When trying to use hdparm to activate the DMA mode, I got the error message

"HDIO_SET_DMA failed".

This could be solved by unsetting _all_ other IDE chipset drivers than the driver for the Intel chipsets (CONFIG_BLK_DEV_PIIX=y), even CONFIG_IDE_GENERIC is not set. If CONFIG_BLK_DEV_IDEDMA_PCI=y and CONFIG_BLK_DEV_IDEDMA_FORCED=y are set, it is not even necessary to use hdparm for the activation.