drm/i915/ringbuffer: Fix context restore upon reset

The discovery with trying to enable full-ppgtt was that we were
completely failing to the load both the mm and context following the
reset. Although we were performing mmio to set the PP_DIR (per-process
GTT) and CCID (context), these were taking no effect (the assumption was
that this would trigger reload of the context and restore the page
tables). It was not until we performed the LRI + MI_SET_CONTEXT in a
following context switch would anything occur.

Since we are then required to reset the context image and PP_DIR using
CS commands, we place those commands into every batch. The hardware
should recognise the no-ops and eliminate the expensive context loads,
but we still have to pay the cost of using cross-powerwell register
writes. In practice, this has no effect on actual context switch times,
and only adds a few hundred nanoseconds to no-op switches. We can improve
the latter by eliminating the w/a around known no-op switches, but there
is an ulterior motive to keeping them.

Always emitting the context switch at the beginning of the request (and
relying on HW to skip unneeded switches) does have one key advantage.
Should we implement request reordering on Haswell, we will not know in
advance what the previous executing context was on the GPU and so we
would not be able to elide the MI_SET_CONTEXT commands ourselves and
always have to emit them. Having our hand forced now actually prepares
us for later.

Now since that context and mm follow the request, we no longer (and not
for a long time since requests took over!) require a trace point to tell
when we write the switch into the ring, since it is always. (This is
even more important when you remember that simply writing into the ring
bears no relation to the current mm.)

v2: Sandybridge has to agree to use LRI as well.

Testcase: igt/drv_selftests/live_hangcheck
Signed-off-by: Chris Wilson <[e-mail]>
Cc: Joonas Lahtinen <[e-mail]>
Cc: Mika Kuoppala <[e-mail]>
Cc: Matthew Auld <[e-mail]>
Cc: Tvrtko Ursulin <[e-mail]>
Reviewed-by: Joonas Lahtinen <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/i915_gem_gtt.c

···
 	wmb();
 }
 
-static inline u32 get_pd_offset(struct i915_hw_ppgtt *ppgtt)
-{
-	GEM_BUG_ON(ppgtt->pd.base.ggtt_offset & 0x3f);
-	return ppgtt->pd.base.ggtt_offset << 10;
-}
-
-static int gen7_mm_switch(struct i915_hw_ppgtt *ppgtt,
-			  struct i915_request *rq)
-{
-	struct intel_engine_cs *engine = rq->engine;
-	u32 *cs;
-
-	/* NB: TLBs must be flushed and invalidated before a switch */
-	cs = intel_ring_begin(rq, 6);
-	if (IS_ERR(cs))
-		return PTR_ERR(cs);
-
-	*cs++ = MI_LOAD_REGISTER_IMM(2);
-	*cs++ = i915_mmio_reg_offset(RING_PP_DIR_DCLV(engine));
-	*cs++ = PP_DIR_DCLV_2G;
-	*cs++ = i915_mmio_reg_offset(RING_PP_DIR_BASE(engine));
-	*cs++ = get_pd_offset(ppgtt);
-	*cs++ = MI_NOOP;
-	intel_ring_advance(rq, cs);
-
-	return 0;
-}
-
-static int gen6_mm_switch(struct i915_hw_ppgtt *ppgtt,
-			  struct i915_request *rq)
-{
-	struct intel_engine_cs *engine = rq->engine;
-	struct drm_i915_private *dev_priv = rq->i915;
-
-	I915_WRITE(RING_PP_DIR_DCLV(engine), PP_DIR_DCLV_2G);
-	I915_WRITE(RING_PP_DIR_BASE(engine), get_pd_offset(ppgtt));
-	return 0;
-}
-
 static void gen8_ppgtt_enable(struct drm_i915_private *dev_priv)
 {
 	struct intel_engine_cs *engine;
···
 	ppgtt->vm.dma = &i915->drm.pdev->dev;
 
 	ppgtt->vm.pte_encode = ggtt->vm.pte_encode;
-	if (IS_GEN6(i915))
-		ppgtt->switch_mm = gen6_mm_switch;
-	else if (IS_GEN7(i915))
-		ppgtt->switch_mm = gen7_mm_switch;
-	else
-		BUG();
 
 	err = gen6_ppgtt_alloc(ppgtt);
 	if (err)

drivers/gpu/drm/i915/i915_gem_gtt.h

···
 
 	gen6_pte_t __iomem *pd_addr;
 
-	int (*switch_mm)(struct i915_hw_ppgtt *ppgtt,
-			 struct i915_request *rq);
 	void (*debug_dump)(struct i915_hw_ppgtt *ppgtt, struct seq_file *m);
 };
 

drivers/gpu/drm/i915/i915_request.c

···
 	/* Keep a second pin for the dual retirement along engine and ring */
 	__intel_context_pin(ce);
 
+	rq->infix = rq->ring->emit; /* end of header; start of user payload */
+
 	/* Check that we didn't interrupt ourselves with a new request */
 	GEM_BUG_ON(rq->timeline->seqno != rq->fence.seqno);
 	return rq;

drivers/gpu/drm/i915/i915_request.h

···
 	/** Position in the ring of the start of the request */
 	u32 head;
 
