linux-kernel - [PATCH] tty: serial: imx: Add fast path when rs485 delays are 0

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Message-Id: <20220119145204.238767-1-hws@denx.de>
Date:   Wed, 19 Jan 2022 15:52:03 +0100
From:   Harald Seiler <hws@...x.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Harald Seiler <hws@...x.de>, Jiri Slaby <jirislaby@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] tty: serial: imx: Add fast path when rs485 delays are 0

Right now, even when `delay_rts_before_send` and `delay_rts_after_send`
are 0, the hrtimer is triggered (with timeout 0) which can introduce a
few 100us of additional overhead on slower i.MX platforms.

Implement a fast path when the delays are 0, where the RTS signal is
toggled immediately instead of going through an hrtimer.  This fast path
behaves identical to the code before delay support was implemented.

Signed-off-by: Harald Seiler <hws@...x.de>
---
 drivers/tty/serial/imx.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index df8a0c8b8b29..67bbbb69229d 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -455,9 +455,14 @@ static void imx_uart_stop_tx(struct uart_port *port)
 	if (port->rs485.flags & SER_RS485_ENABLED) {
 		if (sport->tx_state == SEND) {
 			sport->tx_state = WAIT_AFTER_SEND;
-			start_hrtimer_ms(&sport->trigger_stop_tx,
+
+			if (port->rs485.delay_rts_after_send > 0) {
+				start_hrtimer_ms(&sport->trigger_stop_tx,
 					 port->rs485.delay_rts_after_send);
-			return;
+				return;
+			}
+
+			/* continue without any delay */
 		}
 
 		if (sport->tx_state == WAIT_AFTER_RTS ||
@@ -698,9 +703,14 @@ static void imx_uart_start_tx(struct uart_port *port)
 				imx_uart_stop_rx(port);
 
 			sport->tx_state = WAIT_AFTER_RTS;
-			start_hrtimer_ms(&sport->trigger_start_tx,
+
+			if (port->rs485.delay_rts_before_send > 0) {
+				start_hrtimer_ms(&sport->trigger_start_tx,
 					 port->rs485.delay_rts_before_send);
-			return;
+				return;
+			}
+
+			/* continue without any delay */
 		}
 
 		if (sport->tx_state == WAIT_AFTER_SEND
-- 
2.34.1