5.2.1 hangs on boot where 5.1 does not (ATA issue)

I can boot just fine using 5.1-RELEASE-p11 (see partial bootlog
below).  I cvsuped this morning to 5.2.1, and the boot hangs
somewhere around getting ata1 online.  Warning (error?) messages
occur in this neighborhood with the 5.1 kernel, but at least
it boots.  I'm interested in workarounds, but I also hope
that by providing details here someone will be able to fix this.
It's unfortunate to have something that used to work stop working.

Here are the hand copied messages from just before the freeze,
using 'boot -v' with the 5.2.1 kernel.  Actually, the first
three lines come from a 'boot' not the 'boot -v'; that part
had scrolled off on the 'boot -v' so I couldn't copy it from
there.

---------------------------------------------------------------
GEOM: create disk ad2 dp=0xc2de6160
ad2: 38166MB <ST34081A> [77545/16/63] at ata0-master UDMA100
acd0:DVDROM <DVDROM 12x> at ata1-slave PIO4
ad2: TIMEOUT - READ_DMA retrying (2 retries left) LBA=0
ata1: reiniting channel ..
ata1: reset tp1 mask=03 ostat0=58 ostat1=50
ad2: stat=0x50 err=0x01 lsb=0x00 msb=0x00
acd0:  stat=0x00 err=0x00 lsb=0x14 msb=0xeb
ata1: reset tp2 mask=03 stat0=50 stat1=00 devices=0x9 <ATAPI_SLAVE,ATA_MASTER>
ata1: resetting done
acd0: pio=0x0c wdma=0x22 udma=0x42 cable=40pin
ad2: pio=0x0c wdma=0x22 udma=0x45 cable=40pin
ad2: setting PIO4 on Intel ICH2 chip
ad2: setting UDMA100 on Intel ICH2 chip
acd0: setting IPO4 on Intel ICH2 chip
ad2: setting UDMA100 on Intel ICH2 chip
acd0: setting IPO4 on Intel ICH2 chip
ata1: device config done
ad2: WARNING - READ_DMA interrupt was seen but timeout fired LBA=0
ad2: WARNING - READ_DMA interrupt was seen but taskqueue stalled LBA=0
-----------------------------------------------------------------------

And here's the same area from the boot -v from a successful boot of
the 5.1 kernel:
-----------------------------------------------------------------------
ad0: success setting UDMA100 on Intel ICH2 chip
ad0: <ST340810A/3.34> ATA-6 disk at ata0-master
ad0: 38166MB (78165360 sectors), 77545 C, 16 H, 63 S, 512 B
ad0: 16 secs/int, 1 depth queue, UDMA100
ad0: piomode=12 dmamode=34 udmamode=69 cblid=1
GEOM: new disk ad0
ar: FreeBSD check1 failed
ad2: success setting UDMA100 on Intel ICH2 chip
ad2: <Maxtor 36147H8/BAC51KJ0> ATA-6 disk at ata1-master
ad2: 57241MB (117231408 sectors), 116301 C, 16 H, 63 S, 512 B
ad2: 16 secs/int, 1 depth queue, UDMA100
ad2: piomode=12 dmamode=34 udmamode=69 cblid=0
ad2: READ command timeout tag=0 serv=0 - resetting
ata1: resetting devices ..
ata1: pre reset mask=03 ostat0=58 ostat2=50
ad2: ATAPI 00 00
ata1-slave: ATAPI 14 eb
ata1: after reset mask=03 stat0=50 stat1=00
ad2: ATA 01 a5
ata1: devices=09
ad2: success setting UDMA100 on Intel ICH2 chip
done
ar: FreeBSD read conf failed
ata1-slave: piomode=12 dmamode=34 udmamode=66 dmaflag=1
ad2: READ command timeout tag=0 serv=0 - resetting
ata1: resetting devices ..
ata1: pre reset mask=03 ostat0=58 ostat2=00
ad2: ATAPI 00 00
ata1-slave: ATAPI 14 eb
ata1: after reset mask=03 stat0=50 stat1=00
ad2: ATA 01 a5
ata1: devices=09
ad2: success setting UDMA100 on Intel ICH2 chip
done
ad2: READ command timeout tag=0 serv=0 - resetting
ata1: resetting devices ..
ata1: pre reset mask=03 ostat0=58 ostat2=00
ad2: ATAPI 00 00
ata1-slave: ATAPI 14 eb
ata1: after reset mask=03 stat0=50 stat1=00
ad2: ATA 01 a5
ata1: devices=09
ad2: success setting UDMA100 on Intel ICH2 chip
done
ad2: READ command timeout tag=0 serv=0 - resetting
ad2: success setting PIO4 on Intel ICH2 chip
ad2: trying fallback to PIO mode
ata1: resetting devices ..
ata1: pre reset mask=03 ostat0=50 ostat2=00
ad2: ATAPI 00 00
ata1-slave: ATAPI 14 eb
ata1: after reset mask=03 stat0=50 stat1=00
ad2: ATA 01 a5
ata1: devices=09
ad2: success setting PIO4 on Intel ICH2 chip
done
ata1-slave: success setting PIO4 on Intel ICH2 chip
acd0: <DVDROM 12X/49Z> DVD-ROM drive at ata1 as slave
acd0:  512KB buffer, PIO4
acd0: Reads: CD-R, CD-RW, CD-DA stream, DVD-ROM, DVD-R, packet
acd0: Writes:
acd0: Audio: play, 255 volume levels
acd0: Mechanism: ejectable tray, unlocked
acd0: Medium: no/blank disc
[0] f:80 typ:165 s(CHS):0/1/1 e(CHS):62/254/63 s:63 l:1012032
[1] f:00 typ:12 s(CHS):63/0/1 e(CHS):624/254/63 s:1012095 l:41929650
[2] f:00 typ:165 s(CHS):1023/0/1 e(CHS):1023/254/63 s:42941745 l:35214480
[3] f:00 typ:0 s(CHS):0/0/0 e(CHS):0/0/0 s:0 l:0
GEOM: Configure ad0s1, start 32256 length 518160384 end 518192639
GEOM: Configure ad0s2, start 518192640 length 21467980800 end 21986173439
GEOM: Configure ad0s3, start 21986173440 length 18029813760 end 40015987199
GEOM: new disk ad2
GEOM: Configure ad0s1a, start 0 length 518160384 end 518160383
GEOM: Configure ad0s1c, start 0 length 518160384 end 518160383
GEOM: Configure ad0s3b, start 0 length 1073741824 end 1073741823
GEOM: Configure ad0s3c, start 0 length 18029813760 end 18029813759
GEOM: Configure ad0s3e, start 1073741824 length 16956071936 end 18029813759
[0] f:80 typ:165 s(CHS):0/1/1 e(CHS):1023/254/63 s:63 l:117226242
[1] f:00 typ:0 s(CHS):0/0/0 e(CHS):0/0/0 s:0 l:0
[2] f:00 typ:0 s(CHS):0/0/0 e(CHS):0/0/0 s:0 l:0
[3] f:00 typ:0 s(CHS):0/0/0 e(CHS):0/0/0 s:0 l:0
GEOM: Configure ad2s1, start 32256 length 60019835904 end 60019868159
GEOM: Configure ad2s1c, start 0 length 60019835904 end 60019835903
GEOM: Configure ad2s1e, start 0 length 60019835904 end 60019835903