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List:       freebsd-hackers
Subject:    Re: New warning, should I worry?
From:       john hood <cgull () smoke ! marlboro ! vt ! us>
Date:       1997-08-30 7:27:26
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John S. Dyson writes:
 > john hood said:
 > > 
 > > I've changed the message so it only appears when the bit is set (DMA
 > > has been configured) and the boot -v flag is set.  I'm still modestly
 > > curious to hear reports of motherboards and controller BIOSes that do
 > > set it (they're unusual), but I no longer need to know in order to get
 > > things working.
 > > 
 > Supermicro P6DNF does.  I can turn the message on and off by flipping
 > an ON/OFF in the bios.  I usually prefer for it to be off, but some
 > might like to see it :-).  It depends on my mood :-).
 > 
 > (Actually, the bit appears to be quite useless...)

It isn't, quite.

If the bit is set, that means that no chipset- or drive-specific setup
needs to be done, and the driver can just go ahead and use the DMA
controller.

This means that if you have some chipset that the driver doesn't know
about (most of them, besides Intel's), or your drive uses something
other than the usual DMA modes (very rare), the driver will work.

It'll probably usually work anyway on other chipsets, because the
driver also checks to see whether the drive has been configured to the
usual DMA settings. If it has, the driver assumes that the chipset has
also been configured and that everything will work.  (This is the case
that most people's machines fall into.)

If BIOSes consistently set this bit, I wouldn't need an awful lot of
initialization code and lame heuristics.  But these are PCs we're
talking about here :)

  --jh

-- 
John Hood				cgull@smoke.marlboro.vt.us

Predictably, they all eventually wandered away, rubbing their bruises
and brushing mud out of their hair.  Some went off to work for the
ESA, launching much smaller rockets into low orbits, while others
elected to sit on their front porches drinking Jim Beam from the
bottle and launching bottle rockets from the empties. [Jordan Hubbard]

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