sendfile(2) use with a char driver

I'm looking to use a sendfile(2) with a Xilinx XDMA kernel driver in order to move data from a PCIe board with Xilinx FPGA to the network card with "zero-copy".

Currently I'm getting EINVAL return status from sendfile(2) when providing opened XDMA device file descriptor as input fd.

The device driver provides a character device that can be mmap'ed.

There seem to be other restrictions. Can anyone provide insight on what would be needed to make this work?

Thanks! //hinko