uvc_entity结构体的申请uvc_alloc_entity函数

uvc_entity结构体在使用uvc_alloc_entity申请时，其不仅包含该结构本身，也包括其它额外的内存。
内存的申请使用函数uvc_alloc_entity，其具体代码可见附录。

static struct uvc_entity *uvc_alloc_entity(u16 type, u8 id,
        unsigned int num_pads, unsigned int extra_size)

使用uvc_alloc_entity申请的内存，大体包括4部分：

第一部分为绿色，为uvc_entity结构体本身的大小。sizeof(uvc_entity)

struct uvc_entity {
    struct list_head list;        /* Entity as part of a UVC device. */
    struct list_head chain;        /* Entity as part of a video device
                     * chain. */
    unsigned int flags;

    u8 id;
    u16 type;
    char name[64];

    /* Media controller-related fields. */
    struct video_device *vdev;
    struct v4l2_subdev subdev;
    unsigned int num_pads;
    unsigned int num_links;
    struct media_pad *pads;

    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;
    };

    u8 bNrInPins;
    u8 *baSourceID;

    unsigned int ncontrols;
    struct uvc_control *controls;
};

第二部分为extra_size，但因第二部分后续需跟struct media_pad，故extra_size会对齐struct media_pad的整数倍。

    //对齐到1个sizeof(*entity->pads))大小
    extra_size = roundup(extra_size, sizeof(*entity->pads));

extra_size的内容主要用于存储描述符bmControls，这些bmControls各位代表该单元支持的UVC特定类请求。如对于相机终端描述符其初始化如下：

        if (UVC_ENTITY_TYPE(term) == UVC_ITT_CAMERA) {
            term->camera.bControlSize = n;
            term->camera.bmControls = (u8 *)term + sizeof(*term);
            memcpy(term->camera.bmControls, &buffer[15], n);

其它支持UVC特定类请求的类似，如处理单元描述符，扩展单元描述符。

第三部和第四部分都是media_pad结构体，但第四部分其是额外多申请的一部分，用于存储描述符的baSourceID，故在申请时，都可以看到有+1情况。

unit = uvc_alloc_entity(buffer[2], buffer[3], p + 1, 0);//UVC_VC_SELECTOR_UNIT
term = uvc_alloc_entity(type | UVC_TERM_OUTPUT, buffer[3],1, 0);//UVC_VC_OUTPUT_TERMINAL
term = uvc_alloc_entity(type | UVC_TERM_INPUT, buffer[3],1, n + p);//UVC_VC_INPUT_TERMINAL
unit = uvc_alloc_entity(buffer[2], buffer[3], p + 1, 0);//UVC_VC_SELECTOR_UNIT
unit = uvc_alloc_entity(buffer[2], buffer[3], 2, n);//UVC_VC_PROCESSING_UNIT

至于第三部分，从申请内存来看，只有扩展单元申请到内存,至于怎么用，暂未分析到。

static struct uvc_entity *uvc_alloc_entity(u16 type, u8 id,
        unsigned int num_pads, unsigned int extra_size)
{
    struct uvc_entity *entity;
    unsigned int num_inputs;
    unsigned int size;
    unsigned int i;

    //对齐到1个sizeof(*entity->pads))大小
    extra_size = roundup(extra_size, sizeof(*entity->pads));

    //输入类型则少一个，输出类型保持不变
    num_inputs = (type & UVC_TERM_OUTPUT) ? num_pads : num_pads - 1;

    size = sizeof(*entity) 
        + extra_size 
        + sizeof(*entity->pads) * num_pads
        + num_inputs;

    entity = kzalloc(size, GFP_KERNEL);
    if (entity == NULL)
        return NULL;

    entity->id = id;
    entity->type = type;

    entity->num_links = 0;
    entity->num_pads = num_pads;
    entity->pads = ((void *)(entity + 1)) + extra_size;

    for (i = 0; i < num_inputs; ++i)
        entity->pads[i].flags = MEDIA_PAD_FL_SINK;

    if (!UVC_ENTITY_IS_OTERM(entity))
        entity->pads[num_pads-1].flags = MEDIA_PAD_FL_SOURCE;

    entity->bNrInPins = num_inputs;
    entity->baSourceID = (u8 *)(&entity->pads[num_pads]);

    return entity;
}