SiS 5591 requires hw.ata.ata_dma=0 - Why?

[Yiorgos Adamopoulos <adamo@xxxxxxxxxxxxxx>]

According to ata(4) SiS 5591 is supported at its maximum speed.  However, 
unless I set hw.ata.ata_dma=0 at /boot/loader.conf.local I get:

ad0: READ command timeout tag=0 serv=0 - resetting
ata0: resetting devices ..

although my dmesg shows:

atapci0: <SiS 5591 ATA33 controller> port 0x4000-0x400f,0x374-0x377,0x170-0x177,0x3f4-0x3f7,0x1f0-0x1f7 at device 2.5 on pci0
ata0: at 0x1f0 irq 14 on atapci0
ad0: 76319MB <WDC WD800JB-22JJC0> [155061/16/63] at ata0-master PIO4

I run 1.3-Preview
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