SPI transaction between FPGA and Microcontroller
The thing I didn't have in mind, is that SPI communications suggests that you should send as much data as you want to receive, not more, not less.
So my buffer was noisy because of that. By setting m_length to 1 :     err_code = nrf_drv_spi_transfer(&amp;spi, m_tx, m_length, m_rx_buf, m_length);
I am successfully transmitting one byte, while receiving exactly one byte in my receive buffer m_rx_buf

For the first transfer (command) I'm receiving dummy data that I don't process, for the actual transfer I'm buffering it and processing it.