I have an issue with firmware loading when drm/radeon compiled as module (latest drm-next kernel, RS780). 1 of 10 `modprobe radeon modeset=1` works fine. According to `strace -o log modprobe radeon modeset=1` it looks like some timeout issues:
Code:
open("/lib/modules/2.6.31-rc9-ojab-kms/kernel/drivers/gpu/drm/radeon/radeon.ko", O_RDONLY) = 3
stat("/sys/module", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
stat("/sys/module/radeon", 0x7fff49165050) = -1 ENOENT (No such file or directory)
fcntl(3, F_GETFL)                       = 0x8000 (flags O_RDONLY|O_LARGEFILE)
fstat(3, {st_mode=S_IFREG|0644, st_size=829530, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f2ef433c000
lseek(3, 0, SEEK_CUR)                   = 0
After modprobe /sys/module/radeon dir always exists.

Also I've had a backtrace several times
Code:
Sep 13 13:21:21 ojab kernel: [  165.507267] [drm] radeon kernel modesetting enabled.
Sep 13 13:21:21 ojab kernel: [  165.507397] radeon 0000:01:05.0: PCI INT A -> GSI 18 (level, low) -> IRQ 18
Sep 13 13:21:21 ojab kernel: [  165.507449] radeon 0000:01:05.0: setting latency timer to 64
Sep 13 13:21:21 ojab kernel: [  165.508933] [drm] radeon: Initializing kernel modesetting.
Sep 13 13:21:21 ojab kernel: [  165.509010] [drm] register mmio base: 0xFE9F0000
Sep 13 13:21:21 ojab kernel: [  165.509047] [drm] register mmio size: 65536
Sep 13 13:21:21 ojab kernel: [  165.509568] ATOM BIOS: B27722
Sep 13 13:21:21 ojab kernel: [  165.509607] [drm] Clocks initialized !
Sep 13 13:21:21 ojab kernel: [  165.509654] [drm] radeon: 384M of VRAM memory ready
Sep 13 13:21:21 ojab kernel: [  165.509691] [drm] radeon: 512M of GTT memory ready.
Sep 13 13:21:21 ojab kernel: [  165.509771] [drm] Loading RS780 CP Microcode
Sep 13 13:21:21 ojab kernel: [  165.509817] platform radeon_cp.0: firmware: requesting radeon/RS780_pfp.bin
Sep 13 13:21:21 ojab kernel: [  165.513727] platform radeon_cp.0: firmware: requesting radeon/RS780_me.bin
Sep 13 13:21:21 ojab kernel: [  165.517498] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
Sep 13 13:21:21 ojab kernel: [  165.517602] IP: [<ffffffff8123af3b>] firmware_loading_store+0x5b/0x210
Sep 13 13:21:21 ojab kernel: [  165.517669] PGD 64587067 PUD 66062067 PMD 0 
Sep 13 13:21:21 ojab kernel: [  165.517779] Oops: 0000 [#1] PREEMPT SMP 
Sep 13 13:21:21 ojab kernel: [  165.517888] last sysfs file: /sys/devices/platform/radeon_cp.0/firmware/radeon_cp.0/loading
Sep 13 13:21:21 ojab kernel: [  165.517940] CPU 1 
Sep 13 13:21:21 ojab kernel: [  165.517999] Modules linked in: radeon(+) usbhid hid amd64_edac_mod edac_core evdev ehci_hcd i2c_piix4 r8169 sg ohci_hcd rtc_cmos rtc_core mii processor rtc_lib thermal button af_packet kvm_amd kvm usbcore 
snd_hda_intel snd_hda_codec snd_hwdep snd_pcm snd_timer snd soundcore snd_page_alloc ttm drm_kms_helper drm fb cfbcopyarea cfbimgblt cfbfillrect unix [last unloaded: radeon]
