[Intel-gfx] [PATCH] drm/i915: Prevent machine hang from Broxton's vtd w/a and error capture

[Chris Wilson]

Since capturing the error state requires fiddling around with the GGTT
to read arbitrary buffers and is itself run under stop_machine(), it
deadlocks the machine (effectively a hard hang) when run in conjunction
with Broxton's VTd workaround to serialize GGTT access.

Fixes: 0ef34ad6222a ("drm/i915: Serialize GTT/Aperture accesses on BXT")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Jon Bloomfield <jon.bloomfield at intel.com>
Cc: John Harrison <john.C.Harrison at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
---
 drivers/gpu/drm/i915/i915_gpu_error.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index 48418fb81066..e6c7e8e53815 100644
--- a/drivers/gpu/drm/i915/i915_gpu_error.c
+++ b/drivers/gpu/drm/i915/i915_gpu_error.c
@@ -1813,6 +1813,10 @@ void i915_capture_error_state(struct drm_i915_private *dev_priv,
 	if (!i915_modparams.error_capture)
 		return;
 
+	/* Prevent recursively calling stop_machine() and deadlocking. */
+	if (intel_ggtt_update_needs_vtd_wa(dev_priv))
+		return;
+
 	if (READ_ONCE(dev_priv->gpu_error.first_error))
 		return;
 
-- 
2.15.1