linux-kernel - [PATCH] serial: imx: Raise TX trigger level to 8

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Date: Wed,  8 May 2024 15:37:44 +0200
From: Matthias Schiffer <matthias.schiffer@...tq-group.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>
Cc: Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux@...tq-group.com,
	Matthias Schiffer <matthias.schiffer@...tq-group.com>,
	Michael Krummsdorf <michael.krummsdorf@...group.com>
Subject: [PATCH] serial: imx: Raise TX trigger level to 8

At the default TX trigger level of 2 in non-DMA mode (meaning that an
interrupt is generated when less than 2 characters are left in the
FIFO), we have observed frequent buffer underruns at 115200 Baud on an
i.MX8M Nano. This can cause communication issues if the receiving side
expects a continuous transfer.

Increasing the level to 8 makes the UART trigger an interrupt earlier,
giving the kernel enough time to refill the FIFO, at the cost of
triggering one interrupt per ~24 instead of ~30 bytes of transmitted
data (as the i.MX UART has a 32 byte FIFO).

Signed-off-by: Michael Krummsdorf <michael.krummsdorf@...group.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
---
 drivers/tty/serial/imx.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 79f1503cd75b4..2eb22594960f3 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -1305,7 +1305,7 @@ static void imx_uart_clear_rx_errors(struct imx_port *sport)
 
 }
 
-#define TXTL_DEFAULT 2 /* reset default */
+#define TXTL_DEFAULT 8
 #define RXTL_DEFAULT 8 /* 8 characters or aging timer */
 #define TXTL_DMA 8 /* DMA burst setting */
 #define RXTL_DMA 9 /* DMA burst setting */
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