drm/i915/guc: Fix null pointer dereference when GuC FW is not available

If GuC firmware is not available on the system and we load i915 with enable
GuC, then we hit this null pointer dereference issue:

[   71.098873] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[   71.098938] IP: intel_uc_fw_upload+0x1f/0x360 [i915]
[   71.098947] PGD 0 P4D 0
[   71.098956] Oops: 0000 [#1] PREEMPT SMP PTI
[   71.098965] Modules linked in: i915(O+) netconsole x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel mei_me i2c_i801 prime_numbers mei [last unloaded: i915]
[   71.099005] CPU: 2 PID: 1167 Comm: insmod Tainted: G     U  W  O     4.16.0-rc1+ #337
[   71.099018] Hardware name: /NUC6i5SYB, BIOS SYSKLi35.86A.0065.2018.0103.1000 01/03/2018
[   71.099077] RIP: 0010:intel_uc_fw_upload+0x1f/0x360 [i915]
[   71.099087] RSP: 0018:ffffc90000417aa0 EFLAGS: 00010282
[   71.099097] RAX: 0000000000000000 RBX: ffff88084cad12f8 RCX: ffffffffa03e9357
[   71.099108] RDX: 0000000000000002 RSI: ffffffffa034dba0 RDI: ffff88084cad12f8
[   71.099118] RBP: 0000000000000002 R08: ffff88085344ca90 R09: 0000000000000001
[   71.099128] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88084cad0000
[   71.099139] R13: ffffffffa034dba0 R14: 00000000fffffff5 R15: ffff88084cad12b0
[   71.099151] FS:  00007f7f24ae2740(0000) GS:ffff88085e200000(0000) knlGS:0000000000000000
[   71.099162] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   71.099171] CR2: 0000000000000008 CR3: 0000000855f48001 CR4: 00000000003606e0
[   71.099182] Call Trace:
[   71.099246]  intel_uc_init_hw+0xc8/0x520 [i915]
[   71.099303]  i915_gem_init_hw+0x11f/0x2d0 [i915]
[   71.099364]  i915_gem_init+0x2b9/0x640 [i915]
[   71.099413]  i915_driver_load+0xb74/0x1110 [i915]
[   71.099462]  i915_pci_probe+0x2e/0x90 [i915]
[   71.099476]  pci_device_probe+0xa1/0x130
[   71.099488]  driver_probe_device+0x302/0x470
[   71.099502]  __driver_attach+0xb9/0xe0
[   71.099513]  ? driver_probe_device+0x470/0x470
[   71.099525]  ? driver_probe_device+0x470/0x470
[   71.099538]  bus_for_each_dev+0x64/0x90
[   71.099550]  bus_add_driver+0x164/0x260
[   71.099561]  ? 0xffffffffa04d6000
[   71.099572]  driver_register+0x57/0xc0
[   71.099582]  ? 0xffffffffa04d6000
[   71.099593]  do_one_initcall+0x3b/0x160
[   71.099606]  ? kmem_cache_alloc_trace+0x1c3/0x2a0
[   71.099621]  do_init_module+0x5b/0x1f9
[   71.099635]  load_module+0x2467/0x2a70
[   71.099654]  ? SyS_finit_module+0xbd/0xe0
[   71.099668]  SyS_finit_module+0xbd/0xe0
[   71.099682]  do_syscall_64+0x73/0x1c0
[   71.099694]  entry_SYSCALL_64_after_hwframe+0x26/0x9b
[   71.099706] RIP: 0033:0x7f7f23fb40d9
[   71.099717] RSP: 002b:00007ffda7d67ed8 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[   71.099734] RAX: ffffffffffffffda RBX: 000055f96e2a8870 RCX: 00007f7f23fb40d9
[   71.099748] RDX: 0000000000000000 RSI: 000055f96e2a8260 RDI: 0000000000000003
[   71.099763] RBP: 000055f96e2a8260 R08: 0000000000000000 R09: 00007ffda7d68088
[   71.099777] R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000000
[   71.099791] R13: 000055f96e2a8830 R14: 0000000000000000 R15: 000055f96e2a8260
[   71.099810] Code: 00 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 55 41 54 49 89 f5 55 53 48 c7 c1 57 93 3e a0 48 8b 47 10 48 89 fb 4c 8b 07 <48> 8b 68 08 8b 47 28 85 c0 74 15 83 f8 01 48 c7 c1 5b 93 3e a0
[   71.100004] RIP: intel_uc_fw_upload+0x1f/0x360 [i915] RSP: ffffc90000417aa0
[   71.100020] CR2: 0000000000000008
[   71.100031] ---[ end trace d8ac93c30ceff5b2 ]--

Fixes: 6b0478fb722a ("drm/i915: Implement dynamic GuC WOPCM offset and size calculation")

v2: don't assume it is always GuC FW (Michal)
v3: added a new variable to avoid exceeding the number of characters in the
line (Michal)

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Reported-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jackie Li <yaodong.li@intel.com>
Cc: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Jackie Li <yaodong.li@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180323112319.16293-1-piotr.piorkowski@intel.com

drivers/gpu/drm/i915/intel_uc_fw.c

@@ -199,8 +199,8 @@ int intel_uc_fw_upload(struct intel_uc_fw *uc_fw,
 		       int (*xfer)(struct intel_uc_fw *uc_fw,
 				   struct i915_vma *vma))
 {
-	struct drm_i915_private *i915 = to_i915(uc_fw->obj->base.dev);
 	struct i915_vma *vma;
+	u32 ggtt_pin_bias;
 	int err;
 
 	DRM_DEBUG_DRIVER("%s fw load %s\n",
@@ -222,9 +222,9 @@ int intel_uc_fw_upload(struct intel_uc_fw *uc_fw,
 		goto fail;
 	}
 
+	ggtt_pin_bias = to_i915(uc_fw->obj->base.dev)->guc.ggtt_pin_bias;
 	vma = i915_gem_object_ggtt_pin(uc_fw->obj, NULL, 0, 0,
-				       PIN_OFFSET_BIAS |
-				       i915->guc.ggtt_pin_bias);
+				       PIN_OFFSET_BIAS | ggtt_pin_bias);
 	if (IS_ERR(vma)) {
 		err = PTR_ERR(vma);
 		DRM_DEBUG_DRIVER("%s fw ggtt-pin err=%d\n",