+	/** Position in the ring of the start of the user packets */
+	u32 infix;
+
 	/**
 	 * Position in the ring of the start of the postfix.
 	 * This is required to calculate the maximum available ring space

drivers/gpu/drm/i915/i915_trace.h

···
 	TP_ARGS(ctx)
 );
 
-/**
- * DOC: switch_mm tracepoint
- *
- * This tracepoint allows tracking of the mm switch, which is an important point
- * in the lifetime of the vm in the legacy submission path. This tracepoint is
- * called only if full ppgtt is enabled.
- */
-TRACE_EVENT(switch_mm,
-	TP_PROTO(struct intel_engine_cs *engine, struct i915_gem_context *to),
-
-	TP_ARGS(engine, to),
-
-	TP_STRUCT__entry(
-			__field(u16, class)
-			__field(u16, instance)
-			__field(struct i915_gem_context *, to)
-			__field(struct i915_address_space *, vm)
-			__field(u32, dev)
-	),
-
-	TP_fast_assign(
-			__entry->class = engine->uabi_class;
-			__entry->instance = engine->instance;
-			__entry->to = to;
-			__entry->vm = to->ppgtt ? &to->ppgtt->vm : NULL;
-			__entry->dev = engine->i915->drm.primary->index;
-	),
-
-	TP_printk("dev=%u, engine=%u:%u, ctx=%p, ctx_vm=%p",
-		  __entry->dev, __entry->class, __entry->instance, __entry->to,
-		  __entry->vm)
-);
-
 #endif /* _I915_TRACE_H_ */
 
 /* This part must be outside protection */

drivers/gpu/drm/i915/intel_engine_cs.c

···
 
 	lockdep_assert_held(&engine->i915->drm.struct_mutex);
 
-	engine->legacy_active_context = NULL;
-	engine->legacy_active_ppgtt = NULL;
-
 	ce = fetch_and_zero(&engine->last_retired_context);
 	if (ce)
 		intel_context_unpin(ce);

drivers/gpu/drm/i915/intel_ringbuffer.c

···
 	return i915_gem_find_active_request(engine);
 }
 
-static void reset_ring(struct intel_engine_cs *engine,
-		       struct i915_request *request)
+static void skip_request(struct i915_request *rq)
 {
-	GEM_TRACE("%s seqno=%x\n",
-		  engine->name, request ? request->global_seqno : 0);
+	void *vaddr = rq->ring->vaddr;
+	u32 head;
+
+	head = rq->infix;
+	if (rq->postfix < head) {
+		memset32(vaddr + head, MI_NOOP,
+			 (rq->ring->size - head) / sizeof(u32));
+		head = 0;
+	}
+	memset32(vaddr + head, MI_NOOP, (rq->postfix - head) / sizeof(u32));
+}
+
+static void reset_ring(struct intel_engine_cs *engine, struct i915_request *rq)
+{
+	GEM_TRACE("%s seqno=%x\n", engine->name, rq ? rq->global_seqno : 0);
 
 	/*
 	 * RC6 must be prevented until the reset is complete and the engine
···
 	 * If the request was innocent, we try to replay the request with
 	 * the restored context.
 	 */
-	if (request) {
-		struct drm_i915_private *dev_priv = request->i915;
-		struct intel_context *ce = request->hw_context;
-		struct i915_hw_ppgtt *ppgtt;
-
-		if (ce->state) {
-			I915_WRITE(CCID,
-				   i915_ggtt_offset(ce->state) |
-				   BIT(8) /* must be set! */ |
-				   CCID_EXTENDED_STATE_SAVE |
-				   CCID_EXTENDED_STATE_RESTORE |
-				   CCID_EN);
-		}
-
-		ppgtt = request->gem_context->ppgtt ?: engine->i915->mm.aliasing_ppgtt;
-		if (ppgtt) {
-			u32 pd_offset = ppgtt->pd.base.ggtt_offset << 10;
-
-			I915_WRITE(RING_PP_DIR_DCLV(engine), PP_DIR_DCLV_2G);
-			I915_WRITE(RING_PP_DIR_BASE(engine), pd_offset);
-
-			/* Wait for the PD reload to complete */
-			if (intel_wait_for_register(dev_priv,
-						    RING_PP_DIR_BASE(engine),
-						    BIT(0), 0,
-						    10))
-				DRM_ERROR("Wait for reload of ppgtt page-directory timed out\n");
-
-			ppgtt->pd_dirty_rings &= ~intel_engine_flag(engine);
-		}
-
+	if (rq) {
 		/* If the rq hung, jump to its breadcrumb and skip the batch */
-		if (request->fence.error == -EIO)
-			request->ring->head = request->postfix;
-	} else {
-		engine->legacy_active_context = NULL;
-		engine->legacy_active_ppgtt = NULL;
+		rq->ring->head = intel_ring_wrap(rq->ring, rq->head);
+		if (rq->fence.error == -EIO)
+			skip_request(rq);
 	}
 }
 
