drm/i915/guc: Move check for fast memcpy_wc to relay creation

We only need those fast memcpy_wc when we're using relay to read
continuous GuC log. Let's prevent the user from creating a relay if we
know we won't be able to keep up with GuC.

v2: Adjust the return value (Michał)

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180319095348.9716-6-michal.winiarski@intel.com

drivers/gpu/drm/i915/intel_guc_log.c

@@ -466,16 +466,6 @@ int intel_guc_log_create(struct intel_guc_log *log)
 
 	GEM_BUG_ON(log->vma);
 
-	/*
-	 * We require SSE 4.1 for fast reads from the GuC log buffer and
-	 * it should be present on the chipsets supporting GuC based
-	 * submisssions.
-	 */
-	if (WARN_ON(!i915_has_memcpy_from_wc())) {
-		ret = -EINVAL;
-		goto err;
-	}
-
 	vma = intel_guc_allocate_vma(guc, GUC_LOG_SIZE);
 	if (IS_ERR(vma)) {
 		ret = PTR_ERR(vma);
@@ -574,6 +564,16 @@ int intel_guc_log_relay_open(struct intel_guc_log *log)
 		goto out_unlock;
 	}
 
+	/*
+	 * We require SSE 4.1 for fast reads from the GuC log buffer and
+	 * it should be present on the chipsets supporting GuC based
+	 * submisssions.
+	 */
+	if (!i915_has_memcpy_from_wc()) {
+		ret = -ENXIO;
+		goto out_unlock;
+	}
+
 	ret = guc_log_relay_create(log);
 	if (ret)
 		goto out_unlock;