drm/i915: Trim i915_guc_info() stack usage

i915_guc_info() (part of debugfs output) tries to avoid holding
struct_mutex for a long period by copying onto the stack. This causes a
warning that the stack frame is massive, so stop doing that. We can even
forgo holding the struct_mutex here as that doesn't serialise the values
being read (and the lists used exist for the device lifetime).

v2: Skip printing anything if guc->execbuf_client is disabled (avoids
potential NULL dereference).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129121024.22650-2-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2434,47 +2434,41 @@ static void i915_guc_client_info(struct seq_file *m,
 static int i915_guc_info(struct seq_file *m, void *data)
 {
 	struct drm_i915_private *dev_priv = node_to_i915(m->private);
-	struct drm_device *dev = &dev_priv->drm;
-	struct intel_guc guc;
-	struct i915_guc_client client = {};
+	const struct intel_guc *guc = &dev_priv->guc;
 	struct intel_engine_cs *engine;
 	enum intel_engine_id id;
-	u64 total = 0;
-
-	if (!HAS_GUC_SCHED(dev_priv))
-		return 0;
+	u64 total;
 
-	if (mutex_lock_interruptible(&dev->struct_mutex))
+	if (!guc->execbuf_client) {
+		seq_printf(m, "GuC submission %s\n",
+			   HAS_GUC_SCHED(dev_priv) ?
+			   "disabled" :
+			   "not supported");
 		return 0;
-
-	/* Take a local copy of the GuC data, so we can dump it at leisure */
-	guc = dev_priv->guc;
-	if (guc.execbuf_client)
-		client = *guc.execbuf_client;
-
-	mutex_unlock(&dev->struct_mutex);
+	}
 
 	seq_printf(m, "Doorbell map:\n");
-	seq_printf(m, "\t%*pb\n", GUC_MAX_DOORBELLS, guc.doorbell_bitmap);
-	seq_printf(m, "Doorbell next cacheline: 0x%x\n\n", guc.db_cacheline);
+	seq_printf(m, "\t%*pb\n", GUC_MAX_DOORBELLS, guc->doorbell_bitmap);
+	seq_printf(m, "Doorbell next cacheline: 0x%x\n\n", guc->db_cacheline);
 
-	seq_printf(m, "GuC total action count: %llu\n", guc.action_count);
-	seq_printf(m, "GuC action failure count: %u\n", guc.action_fail);
-	seq_printf(m, "GuC last action command: 0x%x\n", guc.action_cmd);
-	seq_printf(m, "GuC last action status: 0x%x\n", guc.action_status);
-	seq_printf(m, "GuC last action error code: %d\n", guc.action_err);
+	seq_printf(m, "GuC total action count: %llu\n", guc->action_count);
+	seq_printf(m, "GuC action failure count: %u\n", guc->action_fail);
+	seq_printf(m, "GuC last action command: 0x%x\n", guc->action_cmd);
+	seq_printf(m, "GuC last action status: 0x%x\n", guc->action_status);
+	seq_printf(m, "GuC last action error code: %d\n", guc->action_err);
 
+	total = 0;
 	seq_printf(m, "\nGuC submissions:\n");
 	for_each_engine(engine, dev_priv, id) {
-		u64 submissions = guc.submissions[id];
+		u64 submissions = guc->submissions[id];
 		total += submissions;
 		seq_printf(m, "\t%-24s: %10llu, last seqno 0x%08x\n",
-			engine->name, submissions, guc.last_seqno[id]);
+			engine->name, submissions, guc->last_seqno[id]);
 	}
 	seq_printf(m, "\t%s: %llu\n", "Total", total);
 
-	seq_printf(m, "\nGuC execbuf client @ %p:\n", guc.execbuf_client);
-	i915_guc_client_info(m, dev_priv, &client);
+	seq_printf(m, "\nGuC execbuf client @ %p:\n", guc->execbuf_client);
+	i915_guc_client_info(m, dev_priv, guc->execbuf_client);
 
 	i915_guc_log_info(m, dev_priv);