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@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>

diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c
index 1789a66..0e169ad 100644 (file)
--- 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);