drm/i915: Reorganise legacy context switch to cope with late failure

After mi_set_context() succeeds, we need to update the state of the
engine's last_context. This ensures that we hold a pin on the context
whilst the hardware may write to it. However, since we didn't complete
the post-switch setup of the context, we need to force the subsequent
use of the same context to complete the setup (which means updating
should_skip_switch()).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-15-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_context.c

@@ -610,17 +610,19 @@ mi_set_context(struct drm_i915_gem_request *req, u32 hw_flags)
 }
 
 static inline bool skip_rcs_switch(struct intel_engine_cs *engine,
-				   struct intel_context *from,
 				   struct intel_context *to)
 {
 	if (to->remap_slice)
 		return false;
 
-	if (to->ppgtt && from == to &&
-	    !(intel_engine_flag(engine) & to->ppgtt->pd_dirty_rings))
-		return true;
+	if (!to->legacy_hw_ctx.initialized)
+		return false;
 
+	if (to->ppgtt &&
+	    !(intel_engine_flag(engine) & to->ppgtt->pd_dirty_rings))
 		return false;
+
+	return to == engine->last_context;
 }
 
 static bool
@@ -643,18 +645,12 @@ needs_pd_load_pre(struct intel_engine_cs *engine, struct intel_context *to)
 }
 
 static bool
-needs_pd_load_post(struct intel_engine_cs *engine, struct intel_context *to,
-		   u32 hw_flags)
+needs_pd_load_post(struct intel_context *to, u32 hw_flags)
 {
-	struct drm_i915_private *dev_priv = engine->dev->dev_private;
-
 	if (!to->ppgtt)
 		return false;
 
-	if (!IS_GEN8(engine->dev))
-		return false;
-
-	if (engine != &dev_priv->engine[RCS])
+	if (!IS_GEN8(to->i915))
 		return false;
 
 	if (hw_flags & MI_RESTORE_INHIBIT)
@@ -667,12 +663,11 @@ static int do_rcs_switch(struct drm_i915_gem_request *req)
 {
 	struct intel_context *to = req->ctx;
 	struct intel_engine_cs *engine = req->engine;
-	struct intel_context *from = engine->last_context;
-	u32 hw_flags = 0;
-	bool uninitialized = false;
+	struct intel_context *from;
+	u32 hw_flags;
 	int ret, i;
 
-	if (skip_rcs_switch(engine, from, to))
+	if (skip_rcs_switch(engine, to))
 		return 0;
 
 	/* Trying to pin first makes error handling easier. */
@@ -686,23 +681,11 @@ static int do_rcs_switch(struct drm_i915_gem_request *req)
 	 * Pin can switch back to the default context if we end up calling into
 	 * evict_everything - as a last ditch gtt defrag effort that also
 	 * switches to the default context. Hence we need to reload from here.
+	 *
+	 * XXX: Doing so is painfully broken!
 	 */
 	from = engine->last_context;
 
-	if (needs_pd_load_pre(engine, to)) {
-		/* Older GENs and non render rings still want the load first,
-		 * "PP_DCLV followed by PP_DIR_BASE register through Load
-		 * Register Immediate commands in Ring Buffer before submitting
-		 * a context."*/
-		trace_switch_mm(engine, to);
-		ret = to->ppgtt->switch_mm(to->ppgtt, req);
-		if (ret)
-			goto unpin_out;
-
-		/* Doing a PD load always reloads the page dirs */
-		to->ppgtt->pd_dirty_rings &= ~intel_engine_flag(engine);
-	}
-
 	/*
 	 * Clear this page out of any CPU caches for coherent swap-in/out. Note
 	 * that thanks to write = false in this call and us not setting any gpu
@@ -715,53 +698,37 @@ static int do_rcs_switch(struct drm_i915_gem_request *req)
 	if (ret)
 		goto unpin_out;
 
