drm/i915: Deconstruct struct sgt_dma initialiser

gcc-4.4 complains about:

	struct sgt_dma iter = {
		.sg = vma->pages->sgl,
		.dma = sg_dma_address(iter.sg),
		.max = iter.dma + iter.sg->length,
	};

drivers/gpu/drm/i915/i915_gem_gtt.c: In function ‘gen8_ppgtt_insert_4lvl’:
drivers/gpu/drm/i915/i915_gem_gtt.c:938: error: ‘iter.sg’ is used uninitialized in this function
drivers/gpu/drm/i915/i915_gem_gtt.c:939: error: ‘iter.dma’ is used uninitialized in this function

and worse generates invalid code that triggers a GPF:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: gen8_ppgtt_insert_4lvl+0x1b/0x1e0 [i915]
PGD 0
Oops: 0000 [#1] SMP
Modules linked in: snd_aloop nf_conntrack_ipv6 nf_defrag_ipv6 nf_log_ipv6 ip6table_filter ip6_tables ctr ccm xt_state nf_log_ipv4
nf_log_common xt_LOG xt_limit xt_recent xt_owner xt_addrtype iptable_filter ipt_MASQUERADE nf_nat_masquerade_ipv4 iptable_nat
nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack libcrc32c ip_tables dm_mod vhost_net macvtap macvlan vhost tun kvm_intel kvm
irqbypass uas usb_storage hid_multitouch btusb btrtl uvcvideo videobuf2_v4l2 videobuf2_core videodev media videobuf2_vmalloc videobuf2_memops
sg ppdev dell_wmi sparse_keymap mei_wdt sd_mod iTCO_wdt iTCO_vendor_support rtsx_pci_ms memstick rtsx_pci_sdmmc mmc_core dell_smm_hwmon hwmon
dell_laptop dell_smbios dcdbas joydev input_leds hci_uart btintel btqca btbcm bluetooth parport_pc parport i2c_hid
  intel_lpss_acpi intel_lpss pcspkr wmi int3400_thermal acpi_thermal_rel dell_rbtn mei_me mei snd_hda_codec_hdmi snd_hda_codec_realtek
snd_hda_codec_generic ahci libahci acpi_pad xhci_pci xhci_hcd snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device
snd_pcm snd_timer snd soundcore int3403_thermal arc4 e1000e ptp pps_core i2c_i801 iwlmvm mac80211 rtsx_pci iwlwifi cfg80211 rfkill
intel_pch_thermal processor_thermal_device int340x_thermal_zone intel_soc_dts_iosf i915 video fjes
CPU: 2 PID: 2408 Comm: X Not tainted 4.10.0-rc5+ #1
Hardware name: Dell Inc. Latitude E7470/0T6HHJ, BIOS 1.11.3 11/09/2016
task: ffff880219fe4740 task.stack: ffffc90005f98000
RIP: 0010:gen8_ppgtt_insert_4lvl+0x1b/0x1e0 [i915]
RSP: 0018:ffffc90005f9b8c8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8802167d8000 RCX: 0000000000000001
RDX: 00000000ffff7000 RSI: ffff880219f94140 RDI: ffff880228444000
RBP: ffffc90005f9b948 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000080
R13: 0000000000000001 R14: ffffc90005f9bcd7 R15: ffff88020c9a83c0
FS:  00007fb53e1ee920(0000) GS:ffff88024dd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000010 CR3: 000000022ef95000 CR4: 00000000003406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  ppgtt_bind_vma+0x40/0x50 [i915]
  i915_vma_bind+0xcb/0x1c0 [i915]
  __i915_vma_do_pin+0x6e/0xd0 [i915]
  i915_gem_execbuffer_reserve_vma+0x162/0x1d0 [i915]
  i915_gem_execbuffer_reserve+0x4fc/0x510 [i915]
  ? __kmalloc+0x134/0x250
  ? i915_gem_wait_for_error+0x25/0x100 [i915]
  ? i915_gem_wait_for_error+0x25/0x100 [i915]
  i915_gem_do_execbuffer+0x2df/0xa00 [i915]
  ? drm_malloc_gfp.clone.0+0x42/0x80 [i915]
  ? path_put+0x22/0x30
  ? __check_object_size+0x62/0x1f0
  ? terminate_walk+0x44/0x90
  i915_gem_execbuffer2+0x95/0x1e0 [i915]
  drm_ioctl+0x243/0x490
  ? handle_pte_fault+0x1d7/0x220
  ? i915_gem_do_execbuffer+0xa00/0xa00 [i915]
  ? handle_mm_fault+0x10d/0x2a0
  vfs_ioctl+0x18/0x30
  do_vfs_ioctl+0x14b/0x3f0
  SyS_ioctl+0x92/0xa0
  entry_SYSCALL_64_fastpath+0x1a/0xa9
RIP: 0033:0x7fb53b4fcb77
RSP: 002b:00007ffe0c572898 EFLAGS: 00003246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fb53e17c038 RCX: 00007fb53b4fcb77
RDX: 00007ffe0c572900 RSI: 0000000040406469 RDI: 000000000000000b
RBP: 00007fb5376d67e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000028 R11: 0000000000003246 R12: 0000000000000000
R13: 0000000000000000 R14: 000055eecb314d00 R15: 000055eecb315460
Code: 0f 84 5d ff ff ff eb a2 0f 1f 84 00 00 00 00 00 55 48 89 e5 41 57 41 56 41 55 41 54 53 48 83 ec 58 0f 1f 44 00 00 31 c0 89 4d b0 <4c>
8b 60 10 44 8b 70 0c 48 89 d0 4c 8b 2e 48 c1 e8 27 25 ff 01
RIP: gen8_ppgtt_insert_4lvl+0x1b/0x1e0 [i915] RSP: ffffc90005f9b8c8
CR2: 0000000000000010

