From patchwork Mon Jan 24 18:43:44 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg KH X-Patchwork-Id: 536284 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7FE58C433F5 for ; Mon, 24 Jan 2022 19:13:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239723AbiAXTNA (ORCPT ); Mon, 24 Jan 2022 14:13:00 -0500 Received: from ams.source.kernel.org ([145.40.68.75]:37678 "EHLO ams.source.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344191AbiAXTLl (ORCPT ); Mon, 24 Jan 2022 14:11:41 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 90C0EB81235; Mon, 24 Jan 2022 19:11:39 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 93B74C340E5; Mon, 24 Jan 2022 19:11:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1643051498; bh=0NWLm4MpjGOMUQ6L/3GRQzNj72Wo2Ip9yWEX2IoCPiA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=hZIEwKxNHzuHmtvQ8HYkCgMBArjs3WJw3upEg93d8Femu8eA5YG6WhvU/Ut3L5YNA /by5kzFrSgXbyF8clPNCzRmI46b9Iwr1Y5MkUkv2R4YN2m4XYFYr7WN6LV8AA3SGlQ 385rYS4F6Qv3UYNS2f8fKpf0aI8kdLs0bfFJKzDA= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Rafael Gago Castano , Jan Kiszka , Su Bao Cheng , Lukas Wunner Subject: [PATCH 4.14 149/186] serial: Fix incorrect rs485 polarity on uart open Date: Mon, 24 Jan 2022 19:43:44 +0100 Message-Id: <20220124183941.900319614@linuxfoundation.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220124183937.101330125@linuxfoundation.org> References: <20220124183937.101330125@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Lukas Wunner commit d3b3404df318504ec084213ab1065b73f49b0f1d upstream. Commit a6845e1e1b78 ("serial: core: Consider rs485 settings to drive RTS") sought to deassert RTS when opening an rs485-enabled uart port. That way, the transceiver does not occupy the bus until it transmits data. Unfortunately, the commit mixed up the logic and *asserted* RTS instead of *deasserting* it: The commit amended uart_port_dtr_rts(), which raises DTR and RTS when opening an rs232 port. "Raising" actually means lowering the signal that's coming out of the uart, because an rs232 transceiver not only changes a signal's voltage level, it also *inverts* the signal. See the simplified schematic in the MAX232 datasheet for an example: https://www.ti.com/lit/ds/symlink/max232.pdf So, to raise RTS on an rs232 port, TIOCM_RTS is *set* in port->mctrl and that results in the signal being driven low. In contrast to rs232, the signal level for rs485 Transmit Enable is the identity, not the inversion: If the transceiver expects a "high" RTS signal for Transmit Enable, the signal coming out of the uart must also be high, so TIOCM_RTS must be *cleared* in port->mctrl. The commit did the exact opposite, but it's easy to see why given the confusing semantics of rs232 and rs485. Fix it. Fixes: a6845e1e1b78 ("serial: core: Consider rs485 settings to drive RTS") Cc: stable@vger.kernel.org # v4.14+ Cc: Rafael Gago Castano Cc: Jan Kiszka Cc: Su Bao Cheng Signed-off-by: Lukas Wunner Link: https://lore.kernel.org/r/9395767847833f2f3193c49cde38501eeb3b5669.1639821059.git.lukas@wunner.de Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/serial_core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -172,7 +172,7 @@ static void uart_port_dtr_rts(struct uar int RTS_after_send = !!(uport->rs485.flags & SER_RS485_RTS_AFTER_SEND); if (raise) { - if (rs485_on && !RTS_after_send) { + if (rs485_on && RTS_after_send) { uart_set_mctrl(uport, TIOCM_DTR); uart_clear_mctrl(uport, TIOCM_RTS); } else { @@ -181,7 +181,7 @@ static void uart_port_dtr_rts(struct uar } else { unsigned int clear = TIOCM_DTR; - clear |= (!rs485_on || !RTS_after_send) ? TIOCM_RTS : 0; + clear |= (!rs485_on || RTS_after_send) ? TIOCM_RTS : 0; uart_clear_mctrl(uport, clear); } }