struct idr {struct idr_layer *top;struct idr_layer *id_free;int               layers;int               id_free_cnt;spinlock_t        lock;
};

again:if (idr_pre_get(&my_idr, GFP_KERNEL) == 0) {/* No memory, give up entirely */}spin_lock(&my_lock);result = idr_get_new(&my_idr, &target, &id);if (result == -EAGAIN) {sigh();spin_unlock(&my_lock);goto again;}

<linux-2.6.23/drivers/i2c/i2c-core.c>
...
<linux/idr.h>   /* idr头文件 */
...
static DEFINE_IDR(i2c_adapter_idr); /* 声明idr */
...
/* 采用动态总线号声明并注册一个i2c适配器(adapter)，可睡眠针对总线号可动态指定的设备，如基于USB的i2c设备或pci卡*/
int i2c_add_adapter(struct i2c_adapter *adapter)
{int     id, res = 0;
retry:if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)return -ENOMEM;mutex_lock(&core_lists);/* __i2c_first_dynamic_bus_num是当前系统允许的动态总线号的最大值 */res = idr_get_new_above(&i2c_adapter_idr, adapter,                 __i2c_first_dynamic_bus_num, &id);mutex_unlock(&core_lists);if (res < 0) {if (res == -EAGAIN)goto retry;return res;}adapter->nr = id;return i2c_register_adapter(adapter);
}
EXPORT_SYMBOL(i2c_add_adapter);/* 采用静态总线号声明并注册一个i2c适配器(adapter)*/
int i2c_add_numbered_adapter(struct i2c_adapter *adap)
{int     id;int     status;if (adap->nr & ~MAX_ID_MASK)return -EINVAL;
retry:if (idr_pre_get(&i2c_adapter_idr, GFP_KERNEL) == 0)return -ENOMEM;mutex_lock(&core_lists);/* "above" here means "above or equal to", sigh;* we need the "equal to" result to force the result*/status = idr_get_new_above(&i2c_adapter_idr, adap, adap->nr, &id);if (status == 0 && id != adap->nr) {status = -EBUSY;idr_remove(&i2c_adapter_idr, id);}mutex_unlock(&core_lists);if (status == -EAGAIN)goto retry;if (status == 0)status = i2c_register_adapter(adap);return status;
}
EXPORT_SYMBOL_GPL(i2c_add_numbered_adapter);/* 注销一个i2c适配器 */
int i2c_del_adapter(struct i2c_adapter *adap)
{.../* free bus id */idr_remove(&i2c_adapter_idr, adap->nr);...return res;
}
EXPORT_SYMBOL(i2c_del_adapter);/* 通过ID号获得i2c_adapter设备结构体 */
struct i2c_adapter* i2c_get_adapter(int id)
{struct i2c_adapter *adapter;mutex_lock(&core_lists);adapter = (struct i2c_adapter *)idr_find(&i2c_adapter_idr, id);if (adapter && !try_module_get(adapter->owner))adapter = NULL;mutex_unlock(&core_lists);return adapter;
}
EXPORT_SYMBOL(i2c_get_adapter);