[Gllug] Can't set DMA on for CD drive which is only IDE device in system

[John Winters]

I've come across an odd problem in several systems which I've played
with recently.  The common factor is that each has an IDE CD or DVD
drive as the only IDE device in the system.  One has a SCSI HD, one an
SATA HD and two are diskless workstations (apart from the CD drive).

On all of them I'm unable to set DMA access to on for the IDE drive.  If
I try I get:

knight:~# ls -l /dev/cdrom
lrwxrwxrwx  1 root root 3 Oct 16 09:16 /dev/cdrom -> hda
knight:~# hdparm -d 1 /dev/hda

/dev/hda:
 setting using_dma to 1 (on)
 HDIO_SET_DMA failed: Operation not permitted
 using_dma    =  0 (off)
knight:~#

Anyone any idea why this is?  The systems are running Debian Sarge with
an almost-stock 2.6.8 kernel (just re-compiled to suit the particular
processor).  One is an Athlon 64 3000 (32 bit installation), one a
Celeron 2.66 GHz and the other two are Mini-ITX.

I've tried putting an IDE HD into the Athlon box (which is normally
Serial ATA) and then I *can* enable DMA on the DVD drive.

TIA,
John

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