This looks like an issue I've had related to the "backdoor entry" hack for GART_IOMMU – Basically, amd64_agp doesn't utilise module_init or module_exit when CONFIG_GART_IOMMU is enabled, instead stating:

drivers/char/agp/amd64-agp.c@774-775:
/* On AMD64 the PCI driver needs to initialize this driver early
   for the IOMMU, so it has to be called via a backdoor. */

This resulted, on my system, of the agpgart not being initialised despite the module being loaded, and ultimately leads to the crash you describe below.  My "fix" was to remove the #ifndef surrounding the module_init and module_exit calls, however I'm not sure if this is the best way to approach the issue (I have no idea what this backdoor mentioned is, and AFAIK it all works for other drivers).

Regards,
Francis

2009/12/11 Johannes Hirte <johannes.hirte@fem.tu-ilmenau.de>
When the agp driver is not compiled in but is build as module, the kernel oopes on modprobing radeon with modeset=1.

Dec  7 22:08:53 datengrab kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000074
Dec  7 22:08:53 datengrab kernel: IP: [<ffffffffa01f693c>] radeon_agp_init+0x18/0x23c [radeon]
Dec  7 22:08:53 datengrab kernel: PGD 0
Dec  7 22:08:53 datengrab kernel: Oops: 0000 [#1] SMP
Dec  7 22:08:53 datengrab kernel: last sysfs file: /sys/module/agpgart/initstate
Dec  7 22:08:53 datengrab kernel: CPU 0
Dec  7 22:08:53 datengrab kernel: Modules linked in: radeon(+) ttm drm_kms_helper drm i2c_algo_bit snd_seq_midi snd_emu10k1_synth snd_emux_synth snd_seq_virmidi snd_seq_midi_emul snd_seq_oss snd_seq_midi_event snd_seq snd_pcm_oss snd_mixer_oss aes_x86_64 aes_generic xts gf128mul dm_crypt snd_emu10k1 snd_rawmidi snd_ac97_codec ac97_bus snd_pcm snd_seq_device snd_timer snd_page_alloc snd_util_mem snd_hwdep ohci_hcd i2c_amd8111 uhci_hcd sg snd amd64_agp sata_sil k8temp agpgart i2c_amd756 sr_mod hwmon
Dec  7 22:08:53 datengrab kernel: Pid: 2531, comm: work_for_cpu Not tainted 2.6.32 #2 To Be Filled By O.E.M.
Dec  7 22:08:53 datengrab kernel: RIP: 0010:[<ffffffffa01f693c>]  [<ffffffffa01f693c>] radeon_agp_init+0x18/0x23c [radeon]
Dec  7 22:08:53 datengrab kernel: RSP: 0018:ffff8801172c3d80  EFLAGS: 00010282
Dec  7 22:08:53 datengrab kernel: RAX: 0000000000000000 RBX: ffff880118410000 RCX: ffff88011fb97800
Dec  7 22:08:53 datengrab kernel: RDX: 00000000c0000000 RSI: 0000000000000000 RDI: ffff88011e43d800
Dec  7 22:08:53 datengrab kernel: RBP: 0000000000000000 R08: ffffffffa0238201 R09: ffff8801172c3d90
Dec  7 22:08:53 datengrab kernel: R10: ffffffff81589940 R11: 0000000000000400 R12: ffff88011fb97800
Dec  7 22:08:53 datengrab kernel: R13: 0000000000000020 R14: ffffffffa0243240 R15: ffff88011e43dbc0
Dec  7 22:08:53 datengrab kernel: FS:  00007f0e85a7f700(0000) GS:ffff880028200000(0000) knlGS:0000000000000000
Dec  7 22:08:53 datengrab kernel: CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
Dec  7 22:08:53 datengrab kernel: CR2: 0000000000000074 CR3: 0000000001001000 CR4: 00000000000006f0
Dec  7 22:08:53 datengrab kernel: DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Dec  7 22:08:53 datengrab kernel: DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Dec  7 22:08:53 datengrab kernel: Process work_for_cpu (pid: 2531, threadinfo ffff8801172c2000, task ffff8801188d2280)
Dec  7 22:08:53 datengrab kernel: Stack:
Dec  7 22:08:53 datengrab kernel: ffff88011e43dbc0 ffffffff813436f0 ffff880100000008 ffff8801172c3df0
Dec  7 22:08:53 datengrab kernel: <0> ffff8801172c3db0 0000000000000246 ffff880118410000 ffff880118410144
Dec  7 22:08:53 datengrab kernel: <0> 0000000000000282 ffff880118410000 0000000000000000 ffff88011fb97800
Dec  7 22:08:53 datengrab kernel: Call Trace:
Dec  7 22:08:53 datengrab kernel: [<ffffffff813436f0>] ? printk+0x40/0x48
Dec  7 22:08:53 datengrab kernel: [<ffffffffa021f0b0>] ? r600_mc_init+0xf0/0x251 [radeon]
Dec  7 22:08:53 datengrab kernel: [<ffffffffa021fb91>] ? r600_init+0x154/0x2b7 [radeon]
Dec  7 22:08:53 datengrab kernel: [<ffffffffa01f4369>] ? radeon_device_init+0x1fe/0x276 [radeon]
Dec  7 22:08:53 datengrab kernel: [<ffffffffa01f4bda>] ? radeon_driver_load_kms+0xb2/0xec [radeon]
Dec  7 22:08:53 datengrab kernel: [<ffffffffa019ba34>] ? drm_get_dev+0x322/0x42e [drm]
Dec  7 22:08:53 datengrab kernel: [<ffffffff810452d2>] ? do_work_for_cpu+0x0/0x1b
Dec  7 22:08:53 datengrab kernel: [<ffffffff811b9ae1>] ? local_pci_probe+0x12/0x16
Dec  7 22:08:53 datengrab kernel: [<ffffffff810452dd>] ? do_work_for_cpu+0xb/0x1b
Dec  7 22:08:53 datengrab kernel: [<ffffffff8104827c>] ? kthread+0x75/0x7d
Dec  7 22:08:53 datengrab kernel: [<ffffffff8100bc7a>] ? child_rip+0xa/0x20
Dec  7 22:08:53 datengrab kernel: [<ffffffff81048207>] ? kthread+0x0/0x7d
Dec  7 22:08:53 datengrab kernel: [<ffffffff8100bc70>] ? child_rip+0x0/0x20
Dec  7 22:08:53 datengrab kernel: Code: 48 85 c0 74 0c 83 78 74 00 74 06 5a e9 c3 5e fa ff 58 c3 41 55 41 54 55 53 48 89 fb 48 83 ec 48 48 8b 7f 08 48 8b 87 48 03 00 00 <83> 78 74 00 75 1d e8 38 5e fa ff 85 c0 89 c5 74 12 89 c2 48 c7
Dec  7 22:08:53 datengrab kernel: RIP  [<ffffffffa01f693c>] radeon_agp_init+0x18/0x23c [radeon]
Dec  7 22:08:53 datengrab kernel: RSP <ffff8801172c3d80>
Dec  7 22:08:53 datengrab kernel: CR2: 0000000000000074
Dec  7 22:08:53 datengrab kernel: ---[ end trace b9e94e547b237285 ]---

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