Comment 14 for bug 110636

I have/had the same problem, on Ubuntu Feisty, and I have a workaround

My mainboard has a generic PATA-controller (Intel ICH4) and a Promise PATA-fakeraid-controller (PDC20276)
My harddisks are connected to the Promisecontroller and my DVD-drives are connected to the intelcontroller
The harddisks worked ok I think, but the DVD-drives didn´t user DMA and it´s was not possible to enable it.

So I figured out that the problem is the PATA-support of the libata-modules. Before feisty libata wasn´t used for PATA-drives and everything worked fine, so I tried to compile a kernel without libata.

I took the 2.6.21.1 Version from kernel.org but I think it should work with 2.6.20 too.
Use the config of the standard Ubuntu kernel (/boot/config-2.6.20-15-generic) and start menuconfig and deselected the libata-module (Device drivers-Serial ATA (prod) Parallel ATA (experimental) drivers - ATA Device support)
and then select the old ATA modules for your PATA-Controller(s) under Device Drivers-ATA/ATAPI/MFM/RLL support-
(in my case the PROMISE PDC202{68|69|70|71|75|76|77} support) the if it isn´t selected.

then compile and install the kernel, and that´s it, you don´t have to adapt grub or fstab. All devices are recognized as /dev/hdx and hdparm is fully working.

I guess it should be enough to disable the pata-modules of libata and enable the old pata-drivers, but I didn´t test this, because I have no SATA drives, and didn´t need libata at all.