1 概念

总线是设备模型当中的基础组件，简化了驱动的编写和维护，总线可以是物理总线（如PCI，USB）或虚拟总线.

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struct bus_type {
	const char		*name; // 总线类型名称
	const char		*dev_name;
    // 老版本内核这里可能不一样
	const struct attribute_group **bus_groups; // 总线属性
	const struct attribute_group **dev_groups; // 设备属性
	const struct attribute_group **drv_groups; // 驱动属性

	int (*match)(struct device *dev, const struct device_driver *drv); // 匹配检查
	int (*uevent)(const struct device *dev, struct kobj_uevent_env *env);
	int (*probe)(struct device *dev);
	void (*sync_state)(struct device *dev);
	void (*remove)(struct device *dev);
	void (*shutdown)(struct device *dev);
	const struct cpumask *(*irq_get_affinity)(struct device *dev,
			unsigned int irq_vec);

	int (*online)(struct device *dev);
	int (*offline)(struct device *dev);

	int (*suspend)(struct device *dev, pm_message_t state);
	int (*resume)(struct device *dev);

	int (*num_vf)(struct device *dev);

	int (*dma_configure)(struct device *dev);
	void (*dma_cleanup)(struct device *dev);

	const struct dev_pm_ops *pm;
    // 这里之前存在 struct subsys_private *p
    // p 属于 bus 的实现细节，不应该暴露在公共的 bus_type 结构里.
    // 把它从 bus_type 里移走，放进 bus.c 的实现文件中，用 私有封装管理.

	bool need_parent_lock;
};

struct subsys_private {
	struct kset subsys; // 总线在内核的表示
	struct kset *devices_kset; // 属于总线的 device 的 kset
	struct list_head interfaces;
	struct mutex mutex;

	struct kset *drivers_kset; // 属于总线的 driver 的 kset
	struct klist klist_devices;
	struct klist klist_drivers;
	struct blocking_notifier_head bus_notifier;
	unsigned int drivers_autoprobe:1;
	const struct bus_type *bus;
	struct device *dev_root;

	struct kset glue_dirs;
	const struct class *class;

	struct lock_class_key lock_key;
};

我们可以看看每个 /sys/bus 的结构.

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$ ls /sys/bus/gpio/
devices  drivers  drivers_autoprobe  drivers_probe  uevent

2 bus 的注册

我们看看 bus 注册的过程, bus_register.

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bus_register
|-kzalloc => subsys_private 空间
|-kobject_set_name => 设置 bus 名到 kobj
|-kset_register => 在 /sys/bus/ 下创建 kobj 目录及其 attribute 文件
|-bus_create_file => 创建 uevent 文件
|-kset_create_and_add => 创建 /sys/bus/xxx/devices 文件
|-kset_create_and_add => 创建 /sys/bus/xxx/drivers 文件
|-add_probe_files => 创建 drivers_probe 和 drivers_autoprobe 文件
|-sysfs_create_groups => 创建 bus->bus_group 属性文件
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int bus_register(const struct bus_type *bus)
{
	int retval;
	struct subsys_private *priv;
	struct kobject *bus_kobj;
	struct lock_class_key *key;

	priv = kzalloc(sizeof(struct subsys_private), GFP_KERNEL); // kzalloc
	if (!priv)
		return -ENOMEM;

	priv->bus = bus;

	BLOCKING_INIT_NOTIFIER_HEAD(&priv->bus_notifier);

	bus_kobj = &priv->subsys.kobj;
	retval = kobject_set_name(bus_kobj, "%s", bus->name); // kobject_set_name
	if (retval)
		goto out;

	bus_kobj->kset = bus_kset;
	bus_kobj->ktype = &bus_ktype;
	priv->drivers_autoprobe = 1;

	retval = kset_register(&priv->subsys); // kset_register
	if (retval)
		goto out;

	retval = bus_create_file(bus, &bus_attr_uevent); // bus_create_file
	if (retval)
		goto bus_uevent_fail;

	priv->devices_kset = kset_create_and_add("devices", NULL, bus_kobj); // kset_create_and_add 1
	if (!priv->devices_kset) {
		retval = -ENOMEM;
		goto bus_devices_fail;
	}

	priv->drivers_kset = kset_create_and_add("drivers", NULL, bus_kobj); // kset_create_and_add 2
	if (!priv->drivers_kset) {
		retval = -ENOMEM;
		goto bus_drivers_fail;
	}

	INIT_LIST_HEAD(&priv->interfaces);
	key = &priv->lock_key;
	lockdep_register_key(key);
	__mutex_init(&priv->mutex, "subsys mutex", key);
	klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
	klist_init(&priv->klist_drivers, NULL, NULL);

	retval = add_probe_files(bus); // add_probe_files
	if (retval)
		goto bus_probe_files_fail;

	retval = sysfs_create_groups(bus_kobj, bus->bus_groups); // sysfs_create_groups
	if (retval)
		goto bus_groups_fail;

	pr_debug("bus: '%s': registered\n", bus->name);
	return 0;

bus_groups_fail:
	remove_probe_files(bus);
bus_probe_files_fail:
	kset_unregister(priv->drivers_kset);
bus_drivers_fail:
	kset_unregister(priv->devices_kset);
bus_devices_fail:
	bus_remove_file(bus, &bus_attr_uevent);
bus_uevent_fail:
	kset_unregister(&priv->subsys);
	/* Above kset_unregister() will kfree @priv */
	priv = NULL;
out:
	kfree(priv);
	return retval;
}

之后观察如何在 /sys/bus/xxx 下创建一个文件, 创建路径为:

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/sys/bus/<bus_name>/<attr->attr.name>