在linux驱动开发过程中，经常会使用到platform总线来开发，通过platform_driver_register()注册驱动，platform_device_register()来注册设备。如果在同一个platform bus下两次注册同一个device，那么会出现什么情况呢？
1、创建一个platform_device
#define SWLED_DEV "sw_led"
static struct platform_device swled_plfdev = {
.name = SWLED_DEV,
};
struct platform_driver swled_plfdrv = {
.probe = swled_probe,
.driver = {
.owner = THIS_MODULE,
.name = SWLED_DEV,
},
};
2、注册device与driver
ret = platform_device_register(&swled_plfdev);
ret = platform_device_register(&swled_plfdev);
ret = platform_driver_register(&swled_plfdrv);
3、编译软件后烧录到板卡查看结果
系统启动后直接崩溃后重启，崩溃信息如下：
[ 1.808981@2] load module: swoled
[ 1.809829@2] kobject (00000000d497ec8e): tried to init an initialized object, something is seriously wrong.
[ 1.810849@2] CPU: 2 PID: 1 Comm: init Tainted: G WC O 5.4.125-android11-2-g2ac4107d4ee9-dirty-ab1646 #1
[ 1.812142@2] Hardware name: Amlogic (DT)
[ 1.812651@2] Call trace:
[ 1.812996@2] [ffffffc020003980+ 64][<ffffffc0100c63b4>] dump_backtrace+0x0/0x18c
[ 1.813941@2] [ffffffc0200039c0+ 32][<ffffffc0100c6564>] show_stack+0x24/0x34
[ 1.814855@2] [ffffffc0200039e0+ 64][<ffffffc010dcc6b8>] dump_stack+0xbc/0x108
[ 1.815773@2] [ffffffc020003a20+ 32][<ffffffc010dd2774>] kobject_init+0x9c/0xa4
[ 1.816707@2] [ffffffc020003a40+ 32][<ffffffc01076dd90>] device_initialize+0x34/0xe0
[ 1.817692@2] [ffffffc020003a60+ 32][<ffffffc010777014>] platform_device_register+0x20/0x6c
[ 1.818760@2] [ffffffc020003a80+ 400][<ffffffc0099a0660>] swled_dev_init+0x58/0x9c [swoled]
[ 1.819804@2] [ffffffc020003c10+ 64][<ffffffc01008c748>] do_one_initcall+0x130/0x294
[ 1.820791@2] [ffffffc020003c50+ 240][<ffffffc0101aed78>] do_init_module+0x5c/0x21c
[ 1.821753@2] [ffffffc020003d40+ 240][<ffffffc0101aec18>] load_module+0x3a3c/0x3aac
[ 1.822716@2] [ffffffc020003e30+ 48][<ffffffc0101a9964>] __arm64_sys_finit_module+0xbc/0xd8
[ 1.823780@2] [ffffffc020003e60+ 64][<ffffffc0100d1408>] el0_svc_common+0xb0/0x188
[ 1.824743@2] [ffffffc020003ea0+ 336][<ffffffc0100d150c>] el0_svc_compat_handler+0x2c/0x3c
[ 1.825782@2] [ffffffc020003ff0+ 0][<ffffffc010083524>] el0_svc_compat+0x8/0x24
[ 1.826788@2] list_add corruption. prev->next should be next (ffffff80a9224000), but was ffffffc0099a4258. (prev=ffffffc0099a4258).
[ 1.828220@2] ------------[ cut here ]------------
[ 1.828814@2] kernel BUG at lib/list_debug.c:28!
[ 1.829401@2] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
......
