drm/amdgpu/ttm: Fix crash when handling MMIO_REMAP in PDE flags

The MMIO_REMAP BO is a special 4K IO page that does not have a ttm_tt
behind it. However, amdgpu_ttm_tt_pde_flags() was treating it like
normal TT/doorbell/preempt memory and unconditionally accessed
ttm->caching. For the MMIO_REMAP BO, ttm is NULL, so this leads to a
NULL pointer dereference when computing PDE flags.

Fix this by checking that ttm is non-NULL before reading ttm->caching.
This prevents the crash for MMIO_REMAP and also makes the code more
defensive if other BOs ever come through without a ttm_tt.

Fixes: fb5a52d ("drm/amdgpu: Implement TTM handling for MMIO_REMAP placement")
Suggested-by: Jesse Zhang <[email protected]>
Suggested-by: Christian König <[email protected]>
Cc: Alex Deucher <[email protected]>
Signed-off-by: Srinivasan Shanmugam <[email protected]>
Reviewed-by: Jesse Zhang <[email protected]>
Tested-by: Jesse Zhang <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>
(cherry picked from commit 0db94da5a0a1cacda080b9ec8425fcbe4babc141)

Author: srishanm

drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c

@@ -1372,7 +1372,7 @@ uint64_t amdgpu_ttm_tt_pde_flags(struct ttm_tt *ttm, struct ttm_resource *mem)
 		    mem->mem_type == AMDGPU_PL_MMIO_REMAP)) {
 		flags |= AMDGPU_PTE_SYSTEM;
 
-		if (ttm->caching == ttm_cached)
+		if (ttm && ttm->caching == ttm_cached)
 			flags |= AMDGPU_PTE_SNOOPED;
 	}