[Soekris] net5501 ATA UDMA mode problem with FreeBSD 7.0

[Martin Johnson]

I'm upgrading a net5501 with a Travelstar E7K100 IDE drive from  
FreeBSD 6.2 to 7.0.   FreeBSD 6.2 set the IDE controller up in UDMA33  
mode, but unfortunately 7.0 chooses UDMA100.   From my tests, the  
fastest safe setting for this hardware appears to be UMDA66, as  
UDMA100 gives  occasional errors such as "ad0: WARNING - READ_DMA UDMA  
ICRC error (retrying request) LBA=3871".  This is the same on comBIOS  
versions 1.32i and 1.33.

Is there any way to force FreeBSD 7.0 to use UDMA66, or failing that,  
UDMA33 ?

I could disable UDMA by adding 'hw.ata.ata_dma="0"' in /boot/ 
loader.conf, but that would run slowly. I could put a script in /usr/ 
local/etc/rc.d to invoke "atacontrol mode ad0 udma66", but that would  
still leave the system running in an unstable configuration until that  
script was invoked.

Any ideas?   I'm wondering whether I'll need to patch the kernel  
source, and if so, how difficult that will be.    It would be nice if / 
boot/loader.conf had a switch for this, but I haven't spotted any  
documentation for such a thing.

Cheers,

- Martin.