linux-kernel - [PATCH tty-next v5 2/6] serial: 8250: Use frame time to determine timeout

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Message-Id: <20250107212702.169493-3-john.ogness@linutronix.de>
Date: Tue,  7 Jan 2025 22:32:58 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jirislaby@...nel.org>,
	Petr Mladek <pmladek@...e.com>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Esben Haabendal <esben@...nix.com>,
	linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Serge Semin <fancer.lancer@...il.com>
Subject: [PATCH tty-next v5 2/6] serial: 8250: Use frame time to determine timeout

Rather than using a hard-coded per-character Tx-timeout of 10ms,
use the frame time to determine a timeout value. The value is
doubled to ensure that a timeout is only hit during unexpected
circumstances.

Since the frame time may not be available during early printing,
the previous 10ms value is kept as a fallback.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
 drivers/tty/serial/8250/8250_port.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3a946ebe9139..ca8f6f3855eb 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2081,9 +2081,17 @@ static void serial8250_break_ctl(struct uart_port *port, int break_state)
 /* Returns true if @bits were set, false on timeout */
 static bool wait_for_lsr(struct uart_8250_port *up, int bits)
 {
-	unsigned int status, tmout = 10000;
+	unsigned int status, tmout;
+
+	/*
+	 * Wait for a character to be sent. Fallback to a safe default
+	 * timeout value if @frame_time is not available.
+	 */
+	if (up->port.frame_time)
+		tmout = up->port.frame_time * 2 / NSEC_PER_USEC;
+	else
+		tmout = 10000;
 
-	/* Wait up to 10ms for the character(s) to be sent. */
 	for (;;) {
 		status = serial_lsr_in(up);
 
-- 
2.39.5