I've just applied suggested patch to the most recent clone of Jeremy's tree and rebuilt
the JF's 2.6.31.6 kernel. Same error :-

[drm] Initialized drm 1.1.0 20060810
[drm] radeon default to kernel modesetting.
[drm] radeon kernel modesetting enabled.
xen: registering gsi 16 triggering 0 polarity 1
xen_allocate_pirq: returning irq 16 for gsi 16
xen: --> irq=16
xen_set_ioapic_routing: irq 16 gsi 16 vector 152 ioapic 0 pin 16 triggering 1 polarity 1
radeon 0000:01:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
radeon 0000:01:00.0: setting latency timer to 64
[drm] radeon: Initializing kernel modesetting.
[drm:radeon_driver_load_kms] *ERROR* Failed to initialize radeon, disabling IOCTL
radeon 0000:01:00.0: PCI INT A disabled
radeon: probe of 0000:01:00.0 failed with error -22


Boris.



--- On Fri, 1/1/10, Pasi Kärkkäinen <pasik@iki.fi> wrote:

From: Pasi Kärkkäinen <pasik@iki.fi>
Subject: Re: [Fedora-xen] Dom0 kernels Xen Host connected via remote VNC Session
To: "Boris Derzhavets" <bderzhavets@yahoo.com>
Cc: fedora-xen@redhat.com, "M A Young" <m.a.young@durham.ac.uk>
Date: Friday, January 1, 2010, 12:20 PM

On Thu, Dec 31, 2009 at 04:15:02PM -0800, Boris Derzhavets wrote:
>    Switched to initdefault 3 in /etc/inittab and logged into Xen 3.4.2 host
>    with your kernel. Set up VNC session and got the nice dmesg report, which
>    is attached.
>
>    Yes , [drm]  entries for Radeon HD 4650 look good , but via remote VNC
>    session.
>
>    [drm] Initialized drm 1.1.0 20060810
>    [drm] radeon defaulting to kernel modesetting..
>    [drm] radeon kernel modesetting enabled.
>    xen: registering gsi 16 triggering 0 polarity 1
>    xen_allocate_pirq: returning irq 16 for gsi 16
>    xen: --> irq=16
>    xen_set_ioapic_routing: irq 16 gsi 16 vector 152 ioapic 0 pin 16
>    triggering 1 polarity 1
>    radeon 0000:01:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
>    radeon 0000:01:00.0: setting latency timer to 64
>    [drm] radeon: Initializing kernel modesetting.
>    [drm] register mmio base: 0xFE8E0000
>    [drm] register mmio size: 65536
>    ATOM BIOS: 11X
>    [drm] Clocks initialized !
>    mtrr: type mismatch for d0000000,10000000 old: write-back new:
>    write-combining
>    [drm] Detected VRAM RAM=256M, BAR=256M
>    [drm] RAM width 128bits DDR
>    [TTM] Zone  kernel: Available graphics memory: 4074962 kiB.
>    [TTM] Zone   dma32: Available graphics memory: 2097152 kiB.
>    [drm] radeon: 256M of VRAM memory ready
>    [drm] radeon: 512M of GTT memory ready.
>    [drm] Loading RV730 CP Microcode
>    platform radeon_cp.0: firmware: requesting radeon/RV730_pfp.bin
>    platform radeon_cp.0: firmware: requesting radeon/RV730_me.bin
>    [drm] GART: num cpu pages 131072, num gpu pages 131072
>    [drm] ring test succeeded in 1 usecs
>    [drm] radeon: ib pool ready.
>    [drm] ib test succeeded in 0 usecs
>    [drm] Radeon Display Connectors
>    [drm] Connector 0:
>    [drm]   HDMI-A
>    [drm]   DDC: 0x7f10 0x7f10 0x7f14 0x7f14 0x7f18 0x7f18 0x7f1c 0x7f1c
>    [drm]   Encoders:
>    [drm]     DFP2: INTERNAL_UNIPHY1
>    [drm] Connector 1:
>    [drm]   VGA
>    [drm]   DDC: 0x7e20 0x7e20 0x7e24 0x7e24 0x7e28 0x7e28 0x7e2c 0x7e2c
>    [drm]   Encoders:
>    [drm]     CRT2: INTERNAL_KLDSCP_DAC2
>    [drm] Connector 2:
>    [drm]   DVI-I
>    [drm]   DDC: 0x7e40 0x7e40 0x7e44 0x7e44 0x7e48 0x7e48 0x7e4c 0x7e4c
>    [drm]   Encoders:
>    [drm]     CRT1: INTERNAL_KLDSCP_DAC1
>    [drm]     DFP1: INTERNAL_UNIPHY
>    [drm] fb mappable at 0xD0141000
>    [drm] vram apper at 0xD0000000
>    [drm] size 5242880
>    [drm] fb depth is 24
>    [drm]    pitch is 5120
>    executing set pll
>    executing set crtc timing
>    [drm] TV-11: set mode 1280x1024 19
>    Console: switching to colour frame buffer device 160x64
>    fb0: radeondrmfb frame buffer device
>    registered panic notifier
>    [drm] Initialized radeon 2.0.0 20080528 for 0000:01:00..0 on minor 0
>
>    Attempt to run startx locally fails. Keyboard dies.
>    Please, advise.
>

Well, like said, there are known drm/ttm/kms related problems in
Jeremy's tree atm.

I think I got it working with this (temporary) patch:
http://lists.xensource.com/archives/html/xen-devel/2009-10/msg00986.html

-- Pasi