP2_CD_POST; 	} else { 		cd_cap_invert = of_property_read_bool(np, ＂cd-inverted＂); 		if (of_property_read_bool(np, ＂broken-cd＂)) 			host->caps |= MMC_CAP_NEEDS_POLL; 		ret = mmc_gpiod_request_cd(host, ＂cd＂, 0, true, 					   0, &cd_gpio_invert); 		if (!ret) 			dev_info(host->parent, ＂Got CD GPIO ＂); 		else if (ret != -ENOENT && ret != -ENOSYS) 			return ret; 		/* 		 * There are two ways to flag that the CD line is inverted: 		 * through the cd-inverted flag and by the GPIO line itself 		 * being inverted from the GPIO subsystem. This is a leftover 		 * from the times when the GPIO subsystem did not make it 		 * possible to flag a line as inverted. 		 * 		 * If the capability on the host AND the GPIO line are 		 * both inverted, the end result is that the CD line is 		 * not inverted. 		 */ 		if (cd_cap_invert ^ cd_gpio_invert) 			host->caps2 |= MMC_CAP2_CD_ACTIVE_HIGH; 	}     /* 省略 */ }3.2 探卡检测流程
　　mmc_alloc_host函数会创建一个工作队列，mmc_rescan与host->detect绑定。INIT_DELAYED_WORK(&host->detect, mmc_rescan);
　　mmc_rescan就是扫描卡的函数void mmc_rescan(struct work_struct *work) { 	struct mmc_host *host = 		container_of(work, struct mmc_host, detect.work); 	int i; 	if (host->trigger_card_event && host->ops->card_event) { 		host->ops->card_event(host); 		host->trigger_card_event = false; 	} 	if (host->rescan_disable) 		return; 	/* If there is a non-removable card registered, only scan once */ 	if ((host->caps & MMC_CAP_NONREMOVABLE) && host->rescan_entered) 		return; 	host->rescan_entered = 1; 	mmc_bus_get(host); 	/* 	 * if there is a _removable_ card registered, check whether it is 	 * still present 	 */ 	if (host->bus_ops && !host->bus_dead 	    && !(host->caps & MMC_CAP_NONREMOVABLE)) 		host->bus_ops->detect(host); 	host->detect_change = 0; 	/* 	 * 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) { 		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->caps & MMC_CAP_NONREMOVABLE) && host->ops->get_cd && 			host->ops->get_cd(host) == 0) { 		mmc_claim_host(host); 		mmc_power_off(host); 		mmc_release_host(host); 		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))) 			break; 		if (freqs[i] <= host->f_min) 			break; 	} 	mmc_release_host(host);  out: 	if (host->caps & MMC_CAP_NEEDS_POLL) 		mmc_schedule_delayed_work(&host->detect, HZ); }
　　看到最后两行，判断host的能力，如果设置了MMC_CAP_NEEDS_POLL，也就是轮询机制，就会每隔HZ（这是个宏）时间执行一次host->detect，也就是mmc_rescan。总结
　　对于探卡检测，通过在dts里面配置broken-cd 就可以实现轮询探卡检测。
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