linux-kernel - [PATCH v2] serial: atmel: fix incorrect baudrate setup

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Date:   Mon,  9 Jan 2023 08:29:40 +0100
From:   Tobias Schramm <t.schramm@...jaro.org>
To:     Richard Genoud <richard.genoud@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Tobias Schramm <t.schramm@...jaro.org>
Subject: [PATCH v2] serial: atmel: fix incorrect baudrate setup

Commit ba47f97a18f2 ("serial: core: remove baud_rates when serial console
setup") changed uart_set_options to select the correct baudrate
configuration based on the absolute error between requested baudrate and
available standard baudrate settings.
Prior to that commit the baudrate was selected based on which predefined
standard baudrate did not exceed the requested baudrate.
This change of selection logic was never reflected in the atmel serial
driver. Thus the comment left in the atmel serial driver is no longer
accurate.
Additionally the manual rounding up described in that comment and applied
via (quot - 1) requests an incorrect baudrate. Since uart_set_options uses
tty_termios_encode_baud_rate to determine the appropriate baudrate flags
this can cause baudrate selection to fail entirely because
tty_termios_encode_baud_rate will only select a baudrate if relative error
between requested and selected baudrate does not exceed +/-2%.
Fix that by requesting actual, exact baudrate used by the serial.

Fixes: ba47f97a18f2 ("serial: core: remove baud_rates when serial console setup")
Signed-off-by: Tobias Schramm <t.schramm@...jaro.org>
---
 drivers/tty/serial/atmel_serial.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index f1c06e12efa0..9cd7479b03c0 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -2657,13 +2657,7 @@ static void __init atmel_console_get_options(struct uart_port *port, int *baud,
 	else if (mr == ATMEL_US_PAR_ODD)
 		*parity = 'o';

-	/*
-	 * The serial core only rounds down when matching this to a
-	 * supported baud rate. Make sure we don't end up slightly
-	 * lower than one of those, as it would make us fall through
-	 * to a much lower baud rate than we really want.
-	 */
-	*baud = port->uartclk / (16 * (quot - 1));
+	*baud = port->uartclk / (16 * quot);
 }

 static int __init atmel_console_setup(struct console *co, char *options)
-- 
2.30.2