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

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/2381
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007120329.17076-1-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 1bbe2c4a8766f351b7d73a5e643345b64b83437f..6cf8ca2d1b8177752daefbc73abefa39f2dc833e 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -3449,6 +3449,14 @@ 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;