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    Subject[PATCH 5.10 403/563] serial: pl010: Drop CR register reset on set_termios
    Date
    From: Lukas Wunner <lukas@wunner.de>

    [ Upstream commit 08a0c6dff91c965e39905cf200d22db989203ccb ]

    pl010_set_termios() briefly resets the CR register to zero.

    Where does this register write come from?

    The PL010 driver's IRQ handler ambauart_int() originally modified the CR
    register without holding the port spinlock. ambauart_set_termios() also
    modified that register. To prevent concurrent read-modify-writes by the
    IRQ handler and to prevent transmission while changing baudrate,
    ambauart_set_termios() had to disable interrupts. That is achieved by
    writing zero to the CR register.

    However in 2004 the PL010 driver was amended to acquire the port
    spinlock in the IRQ handler, obviating the need to disable interrupts in
    ->set_termios():
    https://git.kernel.org/history/history/c/157c0342e591

    That rendered the CR register write obsolete. Drop it.

    Cc: Russell King <rmk+kernel@armlinux.org.uk>
    Signed-off-by: Lukas Wunner <lukas@wunner.de>
    Link: https://lore.kernel.org/r/fcaff16e5b1abb4cc3da5a2879ac13f278b99ed0.1641128728.git.lukas@wunner.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/tty/serial/amba-pl010.c | 3 ---
    1 file changed, 3 deletions(-)

    diff --git a/drivers/tty/serial/amba-pl010.c b/drivers/tty/serial/amba-pl010.c
    index 3284f34e9dfe1..75d61e038a775 100644
    --- a/drivers/tty/serial/amba-pl010.c
    +++ b/drivers/tty/serial/amba-pl010.c
    @@ -448,14 +448,11 @@ pl010_set_termios(struct uart_port *port, struct ktermios *termios,
    if ((termios->c_cflag & CREAD) == 0)
    uap->port.ignore_status_mask |= UART_DUMMY_RSR_RX;

    - /* first, disable everything */
    old_cr = readb(uap->port.membase + UART010_CR) & ~UART010_CR_MSIE;

    if (UART_ENABLE_MS(port, termios->c_cflag))
    old_cr |= UART010_CR_MSIE;

    - writel(0, uap->port.membase + UART010_CR);
    -
    /* Set baud rate */
    quot -= 1;
    writel((quot & 0xf00) >> 8, uap->port.membase + UART010_LCRM);
    --
    2.34.1


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