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    Subject[PATCH 5.15 203/207] serial: 8250: Fix RTS modem control while in rs485 mode
    Date
    From: Lukas Wunner <lukas@wunner.de>

    commit f85e04503f369b3f2be28c83fc48b74e19936ebc upstream.

    Commit f45709df7731 ("serial: 8250: Don't touch RTS modem control while
    in rs485 mode") sought to prevent user space from interfering with rs485
    communication by ignoring a TIOCMSET ioctl() which changes RTS polarity.

    It did so in serial8250_do_set_mctrl(), which turns out to be too deep
    in the call stack: When a uart_port is opened, RTS polarity is set by
    the rs485-aware function uart_port_dtr_rts(). It calls down to
    serial8250_do_set_mctrl() and that particular RTS polarity change should
    *not* be ignored.

    The user-visible result is that on 8250_omap ports which use rs485 with
    inverse polarity (RTS bit in MCR register is 1 to receive, 0 to send),
    a newly opened port initially sets up RTS for sending instead of
    receiving. That's because omap_8250_startup() sets the cached value
    up->mcr to 0 and omap_8250_restore_regs() subsequently writes it to the
    MCR register. Due to the commit, serial8250_do_set_mctrl() preserves
    that incorrect register value:

    do_sys_openat2
    do_filp_open
    path_openat
    vfs_open
    do_dentry_open
    chrdev_open
    tty_open
    uart_open
    tty_port_open
    uart_port_activate
    uart_startup
    uart_port_startup
    serial8250_startup
    omap_8250_startup # up->mcr = 0
    uart_change_speed
    serial8250_set_termios
    omap_8250_set_termios
    omap_8250_restore_regs
    serial8250_out_MCR # up->mcr written
    tty_port_block_til_ready
    uart_dtr_rts
    uart_port_dtr_rts
    serial8250_set_mctrl
    omap8250_set_mctrl
    serial8250_do_set_mctrl # mcr[1] = 1 ignored

    Fix by intercepting RTS changes from user space in uart_tiocmset()
    instead.

    Link: https://lore.kernel.org/linux-serial/20211027111644.1996921-1-baocheng.su@siemens.com/
    Fixes: f45709df7731 ("serial: 8250: Don't touch RTS modem control while in rs485 mode")
    Cc: Chao Zeng <chao.zeng@siemens.com>
    Cc: stable@vger.kernel.org # v5.7+
    Reported-by: Su Bao Cheng <baocheng.su@siemens.com>
    Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
    Tested-by: Su Bao Cheng <baocheng.su@siemens.com>
    Signed-off-by: Lukas Wunner <lukas@wunner.de>
    Link: https://lore.kernel.org/r/21170e622a1aaf842a50b32146008b5374b3dd1d.1637596432.git.lukas@wunner.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/tty/serial/8250/8250_port.c | 7 -------
    drivers/tty/serial/serial_core.c | 5 +++++
    2 files changed, 5 insertions(+), 7 deletions(-)

    --- a/drivers/tty/serial/8250/8250_port.c
    +++ b/drivers/tty/serial/8250/8250_port.c
    @@ -2024,13 +2024,6 @@ void serial8250_do_set_mctrl(struct uart
    struct uart_8250_port *up = up_to_u8250p(port);
    unsigned char mcr;

    - if (port->rs485.flags & SER_RS485_ENABLED) {
    - if (serial8250_in_MCR(up) & UART_MCR_RTS)
    - mctrl |= TIOCM_RTS;
    - else
    - mctrl &= ~TIOCM_RTS;
    - }
    -
    mcr = serial8250_TIOCM_to_MCR(mctrl);

    mcr = (mcr & up->mcr_mask) | up->mcr_force | up->mