Hang during boot, bisected to commit 96d60303fd

[Marc Dionne]

Since somewhere early in the 2.6.35 merge window I've been seeing a
hang during boot.  The hang last roughly 30 seconds and is followed by
this:

ata4.00: exception Emask 0x0 SAct 0x1 SErr 0x0 action 0x6 frozen
ata4.00: failed command: READ FPDMA QUEUED
ata4.00: cmd 60/08:00:00:00:00/00:00:00:00:00/40 tag 0 ncq 4096 in
         res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
ata4.00: status: { DRDY }
ata4: hard resetting link
ahci_is_device_present: status is: 80
ahci_is_device_present: status(2) is: 3
ata4: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
ata4.00: configured for UDMA/133
ata4.00: device reported invalid CHS sector 0
ata4: EH complete

The system works normally once booted.  I had some time to do a bisect
today and it cleanly ended up pointing to this commit:
    96d60303fd: ahci: Turn off DMA engines when there's no device attached

Reverting the commit makes the system boot cleanly as before.  System
has an Intel ICH10 based board, in AHCI mode, disk is an Intel X25M
SSD.

Marc
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