Recent gccs, such as 4.9, 6.3 or 7.2, do not generate the warning nor do
they explode on use. If we manually create the struct using locals from
the stack, this should eliminate this issue, and does not alter code
generation with gcc-7.2.

Fixes: 894ccebe ("drm/i915: Micro-optimise gen8_ppgtt_insert_entries()")
Reported-by: Kelly French <kfrench@federalhill.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Kelly French <kfrench@federalhill.net>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106211128.12538-1-chris@chris-wilson.co.ukTested-by: Kelly French <kfrench@federalhill.net>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -958,10 +958,14 @@ static void gen8_ppgtt_clear_4lvl(struct i915_address_space *vm,
 	}
 }
 
-struct sgt_dma {
+static inline struct sgt_dma {
 	struct scatterlist *sg;
 	dma_addr_t dma, max;
-};
+} sgt_dma(struct i915_vma *vma) {
+	struct scatterlist *sg = vma->pages->sgl;
+	dma_addr_t addr = sg_dma_address(sg);
+	return (struct sgt_dma) { sg, addr, addr + sg->length };
+}
 
 struct gen8_insert_pte {
 	u16 pml4e;
@@ -1042,11 +1046,7 @@ static void gen8_ppgtt_insert_3lvl(struct i915_address_space *vm,
 				   u32 unused)
 {
 	struct i915_hw_ppgtt *ppgtt = i915_vm_to_ppgtt(vm);
-	struct sgt_dma iter = {
-		.sg = vma->pages->sgl,
-		.dma = sg_dma_address(iter.sg),
-		.max = iter.dma + iter.sg->length,
-	};
+	struct sgt_dma iter = sgt_dma(vma);
 	struct gen8_insert_pte idx = gen8_insert_pte(vma->node.start);
 
 	gen8_ppgtt_insert_pte_entries(ppgtt, &ppgtt->pdp, &iter, &idx,
@@ -1158,11 +1158,7 @@ static void gen8_ppgtt_insert_4lvl(struct i915_address_space *vm,
 				   u32 unused)
 {
 	struct i915_hw_ppgtt *ppgtt = i915_vm_to_ppgtt(vm);
-	struct sgt_dma iter = {
-		.sg = vma->pages->sgl,
-		.dma = sg_dma_address(iter.sg),
-		.max = iter.dma + iter.sg->length,
-	};
+	struct sgt_dma iter = sgt_dma(vma);
 	struct i915_page_directory_pointer **pdps = ppgtt->pml4.pdps;
 
 	if (vma->page_sizes.sg > I915_GTT_PAGE_SIZE) {
@@ -1869,13 +1865,10 @@ static void gen6_ppgtt_insert_entries(struct i915_address_space *vm,
 	unsigned act_pt = first_entry / GEN6_PTES;
 	unsigned act_pte = first_entry % GEN6_PTES;
 	const u32 pte_encode = vm->pte_encode(0, cache_level, flags);
-	struct sgt_dma iter;
+	struct sgt_dma iter = sgt_dma(vma);
 	gen6_pte_t *vaddr;
 
 	vaddr = kmap_atomic_px(ppgtt->pd.page_table[act_pt]);
-	iter.sg = vma->pages->sgl;
-	iter.dma = sg_dma_address(iter.sg);
-	iter.max = iter.dma + iter.sg->length;
 	do {
 		vaddr[act_pte] = pte_encode | GEN6_PTE_ADDR_ENCODE(iter.dma);