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Subject[PATCH v2 03/11] serial: mvebu-uart: correctly calculate minimal possible baudrate
Date
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")
---
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
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