[Intel-gfx] [PATCH 2/2] 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.

v2: Split out the introduction of the variable length MI_SET_CONTEXT
command sequence.

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 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 8a7b61e..a57d49a 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -367,6 +367,8 @@ mi_set_context(struct intel_ring_buffer *ring,
 	case 5: len += 2;
 		break;
 	}
+	if (!new_context->is_initialized)
+		len += 2;
 
 	ret = intel_ring_begin(ring, len);
 	if (ret)
@@ -382,6 +384,22 @@ mi_set_context(struct intel_ring_buffer *ring,
 		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);
 	intel_ring_emit(ring, MI_SET_CONTEXT);
 	intel_ring_emit(ring, i915_gem_obj_ggtt_offset(new_context->obj) |
-- 
1.8.4.rc1