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    Subject[PATCH 5.7 042/477] serial: 8250: Fix max baud limit in generic 8250 port
    Date
    From: Serge Semin <Sergey.Semin@baikalelectronics.ru>

    [ Upstream commit 7b668c064ec33f3d687c3a413d05e355172e6c92 ]

    Standard 8250 UART ports are designed in a way so they can communicate
    with baud rates up to 1/16 of a reference frequency. It's expected from
    most of the currently supported UART controllers. That's why the former
    version of serial8250_get_baud_rate() method called uart_get_baud_rate()
    with min and max baud rates passed as (port->uartclk / 16 / UART_DIV_MAX)
    and ((port->uartclk + tolerance) / 16) respectively. Doing otherwise, like
    it was suggested in commit ("serial: 8250_mtk: support big baud rate."),
    caused acceptance of bauds, which was higher than the normal UART
    controllers actually supported. As a result if some user-space program
    requested to set a baud greater than (uartclk / 16) it would have been
    permitted without truncation, but then serial8250_get_divisor(baud)
    (which calls uart_get_divisor() to get the reference clock divisor) would
    have returned a zero divisor. Setting zero divisor will cause an
    unpredictable effect varying from chip to chip. In case of DW APB UART the
    communications just stop.

    Lets fix this problem by getting back the limitation of (uartclk +
    tolerance) / 16 maximum baud supported by the generic 8250 port. Mediatek
    8250 UART ports driver developer shouldn't have touched it in the first
    place notably seeing he already provided a custom version of set_termios()
    callback in that glue-driver which took into account the extended baud
    rate values and accordingly updated the standard and vendor-specific
    divisor latch registers anyway.

    Fixes: 81bb549fdf14 ("serial: 8250_mtk: support big baud rate.")
    Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
    Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
    Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
    Cc: Paul Burton <paulburton@kernel.org>
    Cc: Ralf Baechle <ralf@linux-mips.org>
    Cc: Arnd Bergmann <arnd@arndb.de>
    Cc: Long Cheng <long.cheng@mediatek.com>
    Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
    Cc: Maxime Ripard <mripard@kernel.org>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Will Deacon <will@kernel.org>
    Cc: Russell King <linux@armlinux.org.uk>
    Cc: linux-mips@vger.kernel.org
    Cc: linux-arm-kernel@lists.infradead.org
    Cc: linux-mediatek@lists.infradead.org
    Link: https://lore.kernel.org/r/20200506233136.11842-2-Sergey.Semin@baikalelectronics.ru
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/tty/serial/8250/8250_port.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
    index f77bf820b7a30..4d83c85a7389b 100644
    --- a/drivers/tty/serial/8250/8250_port.c
    +++ b/drivers/tty/serial/8250/8250_port.c
    @@ -2615,6 +2615,8 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
    struct ktermios *termios,
    struct ktermios *old)
    {
    + unsigned int tolerance = port->uartclk / 100;
    +
    /*
    * Ask the core to calculate the divisor for us.
    * Allow 1% tolerance at the upper limit so uart clks marginally
    @@ -2623,7 +2625,7 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
    */
    return uart_get_baud_rate(port, termios, old,
    port->uartclk / 16 / UART_DIV_MAX,
    - port->uartclk);
    + (port->uartclk + tolerance) / 16);
    }

    void
    --
    2.25.1


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