CMD 64x Series PCI ATA Controllers
The following CMD controllers are supported:
- CMD643 (MWDMA 2)
- CMD646 (MWDMA 2)
- CMD646U (MWDMA 2)
- CMD646U2 (UDMA 2)
- CMD648 (UDMA 4)
- CMD649 (UDMA 5)
For the CMD640 see pata_cmd640
For the CMD/SI680 see pata_sil680
PIO, MWDMA, UDMA
Early CMD646 chips such as CMD646 and CMD646U have UDMA problems. On these chipsets Linux will fall back to the best mode available avoiding UDMA.
CMD646U2 has the same design with CMD643. Both have two IDE channels that share the data lines. In MWDMA modes there is no problem but in UDMA2 mode with four or two drives cable limitations apply. With four drives total cable length of both channels must not exceed 18" at most, while with two drives total cable length of both channels must not exceed 36" at most. Failing to comply with this limitation will result in degraded signal quality.