···
 		intel_ring_reset(engine->buffer, 0);
 }
 
+static int load_pd_dir(struct i915_request *rq,
+		       const struct i915_hw_ppgtt *ppgtt)
+{
+	const struct intel_engine_cs * const engine = rq->engine;
+	u32 *cs;
+
+	cs = intel_ring_begin(rq, 6);
+	if (IS_ERR(cs))
+		return PTR_ERR(cs);
+
+	*cs++ = MI_LOAD_REGISTER_IMM(1);
+	*cs++ = i915_mmio_reg_offset(RING_PP_DIR_DCLV(engine));
+	*cs++ = PP_DIR_DCLV_2G;
+
+	*cs++ = MI_LOAD_REGISTER_IMM(1);
+	*cs++ = i915_mmio_reg_offset(RING_PP_DIR_BASE(engine));
+	*cs++ = ppgtt->pd.base.ggtt_offset << 10;
+
+	intel_ring_advance(rq, cs);
+
+	return 0;
+}
+
 static inline int mi_set_context(struct i915_request *rq, u32 flags)
 {
 	struct drm_i915_private *i915 = rq->i915;
···
 static int switch_context(struct i915_request *rq)
 {
 	struct intel_engine_cs *engine = rq->engine;
-	struct i915_gem_context *to_ctx = rq->gem_context;
-	struct i915_hw_ppgtt *to_mm =
-		to_ctx->ppgtt ?: rq->i915->mm.aliasing_ppgtt;
-	struct i915_gem_context *from_ctx = engine->legacy_active_context;
-	struct i915_hw_ppgtt *from_mm = engine->legacy_active_ppgtt;
+	struct i915_gem_context *ctx = rq->gem_context;
+	struct i915_hw_ppgtt *ppgtt = ctx->ppgtt ?: rq->i915->mm.aliasing_ppgtt;
+	unsigned int unwind_mm = 0;
 	u32 hw_flags = 0;
 	int ret, i;
 
 	lockdep_assert_held(&rq->i915->drm.struct_mutex);
 	GEM_BUG_ON(HAS_EXECLISTS(rq->i915));
 
-	if (to_mm != from_mm ||
-	    (to_mm && intel_engine_flag(engine) & to_mm->pd_dirty_rings)) {
-		trace_switch_mm(engine, to_ctx);
-		ret = to_mm->switch_mm(to_mm, rq);
+	if (ppgtt) {
+		ret = load_pd_dir(rq, ppgtt);
 		if (ret)
 			goto err;
 
-		to_mm->pd_dirty_rings &= ~intel_engine_flag(engine);
-		engine->legacy_active_ppgtt = to_mm;
-		hw_flags = MI_FORCE_RESTORE;
+		if (intel_engine_flag(engine) & ppgtt->pd_dirty_rings) {
+			unwind_mm = intel_engine_flag(engine);
+			ppgtt->pd_dirty_rings &= ~unwind_mm;
+			hw_flags = MI_FORCE_RESTORE;
+		}
 	}
 
-	if (rq->hw_context->state &&
-	    (to_ctx != from_ctx || hw_flags & MI_FORCE_RESTORE)) {
+	if (rq->hw_context->state) {
 		GEM_BUG_ON(engine->id != RCS);
 
 		/*
···
 		 * as nothing actually executes using the kernel context; it
 		 * is purely used for flushing user contexts.
 		 */
-		if (i915_gem_context_is_kernel(to_ctx))
+		if (i915_gem_context_is_kernel(ctx))
 			hw_flags = MI_RESTORE_INHIBIT;
 
 		ret = mi_set_context(rq, hw_flags);
 		if (ret)
 			goto err_mm;
-
-		engine->legacy_active_context = to_ctx;
 	}
 
-	if (to_ctx->remap_slice) {
+	if (ctx->remap_slice) {
 		for (i = 0; i < MAX_L3_SLICES; i++) {
-			if (!(to_ctx->remap_slice & BIT(i)))
+			if (!(ctx->remap_slice & BIT(i)))
 				continue;
 
 			ret = remap_l3(rq, i);
 			if (ret)
-				goto err_ctx;
+				goto err_mm;
 		}
 
-		to_ctx->remap_slice = 0;
+		ctx->remap_slice = 0;
 	}
 
 	return 0;
 
-err_ctx:
-	engine->legacy_active_context = from_ctx;
 err_mm:
-	engine->legacy_active_ppgtt = from_mm;
+	if (unwind_mm)
+		ppgtt->pd_dirty_rings |= unwind_mm;
 err:
 	return ret;
 }

drivers/gpu/drm/i915/intel_ringbuffer.h

···
 	 */
 	struct intel_context *last_retired_context;
 
-	/* We track the current MI_SET_CONTEXT in order to eliminate
-	 * redudant context switches. This presumes that requests are not
-	 * reordered! Or when they are the tracking is updated along with
-	 * the emission of individual requests into the legacy command
-	 * stream (ring).
-	 */
-	struct i915_gem_context *legacy_active_context;
-	struct i915_hw_ppgtt *legacy_active_ppgtt;
-
 	/* status_notifier: list of callbacks for context-switch changes */
 	struct atomic_notifier_head context_status_notifier;