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acpi_get_devices(const char *HID, acpi_walk_callback user_function,	 void *context, void **return_value) 根据形参指定的参数返回一个acpi_device，其中的HID 可能为null其源码分析如下：acpi_statusacpi_get_devices(const char *HID,		 acpi_walk_callback user_function,		 void *context, void **return_value){	acpi_status status;	struct acpi_get_devices_info info;	ACPI_FUNCTION_TRACE(acpi_get_devices);	/* Parameter validation */	#可以看到形参hid可以为null，但是user_function 不能为null	if (!user_function) {		return_ACPI_STATUS(AE_BAD_PARAMETER);	}	/*	 * We're going to call their callback from OUR callback, so we need	 * to know what it is, and their context parameter.	 */	 #用形参给info赋值	info.hid = HID;	info.context = context;	info.user_function = user_function;	/*	 * Lock the namespace around the walk.	 * The namespace will be unlocked/locked around each call	 * to the user function - since this function	 * must be allowed to make Acpi calls itself.	 */	status = acpi_ut_acquire_mutex(ACPI_MTX_NAMESPACE);	if (ACPI_FAILURE(status)) {		return_ACPI_STATUS(status);	}	#开始从root 遍历所有类型为device的设备，来匹配info中的信息以寻找对应的acpi_device，结果保存在return_value中返回	status = acpi_ns_walk_namespace(ACPI_TYPE_DEVICE, ACPI_ROOT_OBJECT,					ACPI_UINT32_MAX, ACPI_NS_WALK_UNLOCK,					acpi_ns_get_device_callback, NULL,					&info, return_value);	(void)acpi_ut_release_mutex(ACPI_MTX_NAMESPACE);	return_ACPI_STATUS(status);}我们继续看看acpi_ns_walk_namespace 中为查找acpi_device所有的callback函数static acpi_statusacpi_ns_get_device_callback(acpi_handle obj_handle,			    u32 nesting_level,			    void *context, void **return_value){	struct acpi_get_devices_info *info = context;	acpi_status status;	struct acpi_namespace_node *node;	u32 flags;	struct acpi_pnp_device_id *hid;	struct acpi_pnp_device_id_list *cid;	u32 i;	u8 found;	int no_match;	status = acpi_ut_acquire_mutex(ACPI_MTX_NAMESPACE);	if (ACPI_FAILURE(status)) {		return (status);	}	#检查handle是否有效	node = acpi_ns_validate_handle(obj_handle);	status = acpi_ut_release_mutex(ACPI_MTX_NAMESPACE);	if (ACPI_FAILURE(status)) {		return (status);	}	if (!node) {		return (AE_BAD_PARAMETER);	}	#search acpi_device分为两种情况，首先查找HID,如果没有查找到HID或者HID 为null，则开始执行user function查找	if (info->hid != NULL) {		#得到node的hid		status = acpi_ut_execute_HID(node, &hid);		if (status == AE_NOT_FOUND) {			return (AE_OK);		} else if (ACPI_FAILURE(status)) {			return (AE_CTRL_DEPTH);		}		#hid就是一个字符串，这里通过strcmp比较字符串是否相等		no_match = strcmp(hid->string, info->hid);		ACPI_FREE(hid);	#如果hid没有找到，则继续找cid，从for循环可以看出一个hid可以对应多个cid		if (no_match) {			/*			 * HID does not match, attempt match within the			 * list of Compatible IDs (CIDs)			 */			status = acpi_ut_execute_CID(node, &cid);			if (status == AE_NOT_FOUND) {				return (AE_OK);			} else if (ACPI_FAILURE(status)) {				return (AE_CTRL_DEPTH);			}			/* Walk the CID list */			#遍历所有的cid			found = FALSE;			for (i = 0; i < cid->count; i++) {				if (strcmp(cid->ids[i].string, info->hid) == 0) {					/* Found a matching CID */					found = TRUE;					break;				}			}			ACPI_FREE(cid);			if (!found) {				return (AE_OK);			}		}	}	/* Run _STA to determine if device is present */	#如果hid和cid 都没有匹配，则执行STA 来判断这个device是否存在	status = acpi_ut_execute_STA(node, &flags);	if (ACPI_FAILURE(status)) {		return (AE_CTRL_DEPTH);	}	if (!(flags & ACPI_STA_DEVICE_PRESENT) &&	    !(flags & ACPI_STA_DEVICE_FUNCTIONING)) {		/*		 * Don't examine the children of the device only when the		 * device is neither present nor functional. See ACPI spec,		 * description of _STA for more information.		 */		return (AE_CTRL_DEPTH);	}	/* We have a valid device, invoke the user function */	#hid和cid 都没有匹配，而且STA返回ok的话，则执行用户指定的user_function来继续查找acpi_device	status = info->user_function(obj_handle, nesting_level,				     info->context, return_value);	return (status);}

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