Radeon (on PPC) with KMS limits pixel clock to 175MHz

In quantal, the support for UMS was removed from the Xorg radeon driver. That's fine, except for the fact that there seems to be something funky with KMS. If I enable it, the pixel clock can't be set above 175MHz. This affects both the console and X. X thinks that it is able to set a mode like 1600x1200@85Hz (the maximum supported by the monitor), but in actual fact it comes out as 65Hz instead...

The limitation is obviously bogus, since the higher dot clock is supported by both the graphics card and the monitor (it works fine without KMS...), but I haven't been able to trace where it comes from or how to override it. I tried setting drm.debug=1, but it didn't provide any real clues.

ProblemType: Bug
DistroRelease: Ubuntu 12.10
Package: linux-image-3.5.0-18-powerpc-smp 3.5.0-18.29
ProcVersionSignature: Ubuntu 3.5.0-18.29-powerpc-smp 3.5.7
Uname: Linux 3.5.0-18-powerpc-smp ppc
ApportVersion: 2.6.1-0ubuntu6
Architecture: powerpc
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: marcus 2989 F.... pulseaudio
CRDA: Error: command ['iw', 'reg', 'get'] failed with exit code 1: nl80211 not found.
Date: Sat Oct 27 23:43:05 2012
HibernationDevice: RESUME=
IwConfig:
 lo no wireless extensions.

 eth0 no wireless extensions.
MarkForUpload: True
ProcFB: 0 radeondrmfb
ProcKernelCmdLine: root=/dev/sda5 ro quiet splash radeon.modeset=1 video=offb:off video=radeonfb:off video=1600x1200-32@85 radeon.agpmode=-1 video=SVIDEO-1:d video=DVI-I-1:e drm.debug=1 drm_kms_helper.edid_firmware=edid/446xpro.bin
RelatedPackageVersions:
 linux-restricted-modules-3.5.0-18-powerpc-smp N/A
 linux-backports-modules-3.5.0-18-powerpc-smp N/A
 linux-firmware 1.95
RfKill:

SourcePackage: linux
UpgradeStatus: Upgraded to quantal on 2012-10-21 (6 days ago)
WpaSupplicantLog: