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=-19.1 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,USER_AGENT_GIT 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 45E19C47080 for ; Mon, 31 May 2021 16:13:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20EF061008 for ; Mon, 31 May 2021 16:13:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233499AbhEaQOE (ORCPT ); Mon, 31 May 2021 12:14:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:37862 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234107AbhEaOjU (ORCPT ); Mon, 31 May 2021 10:39:20 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id DB36D6135D; Mon, 31 May 2021 13:52:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1622469142; bh=po92fyaU31d9OfaH20goi4hugIcHBnKfk/b12o0pnSQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ckoxx8C+w/6lLyD8MgWQc+tkcD9qSyik0l5iwc9LTrEJgmCypfStISXmyDOnV+D5F SzRVc1Q9Ng2ZsKEiBmVTBfeUfsKz5SGy0eLIO75lJJup58uQbntANpoC3htpbYiaoF 6G/B7nHLVWU8H/Kvgf0/Q1orG49o7zFpnLFRqc0E= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, ChiaWei Wang , Jiri Slaby , Andrew Jeffery Subject: [PATCH 5.12 085/296] serial: 8250: Add UART_BUG_TXRACE workaround for Aspeed VUART Date: Mon, 31 May 2021 15:12:20 +0200 Message-Id: <20210531130706.731805073@linuxfoundation.org> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210531130703.762129381@linuxfoundation.org> References: <20210531130703.762129381@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: linux-kernel@vger.kernel.org From: Andrew Jeffery commit df8f2be2fd0b44b2cb6077068f52e05f0ac40897 upstream. 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 Tested-by: ChiaWei Wang Reviewed-by: Jiri Slaby Signed-off-by: Andrew Jeffery Cc: stable Link: https://lore.kernel.org/r/20210520021334.497341-2-andrew@aj.id.au Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250.h | 1 + drivers/tty/serial/8250/8250_aspeed_vuart.c | 1 + drivers/tty/serial/8250/8250_port.c | 12 ++++++++++++ 3 files changed, 14 insertions(+) --- 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 --- a/drivers/tty/serial/8250/8250_aspeed_vuart.c +++ b/drivers/tty/serial/8250/8250_aspeed_vuart.c @@ -403,6 +403,7 @@ static int aspeed_vuart_probe(struct pla 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) --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1815,6 +1815,18 @@ void serial8250_tx_chars(struct uart_825 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 + * 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. Read a register that won't have + * side-effects and discard the result. + */ + serial_in(up, UART_SCR); + } xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1); port->icount.tx++; if (uart_circ_empty(xmit))