[PATCH] cyclic-ids

[Chris Wilson]

---
 drivers/gpu/drm/i915/gt/intel_engine_types.h |  1 +
 drivers/gpu/drm/i915/gt/intel_lrc.c          | 15 ++++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index c400aaa2287b..bfa0199b7a2c 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -330,6 +330,7 @@ struct intel_engine_cs {
 	atomic_t fw_active;
 
 	unsigned long context_tag;
+	unsigned long context_last;
 
 	struct rb_node uabi_node;
 
diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 417f6b0c6c61..19f695f494dd 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -1335,6 +1335,19 @@ static void intel_context_update_runtime(struct intel_context *ce)
 	ce->runtime.total += dt;
 }
 
+static unsigned int next_cyclic_tag(struct intel_engine_cs *engine)
+{
+	unsigned long tag, mask;
+
+	/* Cyclically allocate unused ids */
+	mask = ~0ul << engine->context_last;
+	tag = READ_ONCE(engine->context_tag);
+	tag = ffs((tag & mask) ?: tag);
+	engine->context_last = tag;
+
+	return tag;
+}
+
 static inline struct intel_engine_cs *
 __execlists_schedule_in(struct i915_request *rq)
 {
@@ -1355,7 +1368,7 @@ __execlists_schedule_in(struct i915_request *rq)
 		ce->lrc.ccid = ce->tag;
 	} else {
 		/* We don't need a strict matching tag, just different values */
-		unsigned int tag = ffs(READ_ONCE(engine->context_tag));
+		unsigned int tag = next_cyclic_tag(engine);
 
 		GEM_BUG_ON(tag == 0 || tag >= BITS_PER_LONG);
 		clear_bit(tag - 1, &engine->context_tag);
-- 
2.20.1