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    Subject[PATCH 5.10 03/39] serial: 8250: Fix reporting real baudrate value in c_ospeed field
    Date
    From: Pali Rohár <pali@kernel.org>

    [ Upstream commit 32262e2e429cdb31f9e957e997d53458762931b7 ]

    In most cases it is not possible to set exact baudrate value to hardware.

    So fix reporting real baudrate value which was set to hardware via c_ospeed
    termios field. It can be retrieved by ioctl(TCGETS2) from userspace.

    Real baudrate value is calculated from chosen hardware divisor and base
    clock. It is implemented in a new function serial8250_compute_baud_rate()
    which is inverse of serial8250_get_divisor() function.

    With this change is fixed also UART timeout value (it is updated via
    uart_update_timeout() function), which is calculated from the now fixed
    baudrate value too.

    Cc: stable@vger.kernel.org
    Signed-off-by: Pali Rohár <pali@kernel.org>
    Link: https://lore.kernel.org/r/20210927093704.19768-1-pali@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/tty/serial/8250/8250_port.c | 17 +++++++++++++++++
    1 file changed, 17 insertions(+)

    diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
    index 9d60418e4adb..eaf4eb33a78d 100644
    --- a/drivers/tty/serial/8250/8250_port.c
    +++ b/drivers/tty/serial/8250/8250_port.c
    @@ -2547,6 +2547,19 @@ static unsigned int serial8250_get_divisor(struct uart_port *port,
    return serial8250_do_get_divisor(port, baud, frac);
    }

    +static unsigned int serial8250_compute_baud_rate(struct uart_port *port,
    + unsigned int quot)
    +{
    + if ((port->flags & UPF_MAGIC_MULTIPLIER) && quot == 0x8001)
    + return port->uartclk / 4;
    + else if ((port->flags & UPF_MAGIC_MULTIPLIER) && quot == 0x8002)
    + return port->uartclk / 8;
    + else if (port->type == PORT_NPCM)
    + return DIV_ROUND_CLOSEST(port->uartclk - 2 * (quot + 2), 16 * (quot + 2));
    + else
    + return DIV_ROUND_CLOSEST(port->uartclk, 16 * quot);
    +}
    +
    static unsigned char serial8250_compute_lcr(struct uart_8250_port *up,
    tcflag_t c_cflag)
    {
    @@ -2688,11 +2701,14 @@ void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)

    baud = serial8250_get_baud_rate(port, termios, NULL);
    quot = serial8250_get_divisor(port, baud, &frac);
    + baud = serial8250_compute_baud_rate(port, quot);

    serial8250_rpm_get(up);
    spin_lock_irqsave(&port->lock, flags);

    uart_update_timeout(port, termios->c_cflag, baud);
    + if (tty_termios_baud_rate(termios))
    + tty_termios_encode_baud_rate(termios, baud, baud);

    serial8250_set_divisor(port, baud, quot, frac);
    serial_port_out(port, UART_LCR, up->lcr);
    @@ -2726,6 +2742,7 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,

    baud = serial8250_get_baud_rate(port, termios, old);
    quot = serial8250_get_divisor(port, baud, &frac);
    + baud = serial8250_compute_baud_rate(port, quot);

    /*
    * Ok, we're now changing the port state. Do it with
    --
    2.35.1


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