ADRV9009 TX data width

I'am using zcu102+adrv9009. Now I want to send my customed IP's data to ADRV9009 directly from PL. My own data is I/Q format with 16*2bits. I want to send my data from TX channel 0 of 9009.

As we all know, the data width of RX channel is 64bits=2(two channel)*16(data width per channel)*2(I/Q data format). But I found the dac_data of TX channel is 128 bits width, showing as figure below. In fact there are only two tx channels on ADRV9009. How can I insert my 16*2 bits I/Q data into the 128bits-width dac_data? I tried to Insert my data in dac_data[31:0] and [63:32], but both failed. Please help me. Thank you!