[Intel-gfx] [PATCH] drm/i915: Prevent loading of uninitialized context garbage

[Chris Wilson]

The extended state bits are stored in the LCA register and affect all
updates to the LCA register - i.e. the state on the old context is saved
when SAVE_EX_STATE_EN  is currently set in the old context address before
the update, and the new context is restored when RESTORE_EX_STATE_EN is
set in the new context address. This is irrespective of the
RESTORE_INHIBIT flag in the MI_SET_CONTEXT.

Hence, upon initial loading the contents of the extended state is read
from uninitialised data. To workaround this, on first load we do a dummy
load without the mandatory RESTORE_EX_STATE_EN bit so that the real load
causes us to initialise the extended state of the context before it is
then loaded by the LCA update.

References: https://bugs.freedesktop.org/show_bug.cgi?id=64073
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ben Widawsky <ben at bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 35 +++++++++++++++++++++++++--------
 1 file changed, 27 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 70e9e40..cf35f01 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -351,7 +351,7 @@ mi_set_context(struct intel_ring_buffer *ring,
 	       struct i915_hw_context *new_context,
 	       u32 hw_flags)
 {
-	int ret;
+	int ret, len;
 
 	/* w/a: If Flush TLB Invalidation Mode is enabled, driver must do a TLB
 	 * invalidation prior to MI_SET_CONTEXT. On GEN6 we don't set the value
@@ -364,7 +364,16 @@ mi_set_context(struct intel_ring_buffer *ring,
 			return ret;
 	}
 
-	ret = intel_ring_begin(ring, 6);
+	len = 4;
+	switch (INTEL_INFO(ring->dev)->gen) {
+	case 7:
+	case 5: len += 2;
+		break;
+	}
+	if (!new_context->is_initialized)
+		len += 2;
+
+	ret = intel_ring_begin(ring, len);
 	if (ret)
 		return ret;
 
@@ -376,9 +385,22 @@ mi_set_context(struct intel_ring_buffer *ring,
 	case 5:
 		intel_ring_emit(ring, MI_SUSPEND_FLUSH | MI_SUSPEND_FLUSH_EN);
 		break;
-	default:
-		intel_ring_emit(ring, MI_NOOP);
-		break;
+	}
+
+	if (!new_context->is_initialized) {
+		/* The GPU tries to restore the extended state irrespective
+		 * of RestoreInhibit (since it is part of the LCA switch
+		 * itself rather than the MI_SET_CONTEXT command).
+		 * Since the initial contents may be garbage we do a dummy
+		 * load first then set the mandatory flag for any future
+		 * ring context switches.
+		 */
+		intel_ring_emit(ring, MI_SET_CONTEXT);
+		intel_ring_emit(ring,
+				i915_gem_obj_ggtt_offset(new_context->obj) |
+				MI_MM_SPACE_GTT |
+				MI_SAVE_EXT_STATE_EN |
+				hw_flags);
 	}
 
 	intel_ring_emit(ring, MI_NOOP);
@@ -398,9 +420,6 @@ mi_set_context(struct intel_ring_buffer *ring,
 	case 5:
 		intel_ring_emit(ring, MI_SUSPEND_FLUSH);
 		break;
-	default:
-		intel_ring_emit(ring, MI_NOOP);
-		break;
 	}
 
 	intel_ring_advance(ring);
-- 
1.8.4.rc1