DMA disabled by default for optical drives - why?

[neil woolford]

On Tue, 2005-12-20 at 11:23 +0100, Robert Entner wrote:
> On Tuesday 20 December 2005 07:25, James Gray wrote:

> >
> > All well-and-good, but what if the Ubuntu installer loads the
> > ide-generic driver before your chipset-specific IDE driver resulting in
> > a situation that setting DMA on any drive causes a kernel panic?  Not
> > Ubuntu's fault mind you - it was a known gotcha with early Asus K8VSE
> > motherboard BIOS'es..
> >
> > So enable DMA by default and possibly panic the kernel if the BIOS is a
> > little screwy, or play safe?  Personally, I'd rather have a machine
> > that boots (even at sub-optimal speed) so I can fix the problem over a
> > system that panics before it even reaches run level 1. :)  It's not
> > just old hardware that can e flakey with DMA enabled by deafult -
> > unfortunately.
> 
> How does Windows solve this problem?

I don't know how it solves it automagically out of the box, but I do
know that my Dell 340 came set up with DMA off under Windows 2000, even
though it was capable of using it and turning it on gave me much higher
disc access speeds for read and write.  I suspect the default Windows
install probably plays safe like Ubuntu does, for much the same reasons.

However, there is a caveat;  my Dell did not come direct from Dell, it
was an overstock/return/whatever bought from a third party reseller
(Europc).  So Dell themselves did not install the OEM Windows 2000 that
came with it (fully legitimate, COC on the case, hologram disc etc).
I'm sure a Dell factory install of Windows would have had DMA working,
just as Dell factory installs have drivers for just about every printer
ever built ready to spring into action as soon as the os detects the
hardware.

I think many Windows installs from mainstream vendors are heavily
tweaked and optimised for the hardware they are installed on.  The
generic install probably isn't and will play safe on things like DMA.

Neil
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