drm/radeon: kernel panic in drm_calc_vbltimestamp_from_scanoutpos with 3.18.0-rc6

I was unable too boot 3.18.0-rc6 because of the following kernel
panic in drm_calc_vbltimestamp_from_scanoutpos():

    [drm] Initialized drm 1.1.0 20060810
    [drm] radeon kernel modesetting enabled.
    [drm] initializing kernel modesetting (RV100 0x1002:0x515E 0x15D9:0x8080).
    [drm] register mmio base: 0xC8400000
    [drm] register mmio size: 65536
    radeon 0000:0b:01.0: VRAM: 128M 0x00000000D0000000 - 0x00000000D7FFFFFF (16M used)
    radeon 0000:0b:01.0: GTT: 512M 0x00000000B0000000 - 0x00000000CFFFFFFF
    [drm] Detected VRAM RAM=128M, BAR=128M
    [drm] RAM width 16bits DDR
    [TTM] Zone  kernel: Available graphics memory: 3829346 kiB
    [TTM] Zone   dma32: Available graphics memory: 2097152 kiB
    [TTM] Initializing pool allocator
    [TTM] Initializing DMA pool allocator
    [drm] radeon: 16M of VRAM memory ready
    [drm] radeon: 512M of GTT memory ready.
    [drm] GART: num cpu pages 131072, num gpu pages 131072
    [drm] PCI GART of 512M enabled (table at 0x0000000037880000).
    radeon 0000:0b:01.0: WB disabled
    radeon 0000:0b:01.0: fence driver on ring 0 use gpu addr 0x00000000b0000000 and cpu addr 0xffff8800bbbfa000
    [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
    [drm] Driver supports precise vblank timestamp query.
    [drm] radeon: irq initialized.
    [drm] Loading R100 Microcode
    radeon 0000:0b:01.0: Direct firmware load for radeon/R100_cp.bin failed with error -2
    radeon_cp: Failed to load firmware "radeon/R100_cp.bin"
    [drm:r100_cp_init] *ERROR* Failed to load firmware!
    radeon 0000:0b:01.0: failed initializing CP (-2).
    radeon 0000:0b:01.0: Disabling GPU acceleration
    [drm] radeon: cp finalized
    BUG: unable to handle kernel NULL pointer dereference at 000000000000025c
    IP: [<ffffffff8150423b>] drm_calc_vbltimestamp_from_scanoutpos+0x4b/0x320
    PGD 0
    Oops: 0000 [#1] SMP
    Modules linked in:
    CPU: 1 PID: 1 Comm: swapper/0 Not tainted 3.18.0-rc6-4-default #2649
    Hardware name: Supermicro X7DB8/X7DB8, BIOS 6.00 07/26/2006
    task: ffff880234da2010 ti: ffff880234da4000 task.ti: ffff880234da4000
    RIP: 0010:[<ffffffff8150423b>]  [<ffffffff8150423b>] drm_calc_vbltimestamp_from_scanoutpos+0x4b/0x320
    RSP: 0000:ffff880234da7918  EFLAGS: 00010086
    RAX: ffffffff81557890 RBX: 0000000000000000 RCX: ffff880234da7a48
    RDX: ffff880234da79f4 RSI: 0000000000000000 RDI: ffff880232e15000
    RBP: ffff880234da79b8 R08: 0000000000000000 R09: 0000000000000000
    R10: 000000000000000a R11: 0000000000000001 R12: ffff880232dda1c0
    R13: ffff880232e1518c R14: 0000000000000292 R15: ffff880232e15000
    FS:  0000000000000000(0000) GS:ffff88023fc40000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
    CR2: 000000000000025c CR3: 0000000002014000 CR4: 00000000000007e0
    Stack:
     ffff880234da79d8 0000000000000286 ffff880232dcbc00 0000000000002480
     ffff880234da7958 0000000000000296 ffff880234da7998 ffffffff8151b51d
     ffff880234da7a48 0000000032dcbeb0 ffff880232dcbc00 ffff880232dcbc58
