[PATCH 4.19 219/421] serial: mvebu-uart: correctly calculate minimal possible baudrate

Greg Kroah-Hartman
2021-07-19T16:21:50Z

From: Pali Rohár <pali@kernel.org>

[ Upstream commit deeaf963569a0d9d1b08babb771f61bb501a5704 ]

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@kernel.org>
Fixes: 68a0db1d7da2 ("serial: mvebu-uart: add function to change baudrate")
Link: https://lore.kernel.org/r/20210624224909.6350-4-pali@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 0515b5e6326d..9369b4d42d24 100644
--- a/drivers/tty/serial/mvebu-uart.c
+++ b/drivers/tty/serial/mvebu-uart.c
@@ -471,7 +471,7 @@ 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);

@@ -490,16 +490,21 @@ 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);
--
2.30.2