Linux设备模型分析之bus

前面我们分析了Linux设备模型中kobject、kset以及ktype的使用，它们是设备模型的最基础部分，在其上有更高级一层的bus、device和driver。在这一篇文章中，我们来看一下bus的用法。

一、相关数据结构

首先，我们列出本文涉及的相关数据结构。

[cpp] view plaincopy

51struct bus_type {
52 const char *name;
53 struct bus_attribute *bus_attrs;
54 struct device_attribute *dev_attrs;
55 struct driver_attribute *drv_attrs;
56
57 int (*match)(struct device *dev, struct device_driver *drv);
58 int (*uevent)(struct device *dev, struct kobj_uevent_env *env);
59 int (*probe)(struct device *dev);
60 int (*remove)(struct device *dev);
61 void (*shutdown)(struct device *dev);
62
63 int (*suspend)(struct device *dev, pm_message_t state);
64 int (*resume)(struct device *dev);
65
66 const struct dev_pm_ops *pm;
67
68 struct bus_type_private *p;
69};
38struct bus_attribute {
39 struct attribute attr;
40 ssize_t (*show)(struct bus_type *bus, char *buf);
41 ssize_t (*store)(struct bus_type *bus, const char *buf, size_t count);
42};
336/* interface for exporting device attributes */
337struct device_attribute {
338 struct attribute attr;
339 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
340 char *buf);
341 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
342 const char *buf, size_t count);
343};
162struct driver_attribute {
163 struct attribute attr;
164 ssize_t (*show)(struct device_driver *driver, char *buf);
165 ssize_t (*store)(struct device_driver *driver, const char *buf,
166 size_t count);
167};
2/**
3 * struct bus_type_private - structure to hold the private to the driver core portions of the bus_type structure.
4 *
5 * @subsys - the struct kset that defines this bus. This is the main kobject
6 * @drivers_kset - the list of drivers associated with this bus
7 * @devices_kset - the list of devices associated with this bus
8 * @klist_devices - the klist to iterate over the @devices_kset
9 * @klist_drivers - the klist to iterate over the @drivers_kset
10 * @bus_notifier - the bus notifier list for anything that cares about things
11 * on this bus.
12 * @bus - pointer back to the struct bus_type that this structure is associated
13 * with.
14 *
15 * This structure is the one that is the actual kobject allowing struct
16 * bus_type to be statically allocated safely. Nothing outside of the driver
17 * core should ever touch these fields.
18 */
19struct bus_type_private {
20 struct kset subsys;
21 struct kset *drivers_kset;
22 struct kset *devices_kset;
23 struct klist klist_devices;
24 struct klist klist_drivers;
25 struct blocking_notifier_head bus_notifier;
26 unsigned int drivers_autoprobe:1;
27 struct bus_type *bus;
28};
406struct device {
407 struct device *parent;
408
409 struct device_private *p;
410
411 struct kobject kobj;
412 const char *init_name; /* initial name of the device */
413 struct device_type *type;
414
415 struct mutex mutex; /* mutex to synchronize calls to
416 * its driver.
417 */
418
419 struct bus_type *bus; /* type of bus device is on */
420 struct device_driver *driver; /* which driver has allocated this
421 device */
422 void *platform_data; /* Platform specific data, device
423 core doesn't touch it */
424 struct dev_pm_info power;
425
426#ifdef CONFIG_NUMA
427 int numa_node; /* NUMA node this device is close to */
428#endif
429 u64 *dma_mask; /* dma mask (if dma'able device) */
430 u64 coherent_dma_mask;/* Like dma_mask, but for
431 alloc_coherent mappings as
432 not all hardware supports
433 64 bit addresses for consistent
434 allocations such descriptors. */
435
436 struct device_dma_parameters *dma_parms;
437
438 struct list_head dma_pools; /* dma pools (if dma'ble) */
439
440 struct dma_coherent_mem *dma_mem; /* internal for coherent mem
441 override */
442 /* arch specific additions */
443 struct dev_archdata archdata;
444#ifdef CONFIG_OF
445 struct device_node *of_node;
446#endif
447
448 dev_t devt; /* dev_t, creates the sysfs "dev" */
449
450 spinlock_t devres_lock;
451 struct list_head devres_head;
452
453 struct klist_node knode_class;
454 struct class *class;
455 const struct attribute_group **groups; /* optional groups */
456
457 void (*release)(struct device *dev);
458};
123struct device_driver {
124 const char *name;
125 struct bus_type *bus;
126
127 struct module *owner;
128 const char *mod_name; /* used for built-in modules */
129
130 bool suppress_bind_attrs; /* disables bind/unbind via sysfs */
131
132#if defined(CONFIG_OF)
133 const struct of_device_id *of_match_table;
134#endif
135
136 int (*probe) (struct device *dev);
137 int (*remove) (struct device *dev);
138 void (*shutdown) (struct device *dev);
139 int (*suspend) (struct device *dev, pm_message_t state);
140 int (*resume) (struct device *dev);
141 const struct attribute_group **groups;
142
143 const struct dev_pm_ops *pm;
144
145 struct driver_private *p;
146};

