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

Kamal Mostafa kamal at canonical.com
Fri Nov 20 17:21:53 UTC 2020

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

BugLink: https://bugs.launchpad.net/bugs/1903397

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 at vger.kernel.org> # v5.7+
Suggested-by: Chris Wilson <chris at chris-wilson.co.uk>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2381
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris at 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
(cherry picked from commit d46b60a2e8d246f1f0faa38e52f4f5a73858c338)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
(cherry picked from commit 1664ffee760a5d98952318fdd9b198fae396d660)
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 drivers/gpu/drm/i915/display/intel_display.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 26996e1839e2..4bd383ff0880 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -3431,6 +3431,14 @@ 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:

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