drm/i915/gtt: Relax pd_used assertion

The current assertion tries to make sure that we do not over count the
number of used PDE inside a page directory -- that is with an array of
512 pde, we do not expect more than 512 elements used! However, our
assertion has to take into account that as we pin an element into the
page directory, the caller first pins the page directory so the usage
count is one higher. However, this should be one extra pin per thread,
and the upper bound is that we may have one thread for each entry.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820141218.14714-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -771,7 +771,8 @@ __set_pd_entry(struct i915_page_directory * const pd,
 	       struct i915_page_dma * const to,
 	       u64 (*encode)(const dma_addr_t, const enum i915_cache_level))
 {
-	GEM_BUG_ON(atomic_read(px_used(pd)) > ARRAY_SIZE(pd->entry));
+	/* Each thread pre-pins the pd, and we may have a thread per pde. */
+	GEM_BUG_ON(atomic_read(px_used(pd)) > 2 * ARRAY_SIZE(pd->entry));
 
 	atomic_inc(px_used(pd));
 	pd->entry[idx] = to;