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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E6897C433EF for ; Mon, 24 Jan 2022 20:10:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1378820AbiAXUJy (ORCPT ); Mon, 24 Jan 2022 15:09:54 -0500 Received: from dfw.source.kernel.org ([139.178.84.217]:60156 "EHLO dfw.source.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1376720AbiAXUDo (ORCPT ); Mon, 24 Jan 2022 15:03:44 -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 dfw.source.kernel.org (Postfix) with ESMTPS id 2ECAD6091B; Mon, 24 Jan 2022 20:03:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id F3147C340E5; Mon, 24 Jan 2022 20:03:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1643054622; bh=uAlWWnbqersf2Iz6eORks7ljC27rMrLewZjq8ZOs8Hw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fYjUY/4Fn3f2fKeJXF7YP/152QdTkZe1swQ95VtEaHe16H2cCIq7K076GQWb5jDcJ n8oZLHGkKrDeP4la8FL2y1teFZpCyuHuxvs9LXu5Jq/LCrtLBJfi8+sKbsN4RMlXw4 y7xXb9UYlWD8v9NfybSnKfzFGJMafVMIIhK2BM9E= 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 5.10 449/563] serial: Fix incorrect rs485 polarity on uart open Date: Mon, 24 Jan 2022 19:43:34 +0100 Message-Id: <20220124184039.984110724@linuxfoundation.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220124184024.407936072@linuxfoundation.org> References: <20220124184024.407936072@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: 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 @@ -162,7 +162,7 @@ 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 { @@ -171,7 +171,7 @@ 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); } }