VIA VT6421 w/a (Re: UDMA cables vs speeds)

[perryh at pluto.rain.com]

This is for the archives.  The original posting is here:

http://lists.freebsd.org/pipermail/freebsd-hardware/2011-June/006704.html

> Should an 80-wire Ultra-ATA data cable intended for use at UDMA66
> or UDMA100 also work at UDMA133, or does the higher speed require
> different cabling?

Answer:  as nearly as I've been able to determine, there are only
two kinds of PATA data cables:  the original 40-wire variety that
work for PIO and for UDMA33, and the newer 80-wire variety that
should work for all modes.

> ... I've been experiencing occasional UDMA ICRC errors on the
> IDE port of a PCI SATA/IDE controller card using the VIA VT6421
> chipset, and I've noticed that "atacontrol mode" reports the
> port speed as UDMA133.  The data cable is brand new ...
> I've found several similar reports in the forum and mailing-list
> archives, with no definitive solution but quite a few "it works
> for me" responses, and one observation that the VT6421 seems to
> work well with some drives and not so well with others.  It occurs
> to me that the difference might, perhaps, involve the DMA speeds

DMA speed definitely seems to be a factor.  The errors seem to have
been completely eliminated after I used

  atacontrol mode ad8 UDMA100

to limit the maximum transfer rate.

Throttling the DMA speed is not an ideal solution, and I still have
no clue _why_ it doesn't work reliably at UDMA133, but this does seem
to have gotten around the problem.