From patchwork Wed Mar 16 14:36:40 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Wander Lairson Costa X-Patchwork-Id: 552502 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1B3C4C4332F for ; Wed, 16 Mar 2022 14:37:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232943AbiCPOis (ORCPT ); Wed, 16 Mar 2022 10:38:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36472 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242188AbiCPOir (ORCPT ); Wed, 16 Mar 2022 10:38:47 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id B09F152E2A for ; Wed, 16 Mar 2022 07:37:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1647441452; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=46hIDjyKphqtbInFWiNIIVInDMU/QR9kVXm66RmjIvo=; b=TDbaIDviHwFsBV/4Tp89tkhgq/8FDU0+fwGQOLZisVbwL+fSbRYv0MPE5gccuPdMsI5RYI 8Gm3AZZWszptifH3E0kVUNxVkH3n6Pwkv5UIwdTFHKoK0CUbr2PeaHcq4jM51D8IhQxEp+ dHkKwXz37Nc4vGpWs4NhAXS1zhPIofg= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-286-rXzbfaPKNEqOafpE0uKuUw-1; Wed, 16 Mar 2022 10:37:30 -0400 X-MC-Unique: rXzbfaPKNEqOafpE0uKuUw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B69F23C23FAA; Wed, 16 Mar 2022 14:37:29 +0000 (UTC) Received: from fedora.redhat.com (unknown [10.22.19.38]) by smtp.corp.redhat.com (Postfix) with ESMTP id 853F2404D8F1; Wed, 16 Mar 2022 14:37:26 +0000 (UTC) From: Wander Lairson Costa To: Greg Kroah-Hartman , Jiri Slaby , Wander Lairson Costa , Johan Hovold , "Maciej W. Rozycki" , Serge Semin , Lukas Wunner , =?utf-8?q?Pali_Roh=C3=A1r?= , linux-serial@vger.kernel.org (open list:SERIAL DRIVERS), linux-kernel@vger.kernel.org (open list) Cc: rostedt@goodmis.org, senozhatsky@chromium.org, andre.goddard@gmail.com, sudipm.mukherjee@gmail.com, andy.shevchenko@gmail.com, David.Laight@aculab.com, jonathanh@nvidia.com, phil@raspberrypi.com Subject: [PATCH v4 1/5] serial/8250: Use fifo in 8250 console driver Date: Wed, 16 Mar 2022 11:36:40 -0300 Message-Id: <20220316143646.13301-2-wander@redhat.com> In-Reply-To: <20220316143646.13301-1-wander@redhat.com> References: <20220316143646.13301-1-wander@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org Note: I am using a small test app + driver located at [0] for the problem description. serco is a driver whose write function dispatches to the serial controller. sertest is a user-mode app that writes n bytes to the serial console using the serco driver. While investigating a bug in the RHEL kernel, I noticed that the serial console throughput is way below the configured speed of 115200 bps in a HP Proliant DL380 Gen9. I was expecting something above 10KB/s, but I got 2.5KB/s. $ time ./sertest -n 2500 /tmp/serco real 0m0.997s user 0m0.000s sys 0m0.997s With the help of the function tracer, I then noticed the serial controller was taking around 410us seconds to dispatch one single byte: $ trace-cmd record -p function_graph -g serial8250_console_write \ ./sertest -n 1 /tmp/serco $ trace-cmd report | serial8250_console_write() { 0.384 us | _raw_spin_lock_irqsave(); 1.836 us | io_serial_in(); 1.667 us | io_serial_out(); | uart_console_write() { | serial8250_console_putchar() { | wait_for_xmitr() { 1.870 us | io_serial_in(); 2.238 us | } 1.737 us | io_serial_out(); 4.318 us | } 4.675 us | } | wait_for_xmitr() { 1.635 us | io_serial_in(); | __const_udelay() { 1.125 us | delay_tsc(); 1.429 us | } ... ... ... 1.683 us | io_serial_in(); | __const_udelay() { 1.248 us | delay_tsc(); 1.486 us | } 1.671 us | io_serial_in(); 411.342 us | } In another machine, I measured a throughput of 11.5KB/s, with the serial controller taking between 80-90us to send each byte. That matches the expected throughput for a configuration of 115200 bps. This patch changes the serial8250_console_write to use the 16550 fifo if available. In my benchmarks I got around 25% improvement in the slow machine, and no performance penalty in the fast machine. Signed-off-by: Wander Lairson Costa Link: https://lore.kernel.org/r/20211222112831.1968392-2-wander@redhat.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_port.c | 61 ++++++++++++++++++++++++++--- 1 file changed, 55 insertions(+), 6 deletions(-) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 3b12bfc1ed67..2abb3de11a48 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2056,10 +2056,7 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state) serial8250_rpm_put(up); } -/* - * Wait for transmitter & holding register to empty - */ -static void wait_for_xmitr(struct uart_8250_port *up, int bits) +static void wait_for_lsr(struct uart_8250_port *up, int bits) { unsigned int status, tmout = 10000; @@ -2076,6 +2073,16 @@ static void wait_for_xmitr(struct uart_8250_port *up, int bits) udelay(1); touch_nmi_watchdog(); } +} + +/* + * Wait for transmitter & holding register to empty + */ +static void wait_for_xmitr(struct uart_8250_port *up, int bits) +{ + unsigned int tmout; + + wait_for_lsr(up, bits); /* Wait up to 1s for flow control if necessary */ if (up->port.flags & UPF_CONS_FLOW) { @@ -3325,6 +3332,35 @@ static void serial8250_console_restore(struct uart_8250_port *up) serial8250_out_MCR(up, UART_MCR_DTR | UART_MCR_RTS); } +/* + * Print a string to the serial port using the device FIFO + * + * It sends fifosize bytes and then waits for the fifo + * to get empty. + */ +static void serial8250_console_fifo_write(struct uart_8250_port *up, + const char *s, unsigned int count) +{ + int i; + const char *end = s + count; + unsigned int fifosize = up->port.fifosize; + bool cr_sent = false; + + while (s != end) { + wait_for_lsr(up, UART_LSR_THRE); + + for (i = 0; i < fifosize && s != end; ++i) { + if (*s == '\n' && !cr_sent) { + serial_out(up, UART_TX, '\r'); + cr_sent = true; + } else { + serial_out(up, UART_TX, *s++); + cr_sent = false; + } + } + } +} + /* * Print a string to the serial port trying not to disturb * any possible real use of the port... @@ -3340,7 +3376,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s, struct uart_8250_em485 *em485 = up->em485; struct uart_port *port = &up->port; unsigned long flags; - unsigned int ier; + unsigned int ier, use_fifo; int locked = 1; touch_nmi_watchdog(); @@ -3372,7 +3408,20 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s, mdelay(port->rs485.delay_rts_before_send); } - uart_console_write(port, s, count, serial8250_console_putchar); + use_fifo = (up->capabilities & UART_CAP_FIFO) && + port->fifosize > 1 && + (serial_port_in(port, UART_FCR) & UART_FCR_ENABLE_FIFO) && + /* + * After we put a data in the fifo, the controller will send + * it regardless of the CTS state. Therefore, only use fifo + * if we don't use control flow. + */ + !(up->port.flags & UPF_CONS_FLOW); + + if (likely(use_fifo)) + serial8250_console_fifo_write(up, s, count); + else + uart_console_write(port, s, count, serial8250_console_putchar); /* * Finally, wait for transmitter to become empty