serverworks CSB5 IDE: DMA disabled on secondary channel

[Frantisek Hanzlik <franta@hanzlici.cz>]

Server with Intel SHG2 board, servervorks CSB5 rev. 93: After
upgrade from kernel 2.6.22 (IDE serverworks driver) to 2.6.25
(Fedora 9: libata+pata_serverworks driver), SW RAID1 performance
is very bad. Problem is probably with secondary IDE channel,
which has disabled DMA:

# hdparm -Tt /dev/sda
/dev/sda:
  Timing cached reads:   828 MB in  2.00 seconds = 414.26 MB/sec
  Timing buffered disk reads:  210 MB in  3.01 seconds =  69.83 MB/sec

# hdparm -Tt /dev/sdb
/dev/sdb:
  Timing cached reads:   408 MB in  2.00 seconds = 203.72 MB/sec
  Timing buffered disk reads:    6 MB in  3.05 seconds =   1.96 MB/sec

dmesg output:
...
libata version 3.00 loaded.
PCI: Setting latency timer of device 0000:00:0f.1 to 64
scsi0 : pata_serverworks
scsi1 : pata_serverworks
ata1: PATA max UDMA/100 cmd 0x1f0 ctl 0x3f6 bmdma 0x2440 irq 14
ata2: PATA max UDMA/100 cmd 0x170 ctl 0x376 bmdma 0x2448 irq 15
ata1.00: ATA-7: ST3400620A, 3.AAE, max UDMA/100
ata1.00: 781422768 sectors, multi 16: LBA48
ata1.01: ATAPI: TOSHIBA DVD-ROM SD-M1612, 1004, max UDMA/33
ata1.00: configured for UDMA/100
ata1.01: configured for UDMA/33
ata2.00: ATA-7: ST3400620A, 3.AAE, max UDMA/100
ata2.00: 781422768 sectors, multi 16: LBA48
ata2.01: ATAPI: CD-W540E, 1.0D, max UDMA/33
ata2.00: simplex DMA is claimed by other device, disabling DMA
ata2.01: simplex DMA is claimed by other device, disabling DMA
ata2.00: configured for PIO4
ata2.01: configured for PIO4
...

# lsmod|grep ata
pata_serverworks       10496  8
pata_acpi               8320  0
libata                127328  2 pata_serverworks,pata_acpi
scsi_mod              120948  4 sg,sr_mod,libata,sd_mod

It's some sort of pata_serverworks bug, or is needed any special
setup for this driver to work?
What is "simplex DMA" term?

Thanks, Franta Hanzlik