Linux内核MMC里的轮询机制 _Linux

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。

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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 GPIOn");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 。