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。
代码语言：javascript复制#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。
代码语言：javascript复制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。
代码语言：javascript复制INIT_DELAYED_WORK(&host->detect, mmc_rescan);mmc_rescan就是扫描卡的函数
代码语言：javascript复制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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