二、注册bus

bus的注册接口为bus_register，该函数代码如下：

[cpp] view plaincopy

872/**
873 * bus_register - register a bus with the system.
874 * @bus: bus.
875 *
876 * Once we have that, we registered the bus with the kobject
877 * infrastructure, then register the children subsystems it has:
878 * the devices and drivers that belong to the bus.
879 */
880int bus_register(struct bus_type *bus)
881{
882 int retval;
883 struct bus_type_private *priv;
884
885 priv = kzalloc(sizeof(struct bus_type_private), GFP_KERNEL);
886 if (!priv)
887 return -ENOMEM;
888
889 priv->bus = bus;
890 bus->p = priv;
891
892 BLOCKING_INIT_NOTIFIER_HEAD(&priv->bus_notifier);
893
894 retval = kobject_set_name(&priv->subsys.kobj, "%s", bus->name);
895 if (retval)
896 goto out;
897
898 priv->subsys.kobj.kset = bus_kset;
899 priv->subsys.kobj.ktype = &bus_ktype;
900 priv->drivers_autoprobe = 1;
901
902 retval = kset_register(&priv->subsys);
903 if (retval)
904 goto out;
905
906 retval = bus_create_file(bus, &bus_attr_uevent);
907 if (retval)
908 goto bus_uevent_fail;
909
910 priv->devices_kset = kset_create_and_add("devices", NULL,
911 &priv->subsys.kobj);
912 if (!priv->devices_kset) {
913 retval = -ENOMEM;
914 goto bus_devices_fail;
915 }
916
917 priv->drivers_kset = kset_create_and_add("drivers", NULL,
918 &priv->subsys.kobj);
919 if (!priv->drivers_kset) {
920 retval = -ENOMEM;
921 goto bus_drivers_fail;
922 }
923
924 klist_init(&priv->klist_devices, klist_devices_get, klist_devices_put);
925 klist_init(&priv->klist_drivers, NULL, NULL);
926
927 retval = add_probe_files(bus);
928 if (retval)
929 goto bus_probe_files_fail;
930
931 retval = bus_add_attrs(bus);
932 if (retval)
933 goto bus_attrs_fail;
934
935 pr_debug("bus: '%s': registered\n", bus->name);
936 return 0;
937
938bus_attrs_fail:
939 remove_probe_files(bus);
940bus_probe_files_fail:
941 kset_unregister(bus->p->drivers_kset);
942bus_drivers_fail:
943 kset_unregister(bus->p->devices_kset);
944bus_devices_fail:
945 bus_remove_file(bus, &bus_attr_uevent);
946bus_uevent_fail:
947 kset_unregister(&bus->p->subsys);
948out:
949 kfree(bus->p);
950 bus->p = NULL;
951 return retval;
952}

894行，设置priv->subsys.kobj的名字，priv->subsys是一个kset，代表这个bus。bus是通过一个kset来表示的，而device和device_driver都是通过一个kobject来表示的，由此也可以看出他们的不同。

898行，设置priv->subsys.kobj.kset为bus_kset;

在drivers/base/bus.c文件中，162行有如下定义：

[cpp] view plaincopy

162static struct kset *bus_kset;

同时，在buses_init函数中对bus_kset进行了初始化：

[cpp] view plaincopy

1054int __init buses_init(void)
1055{
1056 bus_kset = kset_create_and_add("bus", &bus_uevent_ops, NULL);
1057 if (!bus_kset)
1058 return -ENOMEM;
1059 return 0;
1060}

