For whatever ingenious reason RedHat decide to purposely turn off DMA on cdrom/dvd-rom drives. I have no idea why you would do that. You have to pass a kernel option to enable it. It boils down to the fact that you cannot playback DVDs without DMA on.

o DMA is disabled on CD-ROM drives in this release in a different but more reliable way than previously. If you are sure that your CD-ROM drive is capable of IDE DMA, place the following line in the /etc/modules.conf file: options ide-cd dma=1
Without this set you will get:

HDIO_SET_DMA failed: Operation not permitted.

when trying to hdparm the drive.