HI3531 VPSS工作模式

我刚开始学习3531，基于sample vdec，写了一个显示jpeg图像到DHD0的程序

程序能正常工作，但是如果我试图调用HI_MPI_VPSS_SetChnMode设置工作模式为VPSS_CHN_MODE_USER，程序不报错，但是显示无图像。

[code]int startVpss() {/*{{{*/
    VPSS_GRP vpssGrp;
    VPSS_CHN vpssChn;
    VPSS_GRP_ATTR_S stGrpAttr;
    VPSS_CHN_ATTR_S stChnAttr;
    VPSS_GRP_PARAM_S stVpssParam;
    VPSS_CHN_MODE_S stChnMode;
    HI_S32 s32Ret;

    stGrpAttr.u32MaxW = 2048;
    stGrpAttr.u32MaxH = 2048;
    stGrpAttr.bDrEn = HI_FALSE;
    stGrpAttr.bDbEn = HI_FALSE;
    stGrpAttr.bIeEn = HI_TRUE;
    stGrpAttr.bNrEn = HI_TRUE;
    stGrpAttr.bHistEn = HI_FALSE;
    stGrpAttr.enDieMode = VPSS_DIE_MODE_AUTO;
    stGrpAttr.enPixFmt = SAMPLE_PIXEL_FORMAT;

    vpssGrp = 0;

    /*** create vpss group ***/
    s32Ret = HI_MPI_VPSS_CreateGrp(vpssGrp, &stGrpAttr);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_CreateGrp failed with 0x%#x!\n", s32Ret);
        return HI_FAILURE;
    }

    /*** set vpss param ***/
    s32Ret = HI_MPI_VPSS_GetGrpParam(vpssGrp, &stVpssParam);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_GetGrpParam failed with 0x%#x!\n", s32Ret);
        return HI_FAILURE;
    }
   
    stVpssParam.u32MotionThresh = 0;
   
    s32Ret = HI_MPI_VPSS_SetGrpParam(vpssGrp, &stVpssParam);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_SetGrpParam failed with 0x%#x!\n", s32Ret);
        return HI_FAILURE;
    }

    /*** enable vpss chn, with frame ***/
    vpssChn = 2;
    /* Set Vpss Chn attr */
    stChnAttr.bSpEn = HI_FALSE;
    stChnAttr.bFrameEn = HI_TRUE;
    stChnAttr.stFrame.u32Color[VPSS_FRAME_WORK_LEFT] = 0xff00;
    stChnAttr.stFrame.u32Color[VPSS_FRAME_WORK_RIGHT] = 0xff00;
    stChnAttr.stFrame.u32Color[VPSS_FRAME_WORK_BOTTOM] = 0xff00;
    stChnAttr.stFrame.u32Color[VPSS_FRAME_WORK_TOP] = 0xff00;
    stChnAttr.stFrame.u32Width[VPSS_FRAME_WORK_LEFT] = 2;
    stChnAttr.stFrame.u32Width[VPSS_FRAME_WORK_RIGHT] = 2;
    stChnAttr.stFrame.u32Width[VPSS_FRAME_WORK_TOP] = 2;
    stChnAttr.stFrame.u32Width[VPSS_FRAME_WORK_BOTTOM] = 2;
   
    s32Ret = HI_MPI_VPSS_SetChnAttr(vpssGrp, vpssChn, &stChnAttr);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_SetChnAttr failed with 0x%#x\n", s32Ret);
        return HI_FAILURE;
    }

    stChnMode.enChnMode = VPSS_CHN_MODE_USER;
    stChnMode.enPixelFormat = SAMPLE_PIXEL_FORMAT;
    stChnMode.bDouble = HI_FALSE;
    stChnMode.u32Width = 1920;
    stChnMode.u32Height = 1080;
    //s32Ret = HI_MPI_VPSS_SetChnMode(vpssGrp, vpssChn, &stChnMode);             // <---------------------- 调用这个就不显示图像
    //if(s32Ret != HI_SUCCESS) {
    //    SAMPLE_PRT("HI_MPI_VPSS_SetChnMode failed with %#x\n", s32Ret);
    //    return HI_FAILURE;
    //}

    s32Ret = HI_MPI_VPSS_EnableChn(vpssGrp, vpssChn);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_EnableChn failed with 0x%#x\n", s32Ret);
        return HI_FAILURE;
    }

    /*** start vpss group ***/
    s32Ret = HI_MPI_VPSS_StartGrp(vpssGrp);
    if (s32Ret != HI_SUCCESS) {
        SAMPLE_PRT("HI_MPI_VPSS_StartGrp failed with 0x%#x\n", s32Ret);
        return HI_FAILURE;
    }

    return HI_SUCCESS;
}/*}}}*/
[/code]


1.默认AUTO模式下，可以正常显示图像。VPSS从哪里自动得到相应的配置？是从绑定到的VO得到的吗？
2.我这里应该如何设置才能正常显示图像呢？

P.S. 我看到文档里说如果要用到虚拟VO，就得设置VPSS的模式为USER，所以我想先在DHD0试试这个API