drm/i915/guc: don't spinwait if the GuC's workqueue is full

Rather than wait to see whether more space becomes available in the GuC
submission workqueue, we can just return -EAGAIN and let the caller try
again in a little while. This gets rid of an uninterruptable sleep in
the polling code :)

We'll also add a counter to the GuC client statistics, to see how often
we find the WQ full.

v2:
    Flag the likely() code path (Tvtrko Ursulin).

v4:
    Add/update comments about failure counters (Tvtrko Ursulin).
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2515,6 +2515,7 @@ static void i915_guc_client_info(struct seq_file *m,
 	seq_printf(m, "\tWQ size %d, offset: 0x%x, tail %d\n",
 		client->wq_size, client->wq_offset, client->wq_tail);
 
+	seq_printf(m, "\tWork queue full: %u\n", client->no_wq_space);
 	seq_printf(m, "\tFailed to queue: %u\n", client->q_fail);
 	seq_printf(m, "\tFailed doorbell: %u\n", client->b_fail);
 	seq_printf(m, "\tLast submission result: %d\n", client->retcode);

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -468,26 +468,22 @@ static void guc_fini_ctx_desc(struct intel_guc *guc,
  */
 int i915_guc_wq_check_space(struct drm_i915_gem_request *request)
 {
-	const size_t size = sizeof(struct guc_wq_item);
+	const size_t wqi_size = sizeof(struct guc_wq_item);
 	struct i915_guc_client *gc = request->i915->guc.execbuf_client;
 	struct guc_process_desc *desc;
-	int ret = -ETIMEDOUT, timeout_counter = 200;
+	u32 freespace;
 
 	GEM_BUG_ON(gc == NULL);
 
 	desc = gc->client_base + gc->proc_desc_offset;
 
-	while (timeout_counter-- > 0) {
-		if (CIRC_SPACE(gc->wq_tail, desc->head, gc->wq_size) >= size) {
-			ret = 0;
-			break;
-		}
+	freespace = CIRC_SPACE(gc->wq_tail, desc->head, gc->wq_size);
+	if (likely(freespace >= wqi_size))
+		return 0;
 
-		if (timeout_counter)
-			usleep_range(1000, 2000);
-	};
+	gc->no_wq_space += 1;
 
-	return ret;
+	return -EAGAIN;
 }
 
 static int guc_add_workqueue_item(struct i915_guc_client *gc,

drivers/gpu/drm/i915/intel_guc.h

@@ -48,9 +48,18 @@ struct drm_i915_gem_request;
  * queue (a circular array of work items), again described in the process
  * descriptor. Work queue pages are mapped momentarily as required.
  *
- * Finally, we also keep a few statistics here, including the number of
- * submissions to each engine, and a record of the last submission failure
- * (if any).
+ * We also keep a few statistics on failures. Ideally, these should all
+ * be zero!
+ *   no_wq_space: times that the submission pre-check found no space was
+ *                available in the work queue (note, the queue is shared,
+ *                not per-engine). It is OK for this to be nonzero, but
+ *                it should not be huge!
+ *   q_fail: failed to enqueue a work item. This should never happen,
+ *           because we check for space beforehand.
+ *   b_fail: failed to ring the doorbell. This should never happen, unless
+ *           somehow the hardware misbehaves, or maybe if the GuC firmware
+ *           crashes? We probably need to reset the GPU to recover.
+ *   retcode: errno from last guc_submit()
  */
 struct i915_guc_client {
 	struct drm_i915_gem_object *client_obj;
@@ -71,12 +80,13 @@ struct i915_guc_client {
 	uint32_t wq_tail;
 	uint32_t unused;		/* Was 'wq_head'		*/
 
-	/* GuC submission statistics & status */
-	uint64_t submissions[GUC_MAX_ENGINES_NUM];
+	uint32_t no_wq_space;
 	uint32_t q_fail;
 	uint32_t b_fail;
 	int retcode;
-	int spare;			/* pad to 32 DWords		*/
+
+	/* Per-engine counts of GuC submissions */
+	uint64_t submissions[GUC_MAX_ENGINES_NUM];
 };
 
 enum intel_guc_fw_status {