AHCI driver prints "D2H reg with I during NCQ" message, what is it?

I see this on boot with kernels 2.6.22.1 and 2.6.20.15:

ata1: D2H reg with I during NCQ, this message won't be printed again

But I can't find anything that tells me what it means, or whether it's a real problem.

x86_64 kernel with controller:

00:1f.2 SATA controller: Intel Corporation 631xESB/632xESB SATA Storage Controller AHCI (rev 09)

and drive:

     ata1.00: ATA-7: HDS728080PLA380, PF2OA63A, max UDMA/133

That's your firmware transmitting unnecessary D2H Reg FIS during NCQ command phase. It's minor protocol violation which can be ignored. No need to worry.

tejun