drm/i915/selftest: assert we get 2M GTT pages

For the LMEM case if we have suitable alignment and 2M physical pages we
should always get 2M GTT pages within the constraints of the hugepages
selftest. If we don't then something might be wrong in our construction
of the backing pages.

References: 330b7d33 ("drm/i915/region: fix order when adding blocks")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130141809.65330-2-matthew.auld@intel.com

drivers/gpu/drm/i915/gem/selftests/huge_pages.c

@@ -368,6 +368,27 @@ static int igt_check_page_sizes(struct i915_vma *vma)
 		err = -EINVAL;
 	}
 
+	/*
+	 * The dma-api is like a box of chocolates when it comes to the
+	 * alignment of dma addresses, however for LMEM we have total control
+	 * and so can guarantee alignment, likewise when we allocate our blocks
+	 * they should appear in descending order, and if we know that we align
+	 * to the largest page size for the GTT address, we should be able to
+	 * assert that if we see 2M physical pages then we should also get 2M
+	 * GTT pages. If we don't then something might be wrong in our
+	 * construction of the backing pages.
+	 *
+	 * Maintaining alignment is required to utilise huge pages in the ppGGT.
+	 */
+	if (i915_gem_object_is_lmem(obj) &&
+	    IS_ALIGNED(vma->node.start, SZ_2M) &&
+	    vma->page_sizes.sg & SZ_2M &&
+	    vma->page_sizes.gtt < SZ_2M) {
+		pr_err("gtt pages mismatch for LMEM, expected 2M GTT pages, sg(%u), gtt(%u)\n",
+		       vma->page_sizes.sg, vma->page_sizes.gtt);
+		err = -EINVAL;
+	}
+
 	if (obj->mm.page_sizes.gtt) {
 		pr_err("obj->page_sizes.gtt(%u) should never be set\n",
 		       obj->mm.page_sizes.gtt);