If this is red hat you have to enter an option in modules.conf to enable dma...
here's a link:
ENABLE DMA ON CD/DVD DRIVES
* DMA was disabled by default in Red Hat 8.0 to protect some people who have systems that become unstable when using DMA on their optical drives. In most cases you can safely re-enable DMA for your optical drives with a simple configuration change and regain better performance. This is needed for example for smooth DVD movie playing.
* In order to enable DMA support, open /etc/modules.conf in any text editor and add a single line: options ide-cd dma=1.
* Making sure that the CD drive is not in use, as root type "rmmod ide-cd". You should now be able to enable DMA on the CD drive. If this causes instability on your system, you may have to change it back.
This is applies to 9 afaik as well