I just kicked off testing some patches on top of 6.8-rc1 and triggered this
immediately:

[ note this happened on both my 32 bit an 64 bit test machines, this is
  just the 32 bit output ]

 BUG: kernel NULL pointer dereference, address: 00000238
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 *pdpt = 0000000000000000 *pde = f000ff53f000ff53 
 Oops: 0000 [#1] PREEMPT SMP PTI
 CPU: 0 PID: 9 Comm: kworker/0:1 Not tainted 6.8.0-rc1-test-00001-g2b44760609e9-dirty #1056
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
 Workqueue: events work_for_cpu_fn
 EIP: ttm_device_init+0xb4/0x274
 Code: 86 10 09 00 00 83 c4 0c 85 c0 0f 84 96 01 00 00 8b 45 ac 8d 9e 94 00 00 00 89 46 08 89 f0 e8 27 05 00 00 8b 55 a8 0f b6 45 98 <8b> 8a 38 02 00 00 50 0f b6 45 9c 50 89 d8 e8 95 ee ff ff 8b 45 a0
 EAX: 00000000 EBX: c135a7e4 ECX: c135a7b0 EDX: 00000000
 ESI: c135a750 EDI: 0007bc1d EBP: c11d7e4c ESP: c11d7de4
 DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00010246
 CR0: 80050033 CR2: 00000238 CR3: 145c4000 CR4: 000006f0
 Call Trace:
  ? show_regs+0x4f/0x58
  ? __die+0x1d/0x58
  ? page_fault_oops+0x171/0x330
  ? lock_acquire+0xa4/0x280
  ? kernelmode_fixup_or_oops.constprop.0+0x7c/0xcc
  ? __bad_area_nosemaphore.constprop.0+0x124/0x1b4
  ? __mutex_lock+0x17f/0xb00
  ? bad_area_nosemaphore+0xf/0x14
  ? do_user_addr_fault+0x140/0x3e4
  ? exc_page_fault+0x5b/0x1d8
  ? pvclock_clocksource_read_nowd+0x130/0x130
  ? handle_exception+0x133/0x133
  ? pvclock_clocksource_read_nowd+0x130/0x130
  ? ttm_device_init+0xb4/0x274
  ? pvclock_clocksource_read_nowd+0x130/0x130
  ? ttm_device_init+0xb4/0x274
  qxl_ttm_init+0x34/0x130
  qxl_bo_init+0xd/0x10
  qxl_device_init+0x52a/0x92c
  qxl_pci_probe+0x91/0x1ac
  local_pci_probe+0x3d/0x84
  work_for_cpu_fn+0x16/0x20
  process_one_work+0x1bc/0x4a0
  worker_thread+0x310/0x3a8
  kthread+0xea/0x110
  ? rescuer_thread+0x2f0/0x2f0
  ? kthread_complete_and_exit+0x1c/0x1c
  ret_from_fork+0x34/0x4c
  ? kthread_complete_and_exit+0x1c/0x1c
  ret_from_fork_asm+0x12/0x18
  entry_INT80_32+0xf0/0xf0
 Modules linked in:
 CR2: 0000000000000238
 ---[ end trace 0000000000000000 ]---

The crash happened here:

int ttm_device_init(struct ttm_device *bdev, const struct ttm_device_funcs *funcs,
		    struct device *dev, struct address_space *mapping,
		    struct drm_vma_offset_manager *vma_manager,
		    bool use_dma_alloc, bool use_dma32)
{
	struct ttm_global *glob = &ttm_glob;
	int ret;

	if (WARN_ON(vma_manager == NULL))
		return -EINVAL;

	ret = ttm_global_init();
	if (ret)
		return ret;

	bdev->wq = alloc_workqueue("ttm",
				   WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_UNBOUND, 16);
	if (!bdev->wq) {
		ttm_global_release();
		return -ENOMEM;
	}

	bdev->funcs = funcs;

	ttm_sys_man_init(bdev);

	ttm_pool_init(&bdev->pool, dev, dev_to_node(dev), use_dma_alloc, use_dma32); <<<------- BUG!

Specifically, it appears that dev is NULL and dev_to_node() doesn't like
having a NULL pointer passed to it.

I currently "fixed" this with a:

	if (!dev)
		return -EINVAL;

at the start of this function just so that I can continue running my tests,
but that is obviously incorrect.

In one of my previous revisions of this patch when I was experimenting, I used something like below. Wonder if that could work in your case and/or in general.

diff --git a/drivers/gpu/drm/ttm/ttm_device.c b/drivers/gpu/drm/ttm/ttm_device.c

index 43e27ab77f95..4c3902b94be4 100644

--- a/drivers/gpu/drm/ttm/ttm_device.c

+++ b/drivers/gpu/drm/ttm/ttm_device.c

@@ -195,6 +195,7 @@ int ttm_device_init(struct ttm_device *bdev, struct ttm_device_funcs *funcs,

                                    bool use_dma_alloc, bool use_dma32)  {

                struct ttm_global *glob = &ttm_glob;

+              bool node_has_cpu = false;

                int ret;

                if (WARN_ON(vma_manager == NULL))

@@ -213,7 +214,12 @@ int ttm_device_init(struct ttm_device *bdev, struct ttm_device_funcs *funcs,

                bdev->funcs = funcs;

                ttm_sys_man_init(bdev);

-              ttm_pool_init(&bdev->pool, dev, NUMA_NO_NODE, use_dma_alloc, use_dma32);

+

+              node_has_cpu = node_state(dev->numa_node, N_CPU);

+              if (node_has_cpu)

+                              ttm_pool_init(&bdev->pool, dev, dev->numa_node, use_dma_alloc, use_dma32);

+              else

+                              ttm_pool_init(&bdev->pool, dev, NUMA_NO_NODE, use_dma_alloc,

+use_dma32);

                bdev->vma_manager = vma_manager;

                spin_lock_init(&bdev->lru_lock);

-- Steve