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=-16.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=unavailable 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 5254DC43461 for ; Mon, 17 May 2021 16:14:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3A64960E0B for ; Mon, 17 May 2021 16:14:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347190AbhEQQPR (ORCPT ); Mon, 17 May 2021 12:15:17 -0400 Received: from mail.kernel.org ([198.145.29.99]:47158 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244766AbhEQPvL (ORCPT ); Mon, 17 May 2021 11:51:11 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 759D861D47; Mon, 17 May 2021 14:46:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1621262782; bh=y7lobXNEl29UJeAhMhyDmSKuFIoD0SytKc1vbhMDieg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=aG62Q6vcuDN92+cqpoea4jwDFSODUNnskkMIjRJEsjP1+JdIveCOmCNHWP7zMOkiW qYM/w8RswSw4noCvt+e8DBwAHhHcf14sl+BUGDfiXfPF+mfyz3y/LbgdW863HW9r65 6xBMmjmmtW9h1bL5QlnN0q/PQ/vuJsGoQ8fdI+Vo= Date: Mon, 17 May 2021 16:15:04 +0200 From: Greg KH To: Andrew Jeffery Cc: linux-serial@vger.kernel.org, jirislaby@kernel.org, joel@jms.id.au, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-aspeed@lists.ozlabs.org, openbmc@lists.ozlabs.org, jenmin_yuan@aspeedtech.com, ryan_chen@aspeedtech.com, miltonm@us.ibm.com, ChiaWei Wang Subject: Re: [PATCH] tty: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART Message-ID: References: <20210517124105.3565860-1-andrew@aj.id.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210517124105.3565860-1-andrew@aj.id.au> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 17, 2021 at 10:11:05PM +0930, Andrew Jeffery wrote: > Aspeed Virtual UARTs directly bridge e.g. the system console UART on the > LPC bus to the UART interface on the BMC's internal APB. As such there's > no RS-232 signalling involved - the UART interfaces on each bus are > directly connected as the producers and consumers of the one set of > FIFOs. > > The APB in the AST2600 generally runs at 100MHz while the LPC bus peaks > at 33MHz. The difference in clock speeds exposes a race in the VUART > design where a Tx data burst on the APB interface can result in a byte > lost on the LPC interface. The symptom is LSR[DR] remains clear on the > LPC interface despite data being present in its Rx FIFO, while LSR[THRE] > remains clear on the APB interface as the host has not consumed the data > the BMC has transmitted. In this state, the UART has stalled and no > further data can be transmitted without manual intervention (e.g. > resetting the FIFOs, resulting in loss of data). > > The recommended work-around is to insert a read cycle on the APB > interface between writes to THR. > > Cc: ChiaWei Wang > Signed-off-by: Andrew Jeffery > --- > drivers/tty/serial/8250/8250.h | 1 + > drivers/tty/serial/8250/8250_aspeed_vuart.c | 1 + > drivers/tty/serial/8250/8250_port.c | 2 ++ > 3 files changed, 4 insertions(+) > > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h > index 52bb21205bb6..4d6f5e0ecd4c 100644 > --- a/drivers/tty/serial/8250/8250.h > +++ b/drivers/tty/serial/8250/8250.h > @@ -88,6 +88,7 @@ struct serial8250_config { > #define UART_BUG_NOMSR (1 << 2) /* UART has buggy MSR status bits (Au1x00) */ > #define UART_BUG_THRE (1 << 3) /* UART has buggy THRE reassertion */ > #define UART_BUG_PARITY (1 << 4) /* UART mishandles parity if FIFO enabled */ > +#define UART_BUG_TXRACE (1 << 5) /* UART Tx fails to set remote DR */ BUG()? > #ifdef CONFIG_SERIAL_8250_SHARE_IRQ > diff --git a/drivers/tty/serial/8250/8250_aspeed_vuart.c b/drivers/tty/serial/8250/8250_aspeed_vuart.c > index a28a394ba32a..4caab8714e2c 100644 > --- a/drivers/tty/serial/8250/8250_aspeed_vuart.c > +++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c > @@ -440,6 +440,7 @@ static int aspeed_vuart_probe(struct platform_device *pdev) > port.port.status = UPSTAT_SYNC_FIFO; > port.port.dev = &pdev->dev; > port.port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_8250_CONSOLE); > + port.bugs |= UART_BUG_TXRACE; > > rc = sysfs_create_group(&vuart->dev->kobj, &aspeed_vuart_attr_group); > if (rc < 0) > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c > index d45dab1ab316..6c032abfc321 100644 > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -1809,6 +1809,8 @@ void serial8250_tx_chars(struct uart_8250_port *up) > count = up->tx_loadsz; > do { > serial_out(up, UART_TX, xmit->buf[xmit->tail]); > + if (up->bugs & UART_BUG_TXRACE) > + serial_in(up, UART_SCR); Can you document why you are doing a call here to serial_in(), otherwise someone running "automated checking scripts" will remove it later as it seems to be doing nothing. thanks, greg k-h