drm/i915: Skip over MI_NOOP when parsing

Though less likely in practice, igt uses MI_NOOP frequently to pad out
its batch buffers. The lookup and valiation of so many MI_NOOP command
descriptions is noticeable, though the side-effect of poisoning the
last-validated-command cache is more likely to impact upon real CS.

Testcase: igt/gen9_exec_parse/bb-large
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201001102632.18789-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_cmd_parser.c

@@ -1452,12 +1452,10 @@ int intel_engine_cmd_parser(struct intel_engine_cs *engine,
 	 * space. Parsing should be faster in some cases this way.
 	 */
 	batch_end = cmd + batch_length / sizeof(*batch_end);
-	do {
-		u32 length;
-
-		if (*cmd == MI_BATCH_BUFFER_END)
-			break;
+	while (*cmd != MI_BATCH_BUFFER_END) {
+		u32 length = 1;
 
+		if (*cmd != MI_NOOP) { /* MI_NOOP == 0 */
 			desc = find_cmd(engine, *cmd, desc, &default_desc);
 			if (!desc) {
 				DRM_DEBUG("CMD: Unrecognized command: 0x%08X\n", *cmd);
@@ -1490,6 +1488,7 @@ int intel_engine_cmd_parser(struct intel_engine_cs *engine,
 						    jump_whitelist);
 				break;
 			}
+		}
 
 		if (!IS_ERR_OR_NULL(jump_whitelist))
 			__set_bit(offset, jump_whitelist);
@@ -1501,7 +1500,7 @@ int intel_engine_cmd_parser(struct intel_engine_cs *engine,
 			ret = -EINVAL;
 			break;
 		}
-	} while (1);
+	}
 
 	if (trampoline) {
 		/*