Fix to my Promise SATA timeout problems

[Rich Wales]

Several days ago, I wrote (in -stable, -current, and -hardware)
about problems I had been having with a Promise SATA300 TX4 card
(PDC40718 chip).

Per a suggestion from Søren, I rechecked my hardware and my BIOS
settings.  I eventually found that I was able to make the timeout
problems disappear by disabling PCI master burst mode in the BIOS
setup of my system.  (For the record, I've been working with an
800-MHz original "Slot A" Athlon on a VIA VAM-0070 motherboard --
a board which appears to be a rebranded EPoX EP-7KXA.)

Not surprisingly, turning off burst mode had a significant impact
on the disk I/O speed of the system, but at least it's stable now,
with no more timeouts or failures.

I'm not sure if the primary culprit here is the Promise card or
the motherboard, but it suggests to me that some systems with iffy
PCI bus tolerances might require suboptimal BIOS settings in order
to work properly with some Promise controllers.  At least, this is
something to try if you encounter problems with a Promise card.

As for whether any effective steps can be taken in the driver to
make it more robust against this problem, I'll have to leave that
to people who are more familiar with the hardware and the code.

I've updated PR 103435 with this info.

Rich Wales
Palo Alto, CA, USA
richw at richw.org
http://www.richw.org