The reference manual for the Ethernet controller on i.MX28 says in chapter 26.2.2:
This section provides the technical details for the uDMA. Additional details required for
legacy mode compatibility is provided in the FEC specification (FEC_BlockGuide.pdf).
Where can I find this FEC specification?
You may use FEC chapters of i.MX25 and i.MX27 Reference Manual - they are based on the same FEC_BlockGuide.pdf.
http://cache.freescale.com/files/dsp/doc/ref_manual/IMX25RM.pdf?fpsp=1
http://cache.freescale.com/files/32bit/doc/ref_manual/MCIMX27RM.pdf?fpsp=1