drm/i915/guc: Fill preempt context once at init time

Since we're inhibiting context save of preempt context, we're no longer
tracking the position of HEAD/TAIL. With GuC, we're adding a new
breadcrumb for each preemption, which means that the HW will do more and
more breadcrumb writes. Eventually the ring is filled, and we're
submitting the preemption context with HEAD==TAIL==0, which won't result
in breadcrumb write, but will trigger hangcheck instead.
Instead of writing a new preempt breadcrumb for each preemption, let's
just fill the ring once at init time (which also saves a couple of
instructions in the tasklet).

v2: Assert that context save restore is inhibited, don't assert on ring
    alignment. (Chris)
v3: Cleanup checkpatch.

Fixes: 517aaffe0c1b ("drm/i915/execlists: Inhibit context save/restore for the fake preempt context")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180226163800.21745-1-michal.winiarski@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -26,8 +26,14 @@
 #include <trace/events/dma_fence.h>
 
 #include "intel_guc_submission.h"
+#include "intel_lrc_reg.h"
 #include "i915_drv.h"
 
+#define GUC_PREEMPT_FINISHED		0x1
+#define GUC_PREEMPT_BREADCRUMB_DWORDS	0x8
+#define GUC_PREEMPT_BREADCRUMB_BYTES	\
+	(sizeof(u32) * GUC_PREEMPT_BREADCRUMB_DWORDS)
+
 /**
  * DOC: GuC-based command submission
  *
@@ -535,8 +541,6 @@ static void flush_ggtt_writes(struct i915_vma *vma)
 		POSTING_READ_FW(GUC_STATUS);
 }
 
-#define GUC_PREEMPT_FINISHED 0x1
-#define GUC_PREEMPT_BREADCRUMB_DWORDS 0x8
 static void inject_preempt_context(struct work_struct *work)
 {
 	struct guc_preempt_work *preempt_work =
@@ -546,37 +550,17 @@ static void inject_preempt_context(struct work_struct *work)
 					     preempt_work[engine->id]);
 	struct intel_guc_client *client = guc->preempt_client;
 	struct guc_stage_desc *stage_desc = __get_stage_desc(client);
-	struct intel_ring *ring = client->owner->engine[engine->id].ring;
 	u32 ctx_desc = lower_32_bits(intel_lr_context_descriptor(client->owner,
 								 engine));
-	u32 *cs = ring->vaddr + ring->tail;
 	u32 data[7];
 
-	if (engine->id == RCS) {
-		cs = gen8_emit_ggtt_write_rcs(cs, GUC_PREEMPT_FINISHED,
-				intel_hws_preempt_done_address(engine));
-	} else {
-		cs = gen8_emit_ggtt_write(cs, GUC_PREEMPT_FINISHED,
-				intel_hws_preempt_done_address(engine));
-		*cs++ = MI_NOOP;
-		*cs++ = MI_NOOP;
-	}
-	*cs++ = MI_USER_INTERRUPT;
-	*cs++ = MI_NOOP;
-
-	GEM_BUG_ON(!IS_ALIGNED(ring->size,
-			       GUC_PREEMPT_BREADCRUMB_DWORDS * sizeof(u32)));
-	GEM_BUG_ON((void *)cs - (ring->vaddr + ring->tail) !=
-		   GUC_PREEMPT_BREADCRUMB_DWORDS * sizeof(u32));
-
-	ring->tail += GUC_PREEMPT_BREADCRUMB_DWORDS * sizeof(u32);
-	ring->tail &= (ring->size - 1);
-
-	flush_ggtt_writes(ring->vma);
-
+	/*
+	 * The ring contains commands to write GUC_PREEMPT_FINISHED into HWSP.
+	 * See guc_fill_preempt_context().
+	 */
 	spin_lock_irq(&client->wq_lock);
 	guc_wq_item_append(client, engine->guc_id, ctx_desc,
-			   ring->tail / sizeof(u64), 0);
+			   GUC_PREEMPT_BREADCRUMB_BYTES / sizeof(u64), 0);
 	spin_unlock_irq(&client->wq_lock);
 
 	/*
@@ -972,6 +956,62 @@ static void guc_client_free(struct intel_guc_client *client)
 	kfree(client);
 }
 
+static inline bool ctx_save_restore_disabled(struct intel_context *ce)
+{
+	u32 sr = ce->lrc_reg_state[CTX_CONTEXT_CONTROL + 1];
+
+#define SR_DISABLED \
+	_MASKED_BIT_ENABLE(CTX_CTRL_ENGINE_CTX_RESTORE_INHIBIT | \
+			   CTX_CTRL_ENGINE_CTX_SAVE_INHIBIT)
+
+	return (sr & SR_DISABLED) == SR_DISABLED;
+
+#undef SR_DISABLED
+}
+
+static void guc_fill_preempt_context(struct intel_guc *guc)
+{
+	struct drm_i915_private *dev_priv = guc_to_i915(guc);
+	struct intel_guc_client *client = guc->preempt_client;
+	struct intel_engine_cs *engine;
+	enum intel_engine_id id;
+
+	for_each_engine(engine, dev_priv, id) {
+		struct intel_context *ce = &client->owner->engine[id];
+		u32 addr = intel_hws_preempt_done_address(engine);
+		u32 *cs;
+
+		GEM_BUG_ON(!ce->pin_count);
+
+		/*
+		 * We rely on this context image *not* being saved after
+		 * preemption. This ensures that the RING_HEAD / RING_TAIL
+		 * remain pointing at initial values forever.
+		 */
+		GEM_BUG_ON(!ctx_save_restore_disabled(ce));
+
+		cs = ce->ring->vaddr;
+		if (id == RCS) {
+			cs = gen8_emit_ggtt_write_rcs(cs,
+						      GUC_PREEMPT_FINISHED,
+						      addr);
+		} else {
+			cs = gen8_emit_ggtt_write(cs,
+						  GUC_PREEMPT_FINISHED,
+						  addr);
+			*cs++ = MI_NOOP;
+			*cs++ = MI_NOOP;
+		}
+		*cs++ = MI_USER_INTERRUPT;
+		*cs++ = MI_NOOP;
+
+		GEM_BUG_ON((void *)cs - ce->ring->vaddr !=
+			   GUC_PREEMPT_BREADCRUMB_BYTES);
+
+		flush_ggtt_writes(ce->ring->vma);
+	}
+}
+
 static int guc_clients_create(struct intel_guc *guc)
 {
 	struct drm_i915_private *dev_priv = guc_to_i915(guc);
@@ -1002,6 +1042,8 @@ static int guc_clients_create(struct intel_guc *guc)
 			return PTR_ERR(client);
 		}
 		guc->preempt_client = client;
+
+		guc_fill_preempt_context(guc);
 	}
 
 	return 0;