drm/i915/gt: Track the context validity explicitly

Rather than assume if and only if the engine->default_state is not set
that the context is invalid, instead track when we know the context has
valid state -- either because we have copied the default_state or we
have completed a context switch to save the HW state.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191203124155.3019926-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_context.c

@@ -147,6 +147,7 @@ static void __intel_context_retire(struct i915_active *active)
 	GEM_TRACE("%s context:%llx retire\n",
 		  ce->engine->name, ce->timeline->fence_context);
 
+	set_bit(CONTEXT_VALID_BIT, &ce->flags);
 	if (ce->state)
 		__context_unpin_state(ce->state);
 

drivers/gpu/drm/i915/gt/intel_context_types.h

@@ -55,6 +55,7 @@ struct intel_context {
 
 	unsigned long flags;
 #define CONTEXT_ALLOC_BIT 0
+#define CONTEXT_VALID_BIT 1
 
 	u32 *lrc_reg_state;
 	u64 lrc_desc;

drivers/gpu/drm/i915/gt/intel_lrc.c

@@ -4102,6 +4102,7 @@ populate_lr_context(struct intel_context *ce,
 
 		memcpy(vaddr, defaults, engine->context_size);
 		i915_gem_object_unpin_map(engine->default_state);
+		__set_bit(CONTEXT_VALID_BIT, &ce->flags);
 		inhibit = false;
 	}
 

drivers/gpu/drm/i915/gt/intel_ring_submission.c

@@ -1319,6 +1319,8 @@ static int ring_context_alloc(struct intel_context *ce)
 			return PTR_ERR(vma);
 
 		ce->state = vma;
+		if (engine->default_state)
+			__set_bit(CONTEXT_VALID_BIT, &ce->flags);
 	}
 
 	return 0;
@@ -1625,7 +1627,7 @@ static int switch_context(struct i915_request *rq)
 	if (ce->state) {
 		GEM_BUG_ON(rq->engine->id != RCS0);
 
-		if (!rq->engine->default_state)
+		if (!test_bit(CONTEXT_VALID_BIT, &ce->flags))
 			hw_flags = MI_RESTORE_INHIBIT;
 
 		ret = mi_set_context(rq, hw_flags);