drm/i915/execlists: Preempt-to-busy

When using a global seqno, we required a precise stop-the-workd event to
handle preemption and unwind the global seqno counter. To accomplish
this, we would preempt to a special out-of-band context and wait for the
machine to report that it was idle. Given an idle machine, we could very
precisely see which requests had completed and which we needed to feed
back into the run queue.

However, now that we have scrapped the global seqno, we no longer need
to precisely unwind the global counter and only track requests by their
per-context seqno. This allows us to loosely unwind inflight requests
while scheduling a preemption, with the enormous caveat that the
requests we put back on the run queue are still _inflight_ (until the
preemption request is complete). This makes request tracking much more
messy, as at any point then we can see a completed request that we
believe is not currently scheduled for execution. We also have to be
careful not to rewind RING_TAIL past RING_HEAD on preempting to the
running context, and for this we use a semaphore to prevent completion
of the request before continuing.

To accomplish this feat, we change how we track requests scheduled to
the HW. Instead of appending our requests onto a single list as we
submit, we track each submission to ELSP as its own block. Then upon
receiving the CS preemption event, we promote the pending block to the
inflight block (discarding what was previously being tracked). As normal
CS completion events arrive, we then remove stale entries from the
inflight tracker.

v2: Be a tinge paranoid and ensure we flush the write into the HWS page
for the GPU semaphore to pick in a timely fashion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620142052.19311-1-chris@chris-wilson.co.uk

1 files changed, 7 insertions, 12 deletions

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index b7e9fddef270..a497cf7acb6a 100644
--- a/drivers/gpu/drm/i915/i915_gpu_error.c
+++ b/drivers/gpu/drm/i915/i915_gpu_error.c
@@ -1248,10 +1248,10 @@ static void error_record_engine_registers(struct i915_gpu_state *error,
 	}
 }
 
-static void record_request(struct i915_request *request,
+static void record_request(const struct i915_request *request,
 			   struct drm_i915_error_request *erq)
 {
-	struct i915_gem_context *ctx = request->gem_context;
+	const struct i915_gem_context *ctx = request->gem_context;
 
 	erq->flags = request->fence.flags;
 	erq->context = request->fence.context;
@@ -1315,20 +1315,15 @@ static void engine_record_requests(struct intel_engine_cs *engine,
 	ee->num_requests = count;
 }
 
-static void error_record_engine_execlists(struct intel_engine_cs *engine,
+static void error_record_engine_execlists(const struct intel_engine_cs *engine,
 					  struct drm_i915_error_engine *ee)
 {
 	const struct intel_engine_execlists * const execlists = &engine->execlists;
-	unsigned int n;
+	struct i915_request * const *port = execlists->active;
+	unsigned int n = 0;
 
-	for (n = 0; n < execlists_num_ports(execlists); n++) {
-		struct i915_request *rq = port_request(&execlists->port[n]);
-
-		if (!rq)
-			break;
-
-		record_request(rq, &ee->execlist[n]);
-	}
+	while (*port)
+		record_request(*port++, &ee->execlist[n++]);
 
 	ee->num_ports = n;
 }