1056行，创建一个名为bus的kset，赋值给bus_kset，对应/sys/bus目录。所有的bus都属性bus_set，即所有的bus对应的sys系统目录都放在/sys/bus目录下。

bus_kset对应的kset_uevent_ops是bus_uevent_ops，其定义如下：

[cpp] view plaincopy

158static const struct kset_uevent_ops bus_uevent_ops = {
159 .filter = bus_uevent_filter,
160};

bus_uevent_filter函数定义如下：

[cpp] view plaincopy

149static int bus_uevent_filter(struct kset *kset, struct kobject *kobj)
150{
151 struct kobj_type *ktype = get_ktype(kobj);
152
153 if (ktype == &bus_ktype)
154 return 1;
155 return 0;
156}

回忆一下上一篇博客《Linux设备模型分析之kset》介绍的内容，如果kset_uevent_ops.filter函数返回0，将不会处理uevent事件。在bus_uevent_filter函数中，如果kobj->ktype不是bus_ktype，则返回0，即不处理Uevent事件。

bus_ktype定义如下：

[cpp] view plaincopy

145static struct kobj_type bus_ktype = {
146 .sysfs_ops = &bus_sysfs_ops,
147};

bus_sysfs_ops定义如下：

[cpp] view plaincopy

119static const struct sysfs_ops bus_sysfs_ops = {
120 .show = bus_attr_show,
121 .store = bus_attr_store,
122};

bus_attr_show和bus_attr_store函数定义如下：

[cpp] view plaincopy

95static ssize_t bus_attr_show(struct kobject *kobj, struct attribute *attr,
96 char *buf)
97{
98 struct bus_attribute *bus_attr = to_bus_attr(attr);
99 struct bus_type_private *bus_priv = to_bus(kobj);
100 ssize_t ret = 0;
101
102 if (bus_attr->show)
103 ret = bus_attr->show(bus_priv->bus, buf);
104 return ret;
105}
106
107static ssize_t bus_attr_store(struct kobject *kobj, struct attribute *attr,
108 const char *buf, size_t count)
109{
110 struct bus_attribute *bus_attr = to_bus_attr(attr);
111 struct bus_type_private *bus_priv = to_bus(kobj);
112 ssize_t ret = 0;
113
114 if (bus_attr->store)
115 ret = bus_attr->store(bus_priv->bus, buf, count);
116 return ret;
117}

bus_attribute结构体定义如下：

[cpp] view plaincopy

38struct bus_attribute {
39 struct attribute attr;
40 ssize_t (*show)(struct bus_type *bus, char *buf);
41 ssize_t (*store)(struct bus_type *bus, const char *buf, size_t count);
42};

回到bus_register函数：

899行，设置priv->subsys.kobj.ktype为 &bus_ktype;回忆一下前面的文章《Linux设备模型分析之kobject》，对kobject属性的读写操作将调用kobject.ktype定义的sysfs_ops函数集指定的show和store函数。所以对bus属性文件的读写操作将会调用bus_ktyp.sysfs_ops指定的bus_attr_show和bus_attr_store函数，这两个函数我们前面已经分析过了，他们将回溯到bus_attr->show和bus_attr->store函数。

900行，设置priv->drivers_autoprobe为1;

902行，调用kset_register注册priv->subsys，即注册代表bus的kset。注册完成后，将在/sys/bus目录下出现对应该bus的目录。

906行，调用bus_create_file(bus, &bus_attr_uevent)，创建uevent属性文件，该函数定义如下：

[cpp] view plaincopy

124int bus_create_file(struct bus_type *bus, struct bus_attribute *attr)
125{
126 int error;
127 if (bus_get(bus)) {
128 error = sysfs_create_file(&bus->p->subsys.kobj, &attr->attr);
129 bus_put(bus);
130 } else
131 error = -EINVAL;
132 return error;
133}

128行，调用sysfs_create_file函数创建了bus_attribute对应的属性文件。

先来看bus_attr_uevent的定义，它是通过宏BUS_ATTR生成的：

[cpp] view plaincopy

870static BUS_ATTR(uevent, S_IWUSR, NULL, bus_uevent_store);

BUS_ATTR定义如下：

[cpp] view plaincopy

44#define BUS_ATTR(_name, _mode, _show, _store) \
45struct bus_attribute bus_attr_##_name = __ATTR(_name, _mode, _show, _store)

