From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5DBEFC433EF for ; Wed, 10 Nov 2021 18:45:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 413FA61241 for ; Wed, 10 Nov 2021 18:45:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233128AbhKJSsC (ORCPT ); Wed, 10 Nov 2021 13:48:02 -0500 Received: from mail.kernel.org ([198.145.29.99]:46890 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232830AbhKJSr2 (ORCPT ); Wed, 10 Nov 2021 13:47:28 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 342DB61247; Wed, 10 Nov 2021 18:44:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1636569880; bh=1Gqceegm+1GqYddfAlC4grL9S7yABjfoSrKw2KQ4diI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=q6e/kBAf5PvU9fWBABL1uj35eiAi4QT2NeUYYQ2qIigkOE2XwxlEp3iQSP/o4Z1yN wYJv6OdfzHYSRaZldG6VI7hhCebnUzYwweT+DIvZKQBF8Eq5xspH0fleoMrNe2x0mJ voBSxt7Ma99/0w4PaS0m8W9denhkG794ZGYdPtHA= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Sergey Senozhatsky , Guenter Roeck , Petr Mladek , Yi Fan Subject: [PATCH 4.9 13/22] printk/console: Allow to disable console output by using console="" or console=null Date: Wed, 10 Nov 2021 19:43:20 +0100 Message-Id: <20211110182002.007984115@linuxfoundation.org> X-Mailer: git-send-email 2.33.1 In-Reply-To: <20211110182001.579561273@linuxfoundation.org> References: <20211110182001.579561273@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Petr Mladek commit 3cffa06aeef7ece30f6b5ac0ea51f264e8fea4d0 upstream. The commit 48021f98130880dd74 ("printk: handle blank console arguments passed in.") prevented crash caused by empty console= parameter value. Unfortunately, this value is widely used on Chromebooks to disable the console output. The above commit caused performance regression because the messages were pushed on slow console even though nobody was watching it. Use ttynull driver explicitly for console="" and console=null parameters. It has been created for exactly this purpose. It causes that preferred_console is set. As a result, ttySX and ttyX are not used as a fallback. And only ttynull console gets registered by default. It still allows to register other consoles either by additional console= parameters or SPCR. It prevents regression because it worked this way even before. Also it is a sane semantic. Preventing output on all consoles should be done another way, for example, by introducing mute_console parameter. Link: https://lore.kernel.org/r/20201006025935.GA597@jagdpanzerIV.localdomain Suggested-by: Sergey Senozhatsky Reviewed-by: Guenter Roeck Tested-by: Guenter Roeck Acked-by: Sergey Senozhatsky Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20201111135450.11214-3-pmladek@suse.com Cc: Yi Fan Signed-off-by: Greg Kroah-Hartman --- kernel/printk/printk.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2035,8 +2035,15 @@ static int __init console_setup(char *st char *s, *options, *brl_options = NULL; int idx; - if (str[0] == 0) + /* + * console="" or console=null have been suggested as a way to + * disable console output. Use ttynull that has been created + * for exacly this purpose. + */ + if (str[0] == 0 || strcmp(str, "null") == 0) { + __add_preferred_console("ttynull", 0, NULL, NULL); return 1; + } if (_braille_console_setup(&str, &brl_options)) return 1;