drm/i915: Stop tracking timeline->inflight_seqnos

In commit 9b6586ae9f6b ("drm/i915: Keep a global seqno per-engine"), we
moved from a global inflight counter to per-engine counters in the
hope that will be easy to run concurrently in future. However, with the
advent of the desire to move requests between engines, we do need a
global counter to preserve the semantics that no engine wraps in the
middle of a submit. (Although this semantic is now only required for gen7
semaphore support, which only supports greater-then comparisons!)

v2: Keep a global counter of all requests ever submitted and force the
reset when it wraps.

References: 9b6586ae9f6b ("drm/i915: Keep a global seqno per-engine")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430131503.5375-1-chris@chris-wilson.co.uk

1 files changed, 17 insertions, 16 deletions

diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
index b692a9f7c357..b1993d4a1a53 100644
--- a/drivers/gpu/drm/i915/i915_request.c
+++ b/drivers/gpu/drm/i915/i915_request.c
@@ -241,6 +241,7 @@ static int reset_all_global_seqno(struct drm_i915_private *i915, u32 seqno)
                                sizeof(timeline->engine[id].global_sync));
         }
 
+        i915->gt.request_serial = seqno;
         return 0;
 }
 
@@ -257,18 +258,22 @@ int i915_gem_set_global_seqno(struct drm_device *dev, u32 seqno)
         return reset_all_global_seqno(i915, seqno - 1);
 }
 
-static int reserve_engine(struct intel_engine_cs *engine)
+static int reserve_gt(struct drm_i915_private *i915)
 {
-        struct drm_i915_private *i915 = engine->i915;
-        u32 active = ++engine->timeline->inflight_seqnos;
-        u32 seqno = engine->timeline->seqno;
         int ret;
 
-        /* Reservation is fine until we need to wrap around */
-        if (unlikely(add_overflows(seqno, active))) {
+        /*
+         * Reservation is fine until we may need to wrap around
+         *
+         * By incrementing the serial for every request, we know that no
+         * individual engine may exceed that serial (as each is reset to 0
+         * on any wrap). This protects even the most pessimistic of migrations
+         * of every request from all engines onto just one.
+         */
+        while (unlikely(++i915->gt.request_serial == 0)) {
                 ret = reset_all_global_seqno(i915, 0);
                 if (ret) {
-                        engine->timeline->inflight_seqnos--;
+                        i915->gt.request_serial--;
                         return ret;
                 }
         }
@@ -279,15 +284,10 @@ static int reserve_engine(struct intel_engine_cs *engine)
         return 0;
 }
 
-static void unreserve_engine(struct intel_engine_cs *engine)
+static void unreserve_gt(struct drm_i915_private *i915)
 {
-        struct drm_i915_private *i915 = engine->i915;
-
         if (!--i915->gt.active_requests)
                 i915_gem_park(i915);
-
-        GEM_BUG_ON(!engine->timeline->inflight_seqnos);
-        engine->timeline->inflight_seqnos--;
 }
 
 void i915_gem_retire_noop(struct i915_gem_active *active,
@@ -362,7 +362,6 @@ static void i915_request_retire(struct i915_request *request)
         list_del_init(&request->link);
         spin_unlock_irq(&engine->timeline->lock);
 
-        unreserve_engine(request->engine);
         advance_ring(request);
 
         free_capture_list(request);
@@ -424,6 +423,8 @@ static void i915_request_retire(struct i915_request *request)
         }
         spin_unlock_irq(&request->lock);
 
+        unreserve_gt(request->i915);
+
         i915_sched_node_fini(request->i915, &request->sched);
         i915_request_put(request);
 }
@@ -642,7 +643,7 @@ i915_request_alloc(struct intel_engine_cs *engine, struct i915_gem_context *ctx)
                 return ERR_CAST(ring);
         GEM_BUG_ON(!ring);
 
-        ret = reserve_engine(engine);
+        ret = reserve_gt(i915);
         if (ret)
                 goto err_unpin;
 
@@ -784,7 +785,7 @@ err_unwind:
 
         kmem_cache_free(i915->requests, rq);
 err_unreserve:
-        unreserve_engine(engine);
+        unreserve_gt(i915);
 err_unpin:
         engine->context_unpin(engine, ctx);
         return ERR_PTR(ret);