SiI3531 SATA port multipler

Hello,

I've been poking around with a SiI3531 sata300 host card, connected to an 
external disk enclosure with an internal SiI4726 card.  There are 3 x SATA 
disks hanging off it at the moment:

> http://www.startech.com/item/SAT3540ERGB-35in-4-Drive-eSATA-Multi-RAID-External-Hard-Drive-Enclosure.aspx

I have assumed that the SiI3531 is command-compatible with the SiI3112, and 
put in the attached patch to see what might happen.  The kernel appears to 
recognise the sata card, but dies before initialising it properly:

> ata2: Portmultiplier (id=47261095 rev=1f06) with 7 ports
> ata2: error writing PM port
> ata2: p6: writing ATA_SC_DET_RESET failed
> ata2: Portmultiplier (id=47261095 rev=1f06) with 7 ports
> ata2: error writing PM port
> ata2: p4: writing ATA_SC_DET_RESET failed
> ata2: error writing PM port
> ata2: p5: writing ATA_SC_DET_RESET failed
> ata2: error writing PM port
> ata2: p6: writing ATA_SC_DET_RESET failed
> ata2: Portmultiplier (id=47261095 rev=1f06) with 7 ports
> ata2: error writing PM port
> ata2: p6: writing ATA_SC_DET_RESET failed
> ata2: Portmultiplier (id=47261095 rev=1f06) with 7 ports
> ata2: error writing PM port
> ata2: p6: writing ATA_SC_DET_RESET failed

I've attached a full dmesg.   This setup is running on an Asus P5Q SE2 
motherboard, in amd64 mode with a -current kernel pulled today.

Any suggestions about what could be done to make this work?

Nick