本文以高通平臺上的android系統為參照展開探討。

1，uevent是什麼呢？
uevent是一種linux裝置模型中的一個組成部分。kset中包含的uevent_ops結構體擁有uevent的操作函式。
uevent可以在裝置發生變化時主動通知應用層。是對普通先註冊裝置後註冊驅動模式的一種補充。一般用作usb裝置的自動驅動載入、電池電量上報等。

2，uevent主動通知應用層的原理是什麼？
有兩種方式，第一種是設定環境變數後使用call_usermodehelper_setup函式直接呼叫應用層程式；第二種是通過netlink嚮應用層傳送訊息，在應用層的守護程序收到訊息後完成相關操作。其中第一種本方式較少使用，以第二種為主。
ps：netlink是一種基於socket的核心空間與使用者空間的雙向通訊機制，十分靈活好用。

3，sys目錄下有許多uevent節點，它們是幹什麼的？

最開始我對uevent的理解感到困惑就是因為這些uevent節點，如cat /sys/devices/soc/qpnp-smbcharger-17/power_supply/battery/uevent節點可以檢視到電池狀態，充放電電流等等資訊，好像uevent就像其他sysfs中的節點一樣僅僅供上層讀取。然而這並不是uevent的全部，在上面解釋的uevent機制中是完全不需要這些節點存在的，事實上上層也並不會去使用這些節點。這些節點的存在僅僅是為了除錯目的，可以提供一種簡單方式檢測到uevent的中間資訊。但在android的電源管理結構中電池的相關資訊是通過讀取/sys/devices/soc/qpnp-smbcharger-17/power_supply/battery/uevent節點獲取的，netlink訊息只發出KOBJ_CHANGE的action kobject_uevent(&cm->dev->kobj, KOBJ_CHANGE)，去觸發上層讀取sys節點。
注意：這些uevent節點與/dev/input/eventX節點雖然名字相似，但其原理和作用是完全不同的。eventX節點是輸入子系統的事件上報介面，需要上層來讀取eventX節點。

4，使用uevent上報電量的例項分析

kernel\drivers\power\qpnp-smbcharger.c
電量狀態變化的中斷函式中呼叫smbchg_charging_status_change函式

static int smbchg_charging_status_change(struct smbchg_chip *chip)
{
	smbchg_vfloat_adjust_check(chip);
	set_property_on_fg(chip, POWER_SUPPLY_PROP_STATUS,
			get_prop_batt_status(chip));
	return 0;
}

set_property_on_fg原始碼位於
kernel\drivers\power\qpnp-smbcharger.c

static int set_property_on_fg(struct smbchg_chip *chip,
		enum power_supply_property prop, int val)
{
	int rc;
	union power_supply_propval ret = {0, };


	if (!chip->bms_psy && chip->bms_psy_name)
		chip->bms_psy =
			power_supply_get_by_name((char *)chip->bms_psy_name);
	if (!chip->bms_psy) {
		pr_smb(PR_STATUS, "no bms psy found\n");
		return -EINVAL;
	}


	ret.intval = val;
	rc = chip->bms_psy->set_property(chip->bms_psy, prop, &ret);
	if (rc)
		pr_smb(PR_STATUS,
			"bms psy does not allow updating prop %d rc = %d\n",
			prop, rc);


	return rc;
}
 

set_property指標賦值位於qpnp-charger.c的prob函式中

static int qpnp_charger_probe(struct spmi_device *spmi)
{

	chip->batt_psy.set_property = qpnp_batt_power_set_property;
}

qpnp_batt_power_set_property函式的實現程式碼位置：kernel\drivers\power\qpnp-charger.c

此函式用於設定uevent節點，並且發出netlink訊息通知上層讀取

static int qpnp_batt_power_set_property(struct power_supply *psy,
				  enum power_supply_property psp,
				  const union power_supply_propval *val)
{
	struct qpnp_chg_chip *chip = container_of(psy, struct qpnp_chg_chip,
								batt_psy);
	int rc = 0;