Sep 13 13:21:21 ojab kernel: [  165.518254] Pid: 1233, comm: firmware.sh Not tainted 2.6.31-rc9-ojab-kms #5 MS-7576
Sep 13 13:21:21 ojab kernel: [  165.518254] RIP: 0010:[<ffffffff8123af3b>]  [<ffffffff8123af3b>] firmware_loading_store+0x5b/0x210
Sep 13 13:21:21 ojab kernel: [  165.518254] RSP: 0018:ffff880063947e58  EFLAGS: 00010202
Sep 13 13:21:21 ojab kernel: [  165.518254] RAX: 0000000000000000 RBX: ffff88006fbaf800 RCX: 000000000000000a
Sep 13 13:21:21 ojab kernel: [  165.518254] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88006511e001
Sep 13 13:21:21 ojab kernel: [  165.518254] RBP: 0000000000000002 R08: 0000000000000028 R09: 000000000000000a
Sep 13 13:21:21 ojab kernel: [  165.518254] R10: 0000000000000001 R11: 0000000000000246 R12: ffff88006d77a400
Sep 13 13:21:21 ojab kernel: [  165.518254] R13: ffffffff81421230 R14: 0000000000000000 R15: ffff88006c945e40
Sep 13 13:21:21 ojab kernel: [  165.518254] FS:  00007f969c2e86f0(0000) GS:ffff88000154f000(0000) knlGS:0000000000000000
Sep 13 13:21:21 ojab kernel: [  165.518254] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Sep 13 13:21:21 ojab kernel: [  165.518254] CR2: 0000000000000008 CR3: 0000000063504000 CR4: 00000000000006e0
Sep 13 13:21:21 ojab kernel: [  165.518254] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Sep 13 13:21:21 ojab kernel: [  165.518254] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Sep 13 13:21:21 ojab kernel: [  165.518254] Process firmware.sh (pid: 1233, threadinfo ffff880063946000, task ffff88006d1c96c0)
Sep 13 13:21:21 ojab kernel: [  165.518254] Stack:
Sep 13 13:21:21 ojab kernel: [  165.518254]  0000000000000002 000000008d98c31a ffff88006c945e60 ffff880063947f50
Sep 13 13:21:21 ojab kernel: [  165.518254] <0> ffff8800634c2fa0 ffffffff81421230 ffff88006d77a410 ffffffff81136c85
Sep 13 13:21:21 ojab kernel: [  165.518254] <0> ffff88006d1c96c0 0000000000000002 ffff88006ea829c0 000000008d98c31a
Sep 13 13:21:21 ojab kernel: [  165.518254] Call Trace:
Sep 13 13:21:21 ojab kernel: [  165.518254]  [<ffffffff81136c85>] ? sysfs_write_file+0xe5/0x190
Sep 13 13:21:21 ojab kernel: [  165.518254]  [<ffffffff810d9734>] ? vfs_write+0xe4/0x1b0
Sep 13 13:21:21 ojab kernel: [  165.518254]  [<ffffffff810d991e>] ? sys_write+0x5e/0xb0
Sep 13 13:21:21 ojab kernel: [  165.518254]  [<ffffffff8100c182>] ? system_call_fastpath+0x16/0x1b
Sep 13 13:21:21 ojab kernel: [  165.518254] Code: 48 89 5c 24 10 4c 89 6c 24 28 48 89 cd 49 8b 9c 24 88 00 00 00 e8 86 20 f7 ff 85 c0 41 89 c6 75 6f f6 43 70 01 74 73 48 8b 43 68 <48> 8b 78 08 e8 0c 13 e8 ff 4c 8b 6b 68 8b b3 80 00 00 00
 31 d2 
Sep 13 13:21:21 ojab kernel: [  165.518254] RIP  [<ffffffff8123af3b>] firmware_loading_store+0x5b/0x210
Sep 13 13:21:21 ojab kernel: [  165.518254]  RSP <ffff880063947e58>
Sep 13 13:21:21 ojab kernel: [  165.518254] CR2: 0000000000000008
Sep 13 13:21:21 ojab kernel: [  165.522528] ---[ end trace 8ccc6e8414991f02 ]---