    Call Trace:
     [<ffffffff8151b51d>] ? drm_vma_offset_remove+0x1d/0x110
     [<ffffffff8152dc98>] radeon_get_vblank_timestamp_kms+0x38/0x60
     [<ffffffff8152076a>] ? ttm_bo_release_list+0xba/0x180
     [<ffffffff81503751>] drm_get_last_vbltimestamp+0x41/0x70
     [<ffffffff81503933>] vblank_disable_and_save+0x73/0x1d0
     [<ffffffff81106b2f>] ? try_to_del_timer_sync+0x4f/0x70
     [<ffffffff81505245>] drm_vblank_cleanup+0x65/0xa0
     [<ffffffff815604fa>] radeon_irq_kms_fini+0x1a/0x70
     [<ffffffff8156c07e>] r100_init+0x26e/0x410
     [<ffffffff8152ae3e>] radeon_device_init+0x7ae/0xb50
     [<ffffffff8152d57f>] radeon_driver_load_kms+0x8f/0x210
     [<ffffffff81506965>] drm_dev_register+0xb5/0x110
     [<ffffffff8150998f>] drm_get_pci_dev+0x8f/0x200
     [<ffffffff815291cd>] radeon_pci_probe+0xad/0xe0
     [<ffffffff8141a365>] local_pci_probe+0x45/0xa0
     [<ffffffff8141b741>] pci_device_probe+0xd1/0x130
     [<ffffffff81633dad>] driver_probe_device+0x12d/0x3e0
     [<ffffffff8163413b>] __driver_attach+0x9b/0xa0
     [<ffffffff816340a0>] ? __device_attach+0x40/0x40
     [<ffffffff81631cd3>] bus_for_each_dev+0x63/0xa0
     [<ffffffff8163378e>] driver_attach+0x1e/0x20
     [<ffffffff81633390>] bus_add_driver+0x180/0x240
     [<ffffffff81634914>] driver_register+0x64/0xf0
     [<ffffffff81419cac>] __pci_register_driver+0x4c/0x50
     [<ffffffff81509bf5>] drm_pci_init+0xf5/0x120
     [<ffffffff821dc871>] ? ttm_init+0x6a/0x6a
     [<ffffffff821dc908>] radeon_init+0x97/0xb5
     [<ffffffff810002fc>] do_one_initcall+0xbc/0x1f0
     [<ffffffff810e3278>] ? __wake_up+0x48/0x60
     [<ffffffff8218e256>] kernel_init_freeable+0x18a/0x215
     [<ffffffff8218d983>] ? initcall_blacklist+0xc0/0xc0
     [<ffffffff818a78f0>] ? rest_init+0x80/0x80
     [<ffffffff818a78fe>] kernel_init+0xe/0xf0
     [<ffffffff818c0c3c>] ret_from_fork+0x7c/0xb0
     [<ffffffff818a78f0>] ? rest_init+0x80/0x80
    Code: 45 ac 0f 88 a8 01 00 00 3b b7 d0 01 00 00 49 89 ff 0f 83 99 01 00 00 48 8b 47 20 48 8b 80 88 00 00 00 48 85 c0 0f 84 cd 01 00 00 <41> 8b b1 5c 02 00 00 41 8b 89 58 02 00 00 89 75 98 41 8b b1 60
    RIP  [<ffffffff8150423b>] drm_calc_vbltimestamp_from_scanoutpos+0x4b/0x320
     RSP <ffff880234da7918>
    CR2: 000000000000025c
    ---[ end trace ad2c0aadf48e2032 ]---
    Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009

It has helped me to add a NULL pointer check that was suggested at
http://lists.freedesktop.org/archives/dri-devel/2014-October/070663.html

I am not familiar with the code. But the change looks sane
and we need something fast at this stage of 3.18 development.
Suggested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Petr Mladek <pmladek@suse.cz>
Tested-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/radeon_kms.c

@@ -795,6 +795,8 @@ int radeon_get_vblank_timestamp_kms(struct drm_device *dev, int crtc,
 
 	/* Get associated drm_crtc: */
 	drmcrtc = &rdev->mode_info.crtcs[crtc]->base;
+	if (!drmcrtc)
+		return -EINVAL;
 
 	/* Helper routine in DRM core does all the work: */
 	return drm_calc_vbltimestamp_from_scanoutpos(dev, crtc, max_error,