drm/i915: OR in the COMMAND read domain for the batch buffer.

The batch buffer may be shared with another read buffer, so we should not
ignore any previously set domains, but just or in the command domain (and
check that the buffer is not writable).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -3369,8 +3369,12 @@ i915_gem_execbuffer(struct drm_device *dev, void *data,
 
 	/* Set the pending read domains for the batch buffer to COMMAND */
 	batch_obj = object_list[args->buffer_count-1];
-	batch_obj->pending_read_domains = I915_GEM_DOMAIN_COMMAND;
-	batch_obj->pending_write_domain = 0;
+	if (batch_obj->pending_write_domain) {
+		DRM_ERROR("Attempting to use self-modifying batch buffer\n");
+		ret = -EINVAL;
+		goto err;
+	}
+	batch_obj->pending_read_domains |= I915_GEM_DOMAIN_COMMAND;
 
 	/* Sanity check the batch buffer, prior to moving objects */
 	exec_offset = exec_list[args->buffer_count - 1].offset;