[ 2.038039@2] Call trace:
[ 2.038378@2] [ffffffc0200038d0+ 16][<ffffffc0105bb6f8>] __list_add_valid+0x8c/0x90[ffffffc0200038e0+ 176][<ffffffc010dd34b0>] kobject_add_internal+0xd4/0x420
[ 2.040380@2] [ffffffc020003990+ 64][<fffft_add+0x88/0xdc
[ 2.041303@2] [ffffffc0200039d0+ 64][<ffffffc01076dfd8>] device_add+0x150/0xa48
[ 2.042234@2] [ff][<ffffffc010777440>] platform_device_add+0x88/0x228
[ 2.043250@2] [ffffffc020003a60+ 32][<ffffffc01077702c>] platform_init+0x58/0x9c [swoled]
[ 2.045366@2] [ffffffc020003c10+ 64][<ffffffc01008c748>] do_one_initcall+0x130/0x294
[ 2.ffffc0101aed78>] do_init_module+0x5c/0x21c
[ 2.047313@2] [ffffffc020003d40+ 240][<ffffffc0101aec18>] load_module+0x3a3c [ffffffc020003e30+ 48][<ffffffc0101a9964>] __arm64_sys_finit_module+0xbc/0xd8
[ 2.049341@2] [ffffffc020003e60+ 64][<n+0xb0/0x188
[ 2.050304@2] [ffffffc020003ea0+ 336][<ffffffc0100d150c>] el0_svc_compat_handler+0x2c/0x3c
[ 2.051343@03ff0+ 0][<ffffffc010083524>] el0_svc_compat+0x8/0x24
[ 2.052288@2] Code: 9138fc00 aa0303e1 aa0803e3 97ee869a (d42100--[ end trace 18856db82685ee12 ]---
[ 2.066232@2] Kernel panic - not syncing: Fatal exception
[ 2.066273@2] SMP: stopping secondary CPUs
[ 2.066629@3] usb 3-1.1: new high-speed USB device number 3 using xhci-hcd
[ 2.067611@2] Kernel Offset: disabled
[ 2.067617@2] CPU features: 0x00010006,2a00aa10
[ 2.067621@2] Memory Limit: none
[ 2.080919@2] Rebooting in 5 seconds..
关键信息：
[ 1.826788@2] list_add corruption. prev->next should be next (ffffff80a9224000), but was ffffffc0099a4258. (prev=ffffffc0099a4258).
[ 1.828220@2] ------------[ cut here ]------------
[ 1.828814@2] kernel BUG at lib/list_debug.c:28!
lib/list_debug.c第28行如下：
可以看到是由于prev->next != next导致，此处涉及到内核双向list操作。因为device在注册到内核时，会将
struct device中的struct kobject kobj成员的struct list_head entry挂载到总线struct bus_type的struct subsys_private *p中的struct kset *devices_kset中的struct list_head list中，便于系统统一管理。
关于kset、kobject的详细内容可参考：
跟着platform_device_register()找到整个过程的函数调用栈：
platform_device_register()
platform_device_add()
|---device_initialize()
| kobject_init()
| kobject_init_internal()
| INIT_LIST_HEAD(&kobj->entry);
|---device_add()
kobject_add()
kobject_add_varg()
kobject_add_internal()
kobj_kset_join()
list_add_tail(&kobj->entry, &kobj->kset->list);
在第一次进行platform_device_register()注册时，假设要加入到链表中的kobj为kobj_B，则将kobj_B->entry加入到kobj_B->kset->list中，得到链表如下：
在第二次进行platform_device_register()注册时，先调用device_initialize()初始化设备。在这个过程中，将kobj_B->entry进行初始化后：
static inline void INIT_LIST_HEAD(struct list_head *list)
{
WRITE_ONCE(list->next, list);
list->prev = list;
}
此时kobj_B->entry的nxet指向了本身。
然后接着在platform_device_add()中调用device_add()添加设备到系统。
|---device_add()
kobject_add()
kobject_add_varg()
kobject_add_internal()
kobj_kset_join()
list_add_tail(&kobj->entry, &kobj->kset->list);
在调用list_add_tail()时，参数new指向kobj_B->entry，head指向kobj_B->kset->list。
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
__list_add(new, head->prev, head);
}
在调用__list_add()时，参数new指向kobj_B->entry，prev指向kobj_B->kset->list->prev（即kobj_B->entry），next指向kobj_B->kset->list。
static inline void __list_add(struct list_head *new,
struct list_head *prev,
struct list_head *next)
{
if (!__list_add_valid(new, prev, next))
return;
next->prev = new;
new->next = next;
new->prev = prev;
WRITE_ONCE(prev->next, new);
}
则在__list_add_valid()中判断prev->next != next时，prev->next指向prev指向kobj_B->kset->list->prev->next（即kobj_B->entry本身），next指向kobj_B->kset->list，故二者不相等。
bool __list_add_valid(struct list_head *new, struct list_head *prev,
struct list_head *next)
{
if (CHECK_DATA_CORRUPTION(next->prev != prev,
"list_add corruption. next->prev should be prev (%px), but was %px. (next=%px).\n",
prev, next->prev, next) ||
CHECK_DATA_CORRUPTION(prev->next != next,
"list_add corruption. prev->next should be next (%px), but was %px. (prev=%px).\n",
next, prev->next, prev) ||
CHECK_DATA_CORRUPTION(new == prev || new == next,
"list_add double add: new=%px, prev=%px, next=%px.\n",
new, prev, next))
return false;
return true;
}
