drm/i915: Mark ininitial fb obj as WT on eLLC machines to avoid rcu lockup during fbdev init

Currently we leave the cache_level of the initial fb obj
set to NONE. This means on eLLC machines the first pin_to_display()
will try to switch it to WT which requires a vma unbind+bind.
If that happens during the fbdev initialization rcu does not
seem operational which causes the unbind to get stuck. To
most appearances this looks like a dead machine on boot.

Avoid the unbind by already marking the object cache_level
as WT when creating it. We still do an excplicit ggtt pin
which will rewrite the PTEs anyway, so they will match whatever
cache level we set.

Cc: <stable@vger.kernel.org> # v5.7+
Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2381Signed-off-by: Ville Syrjälä <ville.syrjala@linux.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/20201007120329.17076-1-ville.syrjala@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20201015122138.30161-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/display/intel_display.c

@@ -3433,6 +3433,14 @@ initial_plane_vma(struct drm_i915_private *i915,
 	if (IS_ERR(obj))
 		return NULL;
 
+	/*
+	 * Mark it WT ahead of time to avoid changing the
+	 * cache_level during fbdev initialization. The
+	 * unbind there would get stuck waiting for rcu.
+	 */
+	i915_gem_object_set_cache_coherency(obj, HAS_WT(i915) ?
+					    I915_CACHE_WT : I915_CACHE_NONE);
+
 	switch (plane_config->tiling) {
 	case I915_TILING_NONE:
 		break;