HI_MPI_VPSS_SetExtChnAttr 返回错误码：0xa0078009

各位大神：请教，
需求：通过crop 来实现类似电子放大的功能；使用扩展通道4；
问题：初始化时就失败了，具体表现为：
VPSS_EXT_CHN_ATTR_S stVpssExtChnAttr;
HI_MPI_VPSS_GetExtChnAttr(VpssGrp1, VpssChn, &stVpssExtChnAttr);
stVpssExtChnAttr.s32BindChn = 2;
stVpssExtChnAttr.u32Width = stSize.u32Width;
stVpssExtChnAttr.u32Height = stSize.u32Height;
stVpssExtChnAttr.s32DstFrameRate = -1;
stVpssExtChnAttr.s32SrcFrameRate = -1;
stVpssExtChnAttr.enPixelFormat = PIXEL_FORMAT_YUV_SEMIPLANAR_420;
stVpssExtChnAttr.enCompressMode = COMPRESS_MODE_NONE;


s32Ret = SAMPLE_COMM_VPSS_EnableChn(VpssGrp1, VpssChn, &stVpssChnAttr, &stVpssChnMode,  &stVpssExtChnAttr);
if (HI_SUCCESS != s32Ret) {
        FM_RECORD_LOG("Enable vpss chn failed!\n");
        goto END_VENC_1080P_CLASSIC_4;
}
执行返回值：
[SAMPLE_COMM_VPSS_EnableChn]-140: SAMPLE_COMM_VPSS_EnableChn failed with 0xa0078009；
查看 /dev/logmpp 提示：
<3>[  vpss] [Func]:VPSS_SetExtChnAttr [Line]:8104 [Info]:<3>[  vpss] src chn2 should be user mode!
chn2 是已经设置了user mode的；