-	if (!to->legacy_hw_ctx.initialized || i915_gem_context_is_default(to)) {
-		hw_flags |= MI_RESTORE_INHIBIT;
+	if (needs_pd_load_pre(engine, to)) {
+		/* Older GENs and non render rings still want the load first,
+		 * "PP_DCLV followed by PP_DIR_BASE register through Load
+		 * Register Immediate commands in Ring Buffer before submitting
+		 * a context."*/
+		trace_switch_mm(engine, to);
+		ret = to->ppgtt->switch_mm(to->ppgtt, req);
+		if (ret)
+			goto unpin_out;
+	}
+
+	if (!to->legacy_hw_ctx.initialized || i915_gem_context_is_default(to))
 		/* NB: If we inhibit the restore, the context is not allowed to
 		 * die because future work may end up depending on valid address
 		 * space. This means we must enforce that a page table load
 		 * occur when this occurs. */
-	} else if (to->ppgtt &&
-		   (intel_engine_flag(engine) & to->ppgtt->pd_dirty_rings)) {
-		hw_flags |= MI_FORCE_RESTORE;
-		to->ppgtt->pd_dirty_rings &= ~intel_engine_flag(engine);
-	}
+		hw_flags = MI_RESTORE_INHIBIT;
+	else if (to->ppgtt &&
+		 intel_engine_flag(engine) & to->ppgtt->pd_dirty_rings)
+		hw_flags = MI_FORCE_RESTORE;
+	else
+		hw_flags = 0;
 
 	/* We should never emit switch_mm more than once */
 	WARN_ON(needs_pd_load_pre(engine, to) &&
-		needs_pd_load_post(engine, to, hw_flags));
+		needs_pd_load_post(to, hw_flags));
 
+	if (to != from || (hw_flags & MI_FORCE_RESTORE)) {
 		ret = mi_set_context(req, hw_flags);
 		if (ret)
 			goto unpin_out;
-
-	/* GEN8 does *not* require an explicit reload if the PDPs have been
-	 * setup, and we do not wish to move them.
-	 */
-	if (needs_pd_load_post(engine, to, hw_flags)) {
-		trace_switch_mm(engine, to);
-		ret = to->ppgtt->switch_mm(to->ppgtt, req);
-		/* The hardware context switch is emitted, but we haven't
-		 * actually changed the state - so it's probably safe to bail
-		 * here. Still, let the user know something dangerous has
-		 * happened.
-		 */
-		if (ret) {
-			DRM_ERROR("Failed to change address space on context switch\n");
-			goto unpin_out;
-		}
-	}
-
-	for (i = 0; i < MAX_L3_SLICES; i++) {
-		if (!(to->remap_slice & (1<<i)))
-			continue;
-
-		ret = i915_gem_l3_remap(req, i);
-		/* If it failed, try again next round */
-		if (ret)
-			DRM_DEBUG_DRIVER("L3 remapping failed\n");
-		else
-			to->remap_slice &= ~(1<<i);
 	}
 
 	/* The backing object for the context is done after switching to the
@@ -786,19 +753,45 @@ static int do_rcs_switch(struct drm_i915_gem_request *req)
 		i915_gem_object_ggtt_unpin(from->legacy_hw_ctx.rcs_state);
 		i915_gem_context_unreference(from);
 	}
-
-	uninitialized = !to->legacy_hw_ctx.initialized;
-	to->legacy_hw_ctx.initialized = true;
-
 	i915_gem_context_reference(to);
 	engine->last_context = to;
 
-	if (uninitialized) {
+	/* GEN8 does *not* require an explicit reload if the PDPs have been
+	 * setup, and we do not wish to move them.
+	 */
+	if (needs_pd_load_post(to, hw_flags)) {
+		trace_switch_mm(engine, to);
+		ret = to->ppgtt->switch_mm(to->ppgtt, req);
+		/* The hardware context switch is emitted, but we haven't
+		 * actually changed the state - so it's probably safe to bail
+		 * here. Still, let the user know something dangerous has
+		 * happened.
+		 */
+		if (ret)
+			return ret;
+	}
+
+	if (to->ppgtt)
+		to->ppgtt->pd_dirty_rings &= ~intel_engine_flag(engine);
+
+	for (i = 0; i < MAX_L3_SLICES; i++) {
+		if (!(to->remap_slice & (1<<i)))
+			continue;
+
+		ret = i915_gem_l3_remap(req, i);
+		if (ret)
+			return ret;
+
+		to->remap_slice &= ~(1<<i);
+	}
+
+	if (!to->legacy_hw_ctx.initialized) {
 		if (engine->init_context) {
 			ret = engine->init_context(req);
 			if (ret)
-				DRM_ERROR("ring init context: %d\n", ret);
+				return ret;
 		}
+		to->legacy_hw_ctx.initialized = true;
 	}
 
 	return 0;