I think that I've found and fixed the problem. There is a copy/paste bug in  
vt6421_set_dma_mode() function which causes wrong values to be written to  
PATA_UDMA_TIMING register.  
 
 
This patch fixes a copy/paste bug that breaks DMA modes on VT6421 PATA port.  
 
Signed-off-by: Ondrej Zary <linux@rain...>  
diff -Nur linux-2.6.21.6/drivers/ata/sata_via.c linux-2.6.21.6-new/drivers/ata/sata_via.c
--- linux-2.6.21.6/drivers/ata/sata_via.c	2007-07-07 06:47:55.000000000 +0200
+++ linux-2.6.21.6-new/drivers/ata/sata_via.c	2007-09-29 11:05:07.000000000 +0200
@@ -380,6 +380,6 @@
 {
 	struct pci_dev *pdev = to_pci_dev(ap->host->dev);
 	static const u8 udma_bits[] = { 0xEE, 0xE8, 0xE6, 0xE4, 0xE2, 0xE1, 0xE0, 0xE0 };
-	pci_write_config_byte(pdev, PATA_UDMA_TIMING, udma_bits[adev->pio_mode - XFER_UDMA_0]);
+	pci_write_config_byte(pdev, PATA_UDMA_TIMING, udma_bits[adev->dma_mode - XFER_UDMA_0]);
 }