	switch (psp) {
	case POWER_SUPPLY_PROP_COOL_TEMP:
		rc = qpnp_chg_configure_jeita(chip, psp, val->intval);
		break;
	case POWER_SUPPLY_PROP_WARM_TEMP:
		rc = qpnp_chg_configure_jeita(chip, psp, val->intval);
		break;
	case POWER_SUPPLY_PROP_CAPACITY:
		chip->fake_battery_soc = val->intval;	//將電量值賦值到fake_battery_soc變數
		power_supply_changed(&chip->batt_psy);	//呼叫power_supply_changed傳送netlink訊息，通知應用層重新讀取
		break;
	...
	default:
		return -EINVAL;
	}


	pr_debug("psy changed batt_psy\n");
	power_supply_changed(&chip->batt_psy);
	return rc;
}

power_supply_changed(&chip->batt_psy);原始碼位於kernel\drivers\power\power_supply_core.c

static void power_supply_changed_work(struct work_struct *work)
{
	unsigned long flags;
	struct power_supply *psy = container_of(work, struct power_supply,
						changed_work);


	dev_dbg(psy->dev, "%s\n", __func__);


	spin_lock_irqsave(&psy->changed_lock, flags);
	/*
	 * Check 'changed' here to avoid issues due to race between
	 * power_supply_changed() and this routine. In worst case
	 * power_supply_changed() can be called again just before we take above
	 * lock. During the first call of this routine we will mark 'changed' as
	 * false and it will stay false for the next call as well.
	 */
	if (likely(psy->changed)) {
		psy->changed = false;
		spin_unlock_irqrestore(&psy->changed_lock, flags);
		class_for_each_device(power_supply_class, NULL, psy,
				      __power_supply_changed_work);
		power_supply_update_leds(psy);
		atomic_notifier_call_chain(&power_supply_notifier,
				PSY_EVENT_PROP_CHANGED, psy);
		kobject_uevent(&psy->dev->kobj, KOBJ_CHANGE);
		spin_lock_irqsave(&psy->changed_lock, flags);
	}


	/*
	 * Hold the wakeup_source until all events are processed.
	 * power_supply_changed() might have called again and have set 'changed'
	 * to true.
	 */
	if (likely(!psy->changed))
		pm_relax(psy->dev);
	spin_unlock_irqrestore(&psy->changed_lock, flags);
}

其中kobject_uevent即傳送netlink訊息到上層kernel\lib\kobject_uevent.c

int kobject_uevent(struct kobject *kobj, enum kobject_action action)
{
	return kobject_uevent_env(kobj, action, NULL);
}
int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
		       char *envp_ext[])
{
	struct kobj_uevent_env *env;
	const char *action_string = kobject_actions[action];
	const char *devpath = NULL;
	const char *subsystem;
	struct kobject *top_kobj;
	struct kset *kset;
	const struct kset_uevent_ops *uevent_ops;
	int i = 0;
	int retval = 0;


	...
#if defined(CONFIG_NET)
	/* send netlink message */
	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
		struct sock *uevent_sock = ue_sk->sk;
		struct sk_buff *skb;
		size_t len;


		if (!netlink_has_listeners(uevent_sock, 1))
			continue;


		/* allocate message with the maximum possible size */
		len = strlen(action_string) + strlen(devpath) + 2;
		skb = alloc_skb(len + env->buflen, GFP_KERNEL);
		if (skb) {
			char *scratch;


			/* add header */
			scratch = skb_put(skb, len);
			sprintf(scratch, "%[email protected]%s", action_string, devpath);


			/* copy keys to our continuous event payload buffer */
			for (i = 0; i < env->envp_idx; i++) {
				len = strlen(env->envp[i]) + 1;
				scratch = skb_put(skb, len);
				strcpy(scratch, env->envp[i]);
			}


			NETLINK_CB(skb).dst_group = 1;
			retval = netlink_broadcast_filtered(uevent_sock, skb,	//傳送netlink訊息
							    0, 1, GFP_KERNEL,
							    kobj_bcast_filter,
							    kobj);
			/* ENOBUFS should be handled in userspace */
			if (retval == -ENOBUFS || retval == -ESRCH)
				retval = 0;
		} else
			retval = -ENOMEM;
	}
#endif
	mutex_unlock(&uevent_sock_mutex);


}