Comment 2 for bug 175022

I have the same problem on Ubuntu Hardy Heron with a Samsung TS-H552B (TSSTcorpCD/DVDW) and 2.6.24-5 Kernel running on an MSI RS480M2/RX480M2 motherboard. If I tried to play a DVD its playback was slow and jerky. Looking on the Ubuntu forums I found that the problem might be that DMA wasn't enabled so I ran:
 $ sudo hdparm -d1 /dev/scd0
     setting using_dma to 1 (on)
     HDIO_SET_DMA failed: Inappropriate ioctl for device

Then following a tip from a forum post I ran
$ dmesg
   ...
   [ 35.286790] ata2.00: ATAPI: TSSTcorpCD/DVDW TS-H552B, GA04, max UDMA/33
   [ 35.286804] ata2.00: simplex DMA is claimed by other device, disabling DMA
   ...

Then searched Ubuntu forums for "simplex DMA" and found these posts http://ubuntuforums.org/showthread.php?t=435706 and http://ubuntuforums.org/showthread.php?t=678153
Folowing the advice in the first post I ran:
$ lsmod | grep -i ^libata”
   libata 176304 4 pata_acpi,sata_sil,ata_generic,pata_atiixp

Then following the advice in the second post in reply #9 I blacklisted the ata_generic module.
WARNING BLACKLISTING THE WRONG MODULE CAN RESULT IN A NON BOOTABLE MACHINE
I did this by opening the file /etc/initramfs-tools/modules and adding these lines at the bottom:
  pata_atiixp
  blacklist ata_generic

Then I recreated the initramfs with
$ sudo update-initramfs -u

Then after rebooting the CDROM drive now has DMA enabled and the DVD's playback smoothly.

From this it appears that there is a bug in the ata_generic module that prevents DMA from being enabled on some CDROM Drives on some computers.