drm/i915: enforce min page size for scratch

If the device needs 64K minimum GTT pages for device local-memory,
like on XEHPSDV, then we need to fail the allocation if we can't
meet it, instead of falling back to 4K pages, otherwise we can't
safely support the insertion of device local-memory pages for
this vm, since the HW expects the correct physical alignment and
size for every PTE, if we mark the page-table as 64K GTT mode.

v2: s/userpsace/userspace [Thomas]
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208141613.7251-5-ramalingam.c@intel.com

drivers/gpu/drm/i915/gt/intel_gtt.c

@@ -337,6 +337,18 @@ int setup_scratch_page(struct i915_address_space *vm)
 		if (size == I915_GTT_PAGE_SIZE_4K)
 			return -ENOMEM;
 
+		/*
+		 * If we need 64K minimum GTT pages for device local-memory,
+		 * like on XEHPSDV, then we need to fail the allocation here,
+		 * otherwise we can't safely support the insertion of
+		 * local-memory pages for this vm, since the HW expects the
+		 * correct physical alignment and size when the page-table is
+		 * operating in 64K GTT mode, which includes any scratch PTEs,
+		 * since userspace can still touch them.
+		 */
+		if (HAS_64K_PAGES(vm->i915))
+			return -ENOMEM;
+
 		size = I915_GTT_PAGE_SIZE_4K;
 	} while (1);
 }