libata/ata_piix stuck in combined mode

[Matteo Brusa <matteo.brusa@gmail.com>]

Hi,
I'm running Sarge on a Dell PE 750, official Debian kernel 2.6.8-2-686,
with 2 SATA disks. The disk controller is an Intel ICH5.
Since I upgraded from kernel 2.4 to 2.6, the DMA disk access doesn't
work anymore, because libata and ata_piix goes in combined mode:

# grep -i ata /var/log/dmesg
 BIOS-e820: 000000003ffc0000 - 000000003ffcfc00 (ACPI data)
Memory: 1031108k/1048320k available (1551k kernel code, 16284k reserved, 690k data, 148k init,
130816k highmem)
libata version 1.02 loaded.
ata_piix version 1.02
ata_piix: combined mode detected
ata1: PATA max UDMA/33 cmd 0x1F0 ctl 0x3F6 bmdma 0xFEA0 irq 14
ata1: dev 0 cfg 49:0f00 82:0000 83:0000 84:0000 85:0000 86:0000 87:0000 88:0407
ata1: dev 0 ATAPI, max UDMA/33
ata1: dev 0 configured for UDMA/33
scsi0 : ata_piix
ata2: SATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0xFEA8 irq 15
ata2: dev 0 cfg 49:2f00 82:7c6b 83:7b09 84:4003 85:7c69 86:3a01 87:4003 88:207f
ata2: dev 0 ATA, max UDMA/133, 156250000 sectors:
ata2: dev 0 configured for UDMA/133
scsi1 : ata_piix
  Vendor: ATA       Model: Maxtor 6Y080M0    Rev: YAR5
EXT3-fs: mounted filesystem with ordered data mode.

I've read some posts here about disabling the PATA/SATA mixed mode in the BIOS; too bad there's no
such a switch in Dell servers.
I tried to disable the IDE cdrom in the BIOS, with no luck. Also kernel 2.6.12 from Knoppix 4.0.2
couldn't help.
Is there a way to force the ata_piix module out of combined mode?

Thanks in advance.
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