From patchwork Wed Dec 9 09:17:28 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Steffen Trumtrar X-Patchwork-Id: 340740 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, URIBL_BLOCKED, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 90108C2BB40 for ; Wed, 9 Dec 2020 09:18:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5BCC7238EB for ; Wed, 9 Dec 2020 09:18:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728843AbgLIJS3 (ORCPT ); Wed, 9 Dec 2020 04:18:29 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44782 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727348AbgLIJS3 (ORCPT ); Wed, 9 Dec 2020 04:18:29 -0500 Received: from metis.ext.pengutronix.de (metis.ext.pengutronix.de [IPv6:2001:67c:670:201:290:27ff:fe1d:cc33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 53944C0617B0 for ; Wed, 9 Dec 2020 01:17:46 -0800 (PST) Received: from dude02.hi.pengutronix.de ([2001:67c:670:100:1d::28]) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1kmvbT-0005wj-Gg; Wed, 09 Dec 2020 10:17:43 +0100 Received: from str by dude02.hi.pengutronix.de with local (Exim 4.92) (envelope-from ) id 1kmvbT-0000cs-6Q; Wed, 09 Dec 2020 10:17:43 +0100 From: Steffen Trumtrar To: Greg Kroah-Hartman Cc: Jiri Slaby , linux-serial@vger.kernel.org Subject: [PATCH 2/2] tty: serial: 8250: wait till transmitter is empty Date: Wed, 9 Dec 2020 10:17:28 +0100 Message-Id: <20201209091728.2357-2-s.trumtrar@pengutronix.de> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20201209091728.2357-1-s.trumtrar@pengutronix.de> References: <20201209091728.2357-1-s.trumtrar@pengutronix.de> MIME-Version: 1.0 X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::28 X-SA-Exim-Mail-From: str@pengutronix.de X-SA-Exim-Scanned: No (on metis.ext.pengutronix.de); SAEximRunCond expanded to false X-PTX-Original-Recipient: linux-serial@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org When only one single character is sent and RS485 signaling is used, the driver runs into timing issues. When serial8250_tx_chars is called the single character is transmitted. The check on uart_circ_empty will be positive and __stop_tx is called. The check on UART_LSR_TEMT in BOTH_EMPTY will then be negativ and the function will return. On the next call to serial8250_tx_chars uart_circ_empty will still be true but the check on BOTH_EMPTY in __stop_tx might still fail. This leads to a deadlock. Use readx_poll_timeout_atomic to allow the shift register to be emptied before checking on BOTH_EMPTY. The timeout value is copied from 8250_dw.c. Signed-off-by: Steffen Trumtrar --- drivers/tty/serial/8250/8250_port.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 3310c2b70138..87daf3758ff0 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -18,6 +18,7 @@ #include #include #include +#include #include #include #include @@ -1519,18 +1520,27 @@ static inline void __do_stop_tx(struct uart_8250_port *p) serial8250_rpm_put_tx(p); } +static unsigned char serial8250_read_lsr(struct uart_8250_port *p) +{ + return serial_in(p, UART_LSR); +} + static inline void __stop_tx(struct uart_8250_port *p) { struct uart_8250_em485 *em485 = p->em485; if (em485) { - unsigned char lsr = serial_in(p, UART_LSR); + unsigned char lsr; + /* * To provide required timeing and allow FIFO transfer, * __stop_tx_rs485() must be called only when both FIFO and * shift register are empty. It is for device driver to enable * interrupt on TEMT. */ + readx_poll_timeout_atomic(serial8250_read_lsr, p, lsr, + lsr & UART_LSR_TEMT, 1, 20000); + if ((lsr & BOTH_EMPTY) != BOTH_EMPTY) return;