int usb_add_function(struct usb_configuration *config,
		struct usb_function *function)
{
	int	value = -EINVAL;

	DBG(config->cdev, "adding '%s'/%p to config '%s'/%p\n",
			function->name, function,
			config->label, config);

	if (!function->set_alt || !function->disable)
		goto done;

	function->config = config;
	list_add_tail(&function->list, &config->functions);

	if (function->bind_deactivated) {
		value = usb_function_deactivate(function);
		if (value)
			goto done;
	}

	/* REVISIT *require* function->bind? */
	if (function->bind) {
		value = function->bind(config, function);
		if (value < 0) {
			list_del(&function->list);
			function->config = NULL;
		}
	} else
		value = 0;

	/* We allow configurations that don't work at both speeds.
	 * If we run into a lowspeed Linux system, treat it the same
	 * as full speed ... it's the function drivers that will need
	 * to avoid bulk and ISO transfers.
	 */
	if (!config->fullspeed && function->fs_descriptors)
		config->fullspeed = true;
	if (!config->highspeed && function->hs_descriptors)
		config->highspeed = true;
	if (!config->superspeed && function->ss_descriptors)
		config->superspeed = true;
	if (!config->superspeed_plus && function->ssp_descriptors)
		config->superspeed_plus = true;

done:
	if (value)
		DBG(config->cdev, "adding '%s'/%p --> %d\n",
				function->name, function, value);
	return value;
}