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=-15.5 required=3.0 tests=BAYES_00,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 4B5BAC433ED for ; Wed, 19 May 2021 06:12:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2748E60FF2 for ; Wed, 19 May 2021 06:12:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235126AbhESGNR (ORCPT ); Wed, 19 May 2021 02:13:17 -0400 Received: from mail-wr1-f53.google.com ([209.85.221.53]:36472 "EHLO mail-wr1-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232769AbhESGM5 (ORCPT ); Wed, 19 May 2021 02:12:57 -0400 Received: by mail-wr1-f53.google.com with SMTP id c14so10840919wrx.3; Tue, 18 May 2021 23:11:34 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=E0IobIzQGFKtYjnVLAhsERrEFaAtOpKnjVXmJUrJllo=; b=VM2jm+CNxIvZklaoBCMn+yOsr2ZCle9el9RmKz84HYfRs8Z5rAvv08LsICuqE5rCuo XWDf3VShmBrAiKKbcBdKhUM1JZJ+cwuUzGuWkTYVZizA2wZFqRN4P94yu48r5vCZcV9M 1YpJPGCIMw17FrMNOlR96iUTcj07ITE2cGyULAUblJHloO1KgG8VndZVony1bAR80USZ FSvtdI9hV6puRngNb4p69Y/HVnRtkaRWI9mCHO9THNcTda0CQ6BqIvL9+A2hiHwNZSWY kgKrVMJwStzveESeGgKU5hJQNo/gxmmMlaswo6hunFb7zzzw5yQSH+46+EXfj03Idqe3 Ne9g== X-Gm-Message-State: AOAM533pDYIKmQNqP77b7WRIzHmc2qrSW1xihxQa6Il4GGIHUb+7G10H 7nPVSPj0fp2IK0RAWuDtBZE= X-Google-Smtp-Source: ABdhPJwIim9jf63DzBY3rwRC398B+hBCfBG7+Rs9X9JQQtLHoS1CD+Yq0iLOe0Ctgxg5qXG/UsNQ8A== X-Received: by 2002:adf:f3cc:: with SMTP id g12mr12461094wrp.300.1621404660316; Tue, 18 May 2021 23:11:00 -0700 (PDT) Received: from ?IPv6:2a0b:e7c0:0:107::70f? ([2a0b:e7c0:0:107::70f]) by smtp.gmail.com with ESMTPSA id m10sm24644065wrr.2.2021.05.18.23.10.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 18 May 2021 23:10:59 -0700 (PDT) Subject: Re: [PATCH v2 1/2] serial: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART To: Andrew Jeffery , linux-serial@vger.kernel.org Cc: gregkh@linuxfoundation.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 References: <20210519000704.3661773-1-andrew@aj.id.au> <20210519000704.3661773-2-andrew@aj.id.au> From: Jiri Slaby Message-ID: <8c14289a-9e81-f8de-b451-664a72ea5963@kernel.org> Date: Wed, 19 May 2021 08:10:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <20210519000704.3661773-2-andrew@aj.id.au> Content-Type: text/plain; charset=iso-8859-2; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19. 05. 21, 2:07, 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 | 10 ++++++++++ > 3 files changed, 12 insertions(+) > > diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h > index 52bb21205bb6..34aa2714f3c9 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 */ > > > #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..9d44b2b2ff18 100644 > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -1809,6 +1809,16 @@ 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) { > + /* The Aspeed BMC virtual UARTs have a bug where data This is not how a multiline comment should start. It should have been: /* * The Aspeed BMC virtual... > + * may get stuck in the BMC's Tx FIFO from bursts of > + * writes on the APB interface. > + * > + * Delay back-to-back writes by a read cycle to avoid > + * stalling the VUART. > + */ > + (void)serial_in(up, UART_SCR); (void) is useless here. It's only syntactic sugar which wouldn't even filter out a warning about unused result (if serial_in was marked w/ __must_check/warn_unused_result attribute). > + } > xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1); > port->icount.tx++; > if (uart_circ_empty(xmit)) > thanks, -- js suse labs