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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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