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 X-Spam-Level: X-Spam-Status: No, score=-13.8 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 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 D9BA5C433E0 for ; Mon, 4 Jan 2021 11:48:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8FA32221F9 for ; Mon, 4 Jan 2021 11:48:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726234AbhADLsJ (ORCPT ); Mon, 4 Jan 2021 06:48:09 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34602 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726129AbhADLsJ (ORCPT ); Mon, 4 Jan 2021 06:48:09 -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 ADBDBC061574 for ; Mon, 4 Jan 2021 03:47:28 -0800 (PST) Received: from ptx.hi.pengutronix.de ([2001:67c:670:100:1d::c0] helo=leviathan.pengutronix.de) by metis.ext.pengutronix.de with esmtps (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1kwOKd-00005p-7M; Mon, 04 Jan 2021 12:47:27 +0100 References: <20201209091728.2357-1-s.trumtrar@pengutronix.de> <20201209091728.2357-2-s.trumtrar@pengutronix.de> User-agent: mu4e 1.4.12; emacs 28.0.50 From: Steffen Trumtrar To: Johan Hovold Cc: Greg Kroah-Hartman , Jiri Slaby , linux-serial@vger.kernel.org Subject: Re: [PATCH 2/2] tty: serial: 8250: wait till transmitter is empty In-reply-to: Date: Mon, 04 Jan 2021 12:47:26 +0100 Message-ID: <87ble4exf5.fsf@pengutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-SA-Exim-Connect-IP: 2001:67c:670:100:1d::c0 X-SA-Exim-Mail-From: s.trumtrar@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 Hi! Johan Hovold writes: > On Wed, Dec 09, 2020 at 10:17:28AM +0100, Steffen Trumtrar wrote: >> 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); > > Tight polling (1 us) for 20 ms with interrupts disabled?! > > Without having looked at the details, there's got to be a better way to > handle this. > I'm sure there is. I just "copied" from 8250_dw.c O:-) Best regards, Steffen Trumtrar -- Pengutronix e.K. | Dipl.-Inform. Steffen Trumtrar | Steuerwalder Str. 21 | https://www.pengutronix.de/ | 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686| Fax: +49-5121-206917-5555 |