linux-kernel - [PATCH v2 3/3] serial: 8250: Set fifo timeout using uart_fifo

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Message-ID: <20240416182741.22514-4-mcpratt@pm.me>
Date: Tue, 16 Apr 2024 18:34:10 +0000
From: Michael Pratt <mcpratt@...me>
To: linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>
Cc: Wander Lairson Costa <wander@...hat.com>, Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Vamshi Gajjela <vamshigajjela@...gle.com>, Michael Pratt <mcpratt@...me>
Subject: [PATCH v2 3/3] serial: 8250: Set fifo timeout using uart_fifo_timeout()

Commit 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
reworked functions for basic 8250 and 16550 type serial devices
in order to enable and use the internal fifo device for buffering,
however the default timeout of 10 ms remained, which is proving
to be insufficient for low baud rates like 9600, causing data overrun.

Unforunately, that commit was written and accepted just before commit
31f6bd7fad3b ("serial: Store character timing information to uart_port")
which introduced the frame_time member of the uart_port struct
in order to store the amount of time it takes to send one uart frame
relative to the baud rate and other serial port configuration,
and commit f9008285bb69 ("serial: Drop timeout from uart_port")
which established function uart_fifo_timeout() in order to
calculate a reasonable timeout to wait until flushing
the fifo device before writing data again when partially filled,
using the now stored frame_time value and size of the buffer.

Fix this by using the stored timeout value made with this new function
to calculate the timeout for the fifo device, when enabled, and when
the buffer is larger than 1 byte (unknown port default).

The previous 2 commits add the struct members used here
in order to store the values, so that the calculations
are offloaded from the functions that are called
during a write operation for better performance.

Tested on a MIPS device (ar934x) at baud rates 625, 9600, 115200.

Fixes: 8f3631f0f6eb ("serial/8250: Use fifo in 8250 console driver")
Signed-off-by: Michael Pratt <mcpratt@...me>
---
V1 -> V2:
 Use stored values instead of calling uart_fifo_timeout()
 or checking capability flags.
 The existence of the timeout value satisfies fifosize > 1.

 drivers/tty/serial/8250/8250_port.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 5b0cfe6bc98c..cf67911a74f5 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2066,7 +2066,10 @@ static void wait_for_lsr(struct uart_8250_port *up, int bits)
 {
 	unsigned int status, tmout = 10000;

-	/* Wait up to 10ms for the character(s) to be sent. */
+	/* Wait for a time relative to buffer size and baud */
+	if (up->fifo_enable && up->port.timeout)
+		tmout = jiffies_to_usecs(up->port.timeout);
+
 	for (;;) {
 		status = serial_lsr_in(up);

-- 
2.30.2