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


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