The ability to use MCLBYTES larger than 2048 is ifdef'd out in the wm
driver, with the somewhat cryptic comment:

       /*
        * XXX MCLBYTES > 2048 causes "Tx packet consumes too many DMA"
        * XXX segments, dropping" -- why?
        */

The message above is probably a result of an MD bug somewhere
(in bus_dma_load_mbuf), the card has no problem using larger buffers.

I have used 4096 bytes mbuf clusters on x86 with many different
wm cards for a long time now without any problem at all, and I 
would like to turn on the ability again. Comments?

-- Ragge