Panic in ata_dmastart after suspend/resume

After an S4 ACPI suspend, my system has panicked on resume in ata_dmastart.  
The system is normally capable of resuming just peachily, and I haven't 
updated the kernel or world in the past few days and it's worked fine until 
just now. :-)

I'm -not- using my ata-suspend patch to ata-disk.c, just to rule that out up 
front. :-)

I'm updating my sources to see if I can reproduce the bug with HEAD.  In the 
meantime, I hope this information proves useful to someone.

-Cliff L. Biffle

panic: ata_dmasetup: transfer active on this device!
panic messages:
---
panic: ata_dmasetup: transfer active on this device!

syncing disks, buffers remaining... 1607 1607 1607 1607 1607 1607 1607 1607 
1607 1607 1607 1607 1607 1607 1607 1607 1607 1607 1607 ad0: READ command 
timeout tag=0 serv=0 - resetting
ata0: resetting devices ..
done
1607 ad0: READ command timeout tag=0 serv=0 - resetting
ata0: resetting devices ..
done

giving up on 392 buffers
Uptime: 14h14m51s
Dumping 159 MB
ata0: resetting devices ..
done
[CTRL-C to abort] [CTRL-C to abort]  16 32 48 64 80 96 112 128 144
---
#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:238
238		dumping++;
#0  doadump () at /usr/src/sys/kern/kern_shutdown.c:238
#1  0xc027f218 in boot (howto=256) at /usr/src/sys/kern/kern_shutdown.c:370
#2  0xc027f514 in panic () at /usr/src/sys/kern/kern_shutdown.c:543
#3  0xc0199104 in ata_dmastart (ch=0xc1c05a00, 
    data=0xc85f8000 "që\r", '\220' <repeats 13 times>, "¾", count=12288, 
dir=1)
    at /usr/src/sys/dev/ata/ata-dma.c:226
#4  0xc018fd5c in ata_pci_dmastart (ch=0xc1c05a00, 
    data=0xc85f8000 "që\r", '\220' <repeats 13 times>, "¾", count=12288, 
dir=1)
    at /usr/src/sys/dev/ata/ata-pci.c:418
#5  0xc0199f31 in ad_transfer (request=0xc24eb7c0)
    at /usr/src/sys/dev/ata/ata-disk.c:478
#6  0xc0189227 in ata_start (ch=0xc1c05a00)
    at /usr/src/sys/dev/ata/ata-all.c:640
#7  0xc019991a in adstrategy (bp=0x0) at /usr/src/sys/dev/ata/ata-disk.c:286
#8  0xc024d685 in g_disk_start (bp=0xc2b42800)
    at /usr/src/sys/geom/geom_disk.c:239
#9  0xc024f62a in g_io_schedule_down (tp=0xc0c6c000)
    at /usr/src/sys/geom/geom_io.c:348
#10 0xc024f8f9 in g_down_procbody () at /usr/src/sys/geom/geom_kern.c:112
#11 0xc026c307 in fork_exit (callout=0xc024f8e0 <g_down_procbody>, arg=0x0, 
    frame=0xc999ed48) at /usr/src/sys/kern/kern_fork.c:793