> So I messed around with this last night (it is a M5229 BTW).  Changing
> 
> 	drvp->n_dmaerrs = NERRS_MAX-1;
> 
> in ata.c to
> 
> 	drvp->n_dmaerrs = NERRS_MAX-2;
> 
> And my disk runs quite happily in UDMA-2 mode.

And it turns out that all the above is rubbish, and I wasn't seeing this 
happen at all.  It all comes of looking for the symptom in the logs -- I 
was looking for the downgrade message and not seeing that, so assumed that 
everything was fine.  What I'd forgotten was that I'd build a kernel with 
DMA entirely disabled for wd0 (part of tracking down a different problem) 
and I'd forgotten to revert the change -- so it was coming up in PIO mode 
4 in the first place, and it certainly wasn't going to downgrade from 
there.

So I'm still stuck with a UDMA-5 disk and a UDMA-2 controller that can't 
agree on running above UDMA-1.

R.