The mainboard MSI E350DM-E33 is advertised with 6 SATA ports.
As it turns out, two of them seem to be driven by on-board
SATA<->PATA converters. If a disk drive is connected to one
of them kernel uses UDMA/33 mode due to cable detection:

[   34.550823] scsi4 : pata_atiixp
[   34.555517] scsi5 : pata_atiixp
[   34.555942] ata5: PATA max UDMA/100 cmd 0x1f0 ctl 0x3f6 bmdma 0xf100 irq 14
[   34.555948] ata6: PATA max UDMA/100 cmd 0x170 ctl 0x376 bmdma 0xf108 irq 15
...
[   35.040799] ata5.00: ATA-8: WDC WD20EADS-00R6B0, 01.00A01, max UDMA/133
[   35.040806] ata5.00: 3907029168 sectors, multi 16: LBA48 NCQ (depth 0/32)
[   35.040817] ata5.00: limited to UDMA/33 due to 40-wire cable
[   35.049166] ata5.00: configured for UDMA/33
[   35.049402] scsi 4:0:0:0: Direct-Access     ATA      WDC WD20EADS-00R 01.0 PQ: 0 ANSI: 5

This patch forces "short cable" mode on this board, as it seems clear that
the on-board SATA<->PATA "cable" is short.
With this patch the disk is configured for UDMA/100:

[    5.976756] ata5.00: ATA-8: WDC WD20EADS-00R6B0, 01.00A01, max UDMA/133
[    5.996434] ata5.00: 3907029168 sectors, multi 16: LBA48 NCQ (depth 0/32)
[    6.024787] ata5.00: configured for UDMA/100

Testing revealed no transfer issues.
Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>