linux-kernel - [PATCH 03/10] serial: mvebu-uart: correctly calculate minimal possible baudrate

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Date:   Fri, 25 Jun 2021 00:49:02 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        Vladimir Vid <vladimir.vid@...tura.hr>,
        Marek Behún <kabel@...nel.org>,
        linux-clk@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: [PATCH 03/10] serial: mvebu-uart: correctly calculate minimal possible baudrate

For default (x16) scheme which is currently used by mvebu-uart.c driver,
maximal divisor of UART base clock is 1023*16. Therefore there is limit for
minimal supported baudrate. This change calculate it correctly and prevents
setting invalid divisor 0 into hardware registers.

Signed-off-by: Pali Rohár <pali@...nel.org>
Fixes: 68a0db1d7da2 ("serial: mvebu-uart: add function to change baudrate")
---
 drivers/tty/serial/mvebu-uart.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index dc0c26824ddb..f8b0016db847 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -481,7 +481,7 @@ static void mvebu_uart_set_termios(struct uart_port *port,
 				   struct ktermios *old)
 {
 	unsigned long flags;
-	unsigned int baud;
+	unsigned int baud, min_baud, max_baud;
 
 	spin_lock_irqsave(&port->lock, flags);
 
@@ -500,16 +500,21 @@ static void mvebu_uart_set_termios(struct uart_port *port,
 		port->ignore_status_mask |= STAT_RX_RDY(port) | STAT_BRK_ERR;
 
 	/*
+	 * Maximal divisor is 1023 * 16 when using default (x16) scheme.
 	 * Maximum achievable frequency with simple baudrate divisor is 230400.
 	 * Since the error per bit frame would be of more than 15%, achieving
 	 * higher frequencies would require to implement the fractional divisor
 	 * feature.
 	 */
-	baud = uart_get_baud_rate(port, termios, old, 0, 230400);
+	min_baud = DIV_ROUND_UP(port->uartclk, 1023 * 16);
+	max_baud = 230400;
+
+	baud = uart_get_baud_rate(port, termios, old, min_baud, max_baud);
 	if (mvebu_uart_baud_rate_set(port, baud)) {
 		/* No clock available, baudrate cannot be changed */
 		if (old)
-			baud = uart_get_baud_rate(port, old, NULL, 0, 230400);
+			baud = uart_get_baud_rate(port, old, NULL,
+						  min_baud, max_baud);
 	} else {
 		tty_termios_encode_baud_rate(termios, baud, baud);
 		uart_update_timeout(port, termios->c_cflag, baud);
-- 
2.20.1