__ATTR定义如下：

[cpp] view plaincopy

70#define __ATTR(_name,_mode,_show,_store) { \
71 .attr = {.name = __stringify(_name), .mode = _mode }, \
72 .show = _show, \
73 .store = _store, \
74}

从bus_attr_uevent的定义可以看出，对应的uevent属性只有store接口，没有实现show接口。store接口函数bus_uevent_store定义如下：

[cpp] view plaincopy

861static ssize_t bus_uevent_store(struct bus_type *bus,
862 const char *buf, size_t count)
863{
864 enum kobject_action action;
865
866 if (kobject_action_type(buf, count, &action) == 0)
867 kobject_uevent(&bus->p->subsys.kobj, action);
868 return count;
869}

回到bus_register函数：

910 - 915行，创建名为devices的kset，其parent是priv->subsys.kobj，即在相应bus目录下创建一个devices目录。

917 - 922行，创建名为drivers的kset，其parent是priv->subsys.kobj，即在相应bus目录下创建一个drivers目录。

924行，初始化priv->klist_devices，即devices链表。

925行，初始化priv->klist_drivers，即drivers链表。

927行，调用add_probe_files(bus)函数创建drivers_probe和drivers_autoprobe两个属性文件，该函数定义如下：

[cpp] view plaincopy

606static int add_probe_files(struct bus_type *bus)
607{
608 int retval;
609
610 retval = bus_create_file(bus, &bus_attr_drivers_probe);
611 if (retval)
612 goto out;
613
614 retval = bus_create_file(bus, &bus_attr_drivers_autoprobe);
615 if (retval)
616 bus_remove_file(bus, &bus_attr_drivers_probe);
617out:
618 return retval;
619}

bus_create_file函数我们前面已经分析过了，610行，创建drivers_probe属性文件，614行，创建drivers_autoprobe属性文件。其对应的bus_attribute定义如下：

[cpp] view plaincopy

602static BUS_ATTR(drivers_probe, S_IWUSR, NULL, store_drivers_probe);
603static BUS_ATTR(drivers_autoprobe, S_IWUSR | S_IRUGO,
604 show_drivers_autoprobe, store_drivers_autoprobe);

属性文件对应的操作函数定义如下：

[cpp] view plaincopy

225static ssize_t show_drivers_autoprobe(struct bus_type *bus, char *buf)
226{
227 return sprintf(buf, "%d\n", bus->p->drivers_autoprobe);
228}
229
230static ssize_t store_drivers_autoprobe(struct bus_type *bus,
231 const char *buf, size_t count)
232{
233 if (buf[0] == '0')
234 bus->p->drivers_autoprobe = 0;
235 else
236 bus->p->drivers_autoprobe = 1;
237 return count;
238}
239
240static ssize_t store_drivers_probe(struct bus_type *bus,
241 const char *buf, size_t count)
242{
243 struct device *dev;
244
245 dev = bus_find_device_by_name(bus, NULL, buf);
246 if (!dev)
247 return -ENODEV;
248 if (bus_rescan_devices_helper(dev, NULL) != 0)
249 return -EINVAL;
250 return count;
251}

show_drivers_autoprobe函数用于显示bus->p->drivers_autoprobe的值。

store_drivers_autoprobe函数根据用户的要求设置bus->p->drivers_autoprobe的值。

store_drivers_probe函数245行调用bus_find_device_by_name 在bus上查找用户指定的device，并且在248行调用bus_rescan_devices_helper函数查找匹配device_driver.

回到bus_register函数：

931行，调用bus_add_attrs函数创建bus默认属性文件，该函数定义如下：

[cpp] view plaincopy

810/**
811 * bus_add_attrs - Add default attributes for this bus.
812 * @bus: Bus that has just been registered.
813 */
814
815static int bus_add_attrs(struct bus_type *bus)
816{
817 int error = 0;
818 int i;
819
820 if (bus->bus_attrs) {
821 for (i = 0; attr_name(bus->bus_attrs[i]); i++) {
822 error = bus_create_file(bus, &bus->bus_attrs[i]);
823 if (error)
824 goto err;
825 }
826 }
827done:
828 return error;
829err:
830 while (--i >= 0)
831 bus_remove_file(bus, &bus->bus_attrs[i]);
832 goto done;
833}

820 - 826行，如果bus->bus_attrs指定了bus的默认属性，则创建对应的属性文件。

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