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([2a0b:e7c0:0:107::70f]) by smtp.gmail.com with ESMTPSA id r5sm1605843wmh.23.2021.05.19.22.24.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 May 2021 22:24:46 -0700 (PDT) Subject: Re: [PATCH v3 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: <20210520021334.497341-1-andrew@aj.id.au> <20210520021334.497341-2-andrew@aj.id.au> From: Jiri Slaby Message-ID: Date: Thu, 20 May 2021 07:24:45 +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: <20210520021334.497341-2-andrew@aj.id.au> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210519_222449_067925_4EBA3F87 X-CRM114-Status: GOOD ( 30.84 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 20. 05. 21, 4:13, 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 Reviewed-by: Jiri Slaby > --- > 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(+) > > 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..fc5ab2032282 100644 > --- a/drivers/tty/serial/8250/8250_port.c > +++ b/drivers/tty/serial/8250/8250_port.c > @@ -1809,6 +1809,18 @@ 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 > + * 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)) > -- js suse labs _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel