NCQ on Promise SATA300 TX4

[Tim Besard <tim.besard@gmail.com>]

Hi,

I recently bough a Promise SATA300 TX4 controller, which I am using in
combination with an NCQ-capable harddrive. As the controller supports
NCQ [1], and the libata webpages seems to indicate [2] that the
sata_promise driver does as well, I am currently trying to enable NCQ
(kernel 2.6.27).

Without any luck however, my dmesg log indicates the following:
[    4.244047] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
[    4.253636] ata1.00: ATA-7: ST3500320AS, SD04, max UDMA/133
[    4.253639] ata1.00: 976773168 sectors, multi 0: LBA48 NCQ (depth
0/32)
[    4.268850] ata1.00: configured for UDMA/133

And the NCQ queue depth flag seems read-only:
tim@tim-desktop:~$ sudo echo 31 > /sys/block/sdb/device/queue_depth
bash: /sys/block/sdb/device/queue_depth: Permission denied

The read-only nature of the queue_depth field seems to indicate that the
SATA module doesn't have NCQ support, but this contradicts with what I
read on the libata page.

Could you shed some light on this issue? And if the driver doesn't
support NCQ for this card, is there another way to accomplish this
(without having to buy a new controller)?

References:
1:
http://www.promise.com/product/product_detail_eng.asp?segment=Non-RAID%
20HBAs&product_id=139
2: http://linux-ata.org/driver-status.html#tx2

Thanks in advance,
Tim