蓝牙学习（2）USB Adapter

主要分析一下蓝牙USB Adapter使用USB接口传输HCI包的实现及过程。

参照上面的Bluetooth core system architecture图， 蓝牙USB Adapter作为Bluetooth controller以USB的物理形式连接到Linux host processor上，通过HCI protocol和Host通信。

bluetooth in Linux kernel

Linux kernel side主要包括：

• Bluetooth Core: （netluetooth*)
  
  • HCI (Host Controller Interface) device and connection manager, schedule
  • SCO (Synchronous Connection Oriented) audio links
  • L2CAP (Logical Link Control and Adaptation Protocol)
  • SMP (Security Manager Protocol) on LE (Low Energy) links
• HCI Device drivers (Interface to the hardware)
  
  • USB (btusb)
  • UART (hciuart)
  • SDIO
• RFCOMM (RFCOMM Protocol for serial communication) Module (creates /dev/rfcomm serial devices)
• BNEP (Bluetooth Network Encapsulation Protocol) Module (creates /sys/class/net/bnep network interfaces)
• CMTP (CAPI Message Transport Protocol) Module
• HIDP (Human Interface Device Protocol) Module (creates /sys/class/input devices)

HCI device driver

HCI：Host Controller Interface

HCI提供了访问Controller的统一接口

Controller主要包含下面几种类型

• BR/EDR Controller
• BD/EDR/LE Controller
• LE Controller
• AMP Controller (Alternate MAC/PHY)

include/net/bluetooth/hci.h 中定义的HCI bus 接口类型包括：

/* HCI bus types */
#define HCI_VIRTUAL 0
#define HCI_USB     1
#define HCI_PCCARD  2
#define HCI_UART    3
#define HCI_RS232   4
#define HCI_PCI     5
#define HCI_SDIO    6
#define HCI_SPI     7
#define HCI_I2C     8
#define HCI_SMD     9

btusb

bluetooth USB adapter是作为usb device挂载到USB总线上的。因此是通过usb_driver提供的机制去probe，而不是直接通过platform_driver.
这点和i2c, SPI 等设备驱动都是类似的。

static struct usb_driver btusb_driver = {
    .name       = "btusb",
    .probe      = btusb_probe,
    .disconnect = btusb_disconnect,
#ifdef CONFIG_PM
    .suspend    = btusb_suspend,
    .resume     = btusb_resume,
#endif
    .id_table   = btusb_table,
    .supports_autosuspend = 1,
    .disable_hub_initiated_lpm = 1,
};

probe

在probe函数中, hci device的operators函数指针被赋值

static int btusb_probe(struct usb_interface *intf,
               const struct usb_device_id *id)
{
    //...
    hdev->open   = btusb_open;
    hdev->close  = btusb_close;
    hdev->flush  = btusb_flush;
    hdev->send   = btusb_send_frame;
    hdev->notify = btusb_notify;
    //...
}

其中HCI Device数据结构定义, include/net/bluetooth/hci_core.h

struct hci_dev {
    struct list_head list;
    struct mutex    lock;
    char        name[8];
    unsigned long   flags;
    __u16       id;
    __u8        bus;
    __u8        dev_type;

    //...

    int (*open)(struct hci_dev *hdev);
    int (*close)(struct hci_dev *hdev);
    int (*flush)(struct hci_dev *hdev);
    int (*setup)(struct hci_dev *hdev);
    int (*shutdown)(struct hci_dev *hdev);
    int (*send)(struct hci_dev *hdev, struct sk_buff *skb);
    void (*notify)(struct hci_dev *hdev, unsigned int evt);

    //...
    }  

接收发送数据

Bluetooth USB设备定义了不同的pipe用于不同类型的数据传输
- Control pipes are used to transport HCI commands.
- Interrupt pipes are responsible for carrying HCI events.
- Bulk pipes transfer asynchronous connectionless (ACL) Bluetooth data.
- Isochronous pipes carry SCO audio data.

static int btusb_send_frame(struct hci_dev *hdev, struct sk_buff *skb)
{
    struct urb *urb;
    BT_DBG("%s", hdev->name);
    switch (hci_skb_pkt_type(skb)) {
    case HCI_COMMAND_PKT:
        urb = alloc_ctrl_urb(hdev, skb);
        if (IS_ERR(urb))
            return PTR_ERR(urb);
        hdev->stat.cmd_tx++;
        return submit_or_queue_tx_urb(hdev, urb);
    case HCI_ACLDATA_PKT:
        urb = alloc_bulk_urb(hdev, skb);
        if (IS_ERR(urb))
            return PTR_ERR(urb);
        hdev->stat.acl_tx++;
        return submit_or_queue_tx_urb(hdev, urb);
    case HCI_SCODATA_PKT:
        if (hci_conn_num(hdev, SCO_LINK) < 1)
            return -ENODEV;
        urb = alloc_isoc_urb(hdev, skb);
        if (IS_ERR(urb))
            return PTR_ERR(urb);
        hdev->stat.sco_tx++;
        return submit_tx_urb(hdev, urb);
    }
    return -EILSEQ;
}

接收中断处理：
注册

btusb_open -->
btusb_submit_intr_urb-->
//initialize a interrupt urb
usb_fill_int_urb(urb, data->udev, pipe, buf, size,
             btusb_intr_complete, hdev, data->intr_ep->bInterval);

usb_complete_t 回调函数btusb_intr_complete被注册

    if (btusb_recv_intr(data, urb->transfer_buffer,
                    urb->actual_length) < 0) {
            bt_dev_err(hdev, "corrupted event packet");
            hdev->stat.err_rx++;
        }

在btusb_recv_intr函数中，数据被copy到skb. 内核中所有network相关的queue, buffer都用这个通用的结构体。

Reference

https://iotbreaks.com/base-knowledge-about-bluetooth/

http://www.embeddedlinux.org.cn/essentiallinuxdevicedrivers/final/ch16lev1sec1.html#ch16lev1sec1

https://wiki.linuxfoundation.org/networking/sk_buff