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Plextor DMA problem on Linux


I've a puzzling problem and hope someone's experience
can help me.
I have quite a recent Plextor 48/24/48 burner attached to my Linux
system (currently 2.4.23-rc2)

This drive has been running initially with my kernel which enables
DMA by default.
Now I cannot use the drive anymore under such a kernel.
readcd dev=0,1,0 -f /dev/null  works but that's special cased since
readcd dev=0,1,0 -f - | cat >/dev/null  DOESN'T work and even
a normal 'mount' doesn't work. Always, I see a DMA timeout error on
I've tried various things like the boot parameters acpi=off and I've 
connected the drive to the other cable of my VIA vt82c686a onboard
controller. Nothing helped.
Then I regenerated the kernel with the option 'DMA for disks only'
and now I have no problems with the drive except a simple
readcd now produces a system load of more than 90% (on an otherwise
idle system) and the read speed drops to 4250 kb/s instead of
7056 kb/s as reported by readcd initially.

Is my drive defect now (just the DMA logic) ?
This occurred before and after upgrading the drive's BIOS to the
most recent version. Or is it a Linux problem, but that's hard
to believe since even an older kernel which has run fine with this
drive doesn't run anymore with this drive.

Any ideas and comments are more than welcome,

Helmut Jarausch

Lehrstuhl fuer Numerische Mathematik
RWTH - Aachen University
D 52056 Aachen, Germany

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