Here are five possible code completions for the given function:

### 1. Using kfree to free the memory

```c
void hci_unregister_dev(struct hci_dev *hdev)
{
    BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);
    kfree(hdev);
}
```

### 2. Using put_device to free the memory

```c
void hci_unregister_dev(struct hci_dev *hdev)
{
    BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);
    put_device(&hdev->dev);
}
```

### 3. Using kfree_rcu to free the memory after a grace period

```c
void hci_unregister_dev(struct hci_dev *hdev)
{
    BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);
    kfree_rcu(hdev, rcu_head);
}
```

### 4. Using a custom free function

```c
void hci_unregister_dev(struct hci_dev *hdev)
{
    BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);
    hci_dev_free(hdev);
}

// Define the custom free function
void hci_dev_free(struct hci_dev *hdev)
{
    // Free any resources held by hdev
    kfree(hdev);
}
```

### 5. Using a combination of put_device and kfree

```c
void hci_unregister_dev(struct hci_dev *hdev)
{
    BT_DBG("%p name %s bus %d", hdev, hdev->name, hdev->bus);
    put_device(&hdev->dev);
    if (atomic_dec_and_test(&hdev->refcount))
        kfree(hdev);
}
```

Note: The actual implementation will depend on the specific requirements of your code and the memory management strategy used in your project.