LINUX&UVC输入终端描述符解析

UVC输入终端是UVC设备拓扑结构中数据流的起始节点。UVC输入终端使用UVC输入终端描述符来描述.
关于该描述符详细的字段描述详见：https://www.usbzh.com/article/detail-95.html
关于相机终端描述符详见：https://www.usbzh.com/article/detail-47.html

根据前文可知，输入终端按其类型可分为：

一般最常见的UVC相机就是ITT_CAMERA类型，值为0x201。

uvc_parse_standard_control函数中关于输入终端的代码解析如下：

case UVC_VC_INPUT_TERMINAL:
        if (buflen < 8) {
            uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
                "interface %d INPUT_TERMINAL error\n",
                udev->devnum, alts->desc.bInterfaceNumber);
            return -EINVAL;
        }

        /*
         * Reject invalid terminal types that would cause issues:
         *
         * - The high byte must be non-zero, otherwise it would be
         *   confused with a unit.
         *
         * - Bit 15 must be 0, as we use it internally as a terminal
         *   direction flag.
         *
         * Other unknown types are accepted.
         */
        type = get_unaligned_le16(&buffer[4]);
        if ((type & 0x7f00) == 0 || (type & 0x8000) != 0) {
            uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
                "interface %d INPUT_TERMINAL %d has invalid "
                "type 0x%04x, skipping\n", udev->devnum,
                alts->desc.bInterfaceNumber,
                buffer[3], type);
            return 0;
        }

        n = 0;
        p = 0;
        len = 8;

        if (type == UVC_ITT_CAMERA) {
            n = buflen >= 15 ? buffer[14] : 0;
            len = 15;

        } else if (type == UVC_ITT_MEDIA_TRANSPORT_INPUT) {
            n = buflen >= 9 ? buffer[8] : 0;
            p = buflen >= 10 + n ? buffer[9+n] : 0;
            len = 10;
        }

        if (buflen < len + n + p) {
            uvc_trace(UVC_TRACE_DESCR, "device %d videocontrol "
                "interface %d INPUT_TERMINAL error\n",
                udev->devnum, alts->desc.bInterfaceNumber);
            return -EINVAL;
        }

        term = uvc_alloc_entity(type | UVC_TERM_INPUT, buffer[3],
                    1, n + p);
        if (term == NULL)
            return -ENOMEM;

        if (UVC_ENTITY_TYPE(term) == UVC_ITT_CAMERA) {
            term->camera.bControlSize = n;
            term->camera.bmControls = (u8 *)term + sizeof(*term);
            term->camera.wObjectiveFocalLengthMin =    get_unaligned_le16(&buffer[8]);
            term->camera.wObjectiveFocalLengthMax =    get_unaligned_le16(&buffer[10]);
            term->camera.wOcularFocalLength = get_unaligned_le16(&buffer[12]);
            memcpy(term->camera.bmControls, &buffer[15], n);
        } else if (UVC_ENTITY_TYPE(term) == UVC_ITT_MEDIA_TRANSPORT_INPUT) {
            term->media.bControlSize = n;
            term->media.bmControls = (u8 *)term + sizeof(*term);
            term->media.bTransportModeSize = p;
            term->media.bmTransportModes = (u8 *)term + sizeof(*term) + n;
            memcpy(term->media.bmControls, &buffer[9], n);
            memcpy(term->media.bmTransportModes, &buffer[10+n], p);
        }

        if (buffer[7] != 0)
            usb_string(udev, buffer[7], term->name,sizeof(term->name));
        else if (UVC_ENTITY_TYPE(term) == UVC_ITT_CAMERA)
            sprintf(term->name, "Camera %u", buffer[3]);
        else if (UVC_ENTITY_TYPE(term) == UVC_ITT_MEDIA_TRANSPORT_INPUT)
            sprintf(term->name, "Media %u", buffer[3]);
        else
            sprintf(term->name, "Input %u", buffer[3]);

        list_add_tail(&term->list, &dev->entities);
        break;

代码看着其实挺乱的，不过如果对代码整理，我相信大部分都会很看地很清楚的：
先定义3个结构体，分别为输入终端描述符，相机终端描述符和序列媒体描述符。

// Input Terminal Descriptor
typedef struct _USB_IT_DESCRIPTOR
{
   UINT8  bLength;
   UINT8  bDescriptorType;
   UINT8  bDescriptorSubtype;
   UINT8  bTerminalID;
   UINT16 wTerminalType;
   UINT8  bAssocTerminal;
   UINT8  iTerminal;
   UINT8  reserved[0];
} USB_IT_DESCRIPTOR;

typedef struct _CAMERA_TERMINAL_DESCRIPTOR
   UINT8  bLength;
   UINT8  bDescriptorType;
   UINT8  bDescriptorSubtype;
   UINT8  bTerminalID;
   UINT16 wTerminalType;
   UINT8  bAssocTerminal;
   UINT8  iTerminal;
   UINT16 wObjectiveFocalLengthMin;
   UIN16 wObjectiveFocalLengthMax;
   UIN16 wOcularFocalLength;
   UINT8 bControlSize;
   UINT8  bmControls[3];
 }CAMERA_TERMINAL_DESCRIPTOR;

然后根据不同的bTerminalID，使用不同的结构体处理：

 if (UVC_ENTITY_TYPE(term) == UVC_TERM_INPUT) {
 ...
 }
else if (UVC_ENTITY_TYPE(term) == UVC_ITT_CAMERA){
...
}
else if (UVC_ENTITY_TYPE(term) == UVC_ITT_MEDIA_TRANSPORT_INPUT){
...
}

至于申请的通用结构体uvc_entity，内部根据类型的不同，其用不同的结构体而已。这种情况在windows内核中太常见了，Linux中我也是第一次见。

struct uvc_entity {
...
    u8 id;
    u16 type;
...
union {
        struct {
            u16 wObjectiveFocalLengthMin;
            u16 wObjectiveFocalLengthMax;
            u16 wOcularFocalLength;
            u8  bControlSize;
            u8  *bmControls;
        } camera;

        struct {
            u8  bControlSize;
            u8  *bmControls;
            u8  bTransportModeSize;
            u8  *bmTransportModes;
        } media;

        struct {
        } output;

        struct {
            u16 wMaxMultiplier;
            u8  bControlSize;
            u8  *bmControls;
            u8  bmVideoStandards;
        } processing;

        struct {
        } selector;

        struct {
            u8  guidExtensionCode[16];
            u8  bNumControls;
            u8  bControlSize;
            u8  *bmControls;
            u8  *bmControlsType;
        } extension;
    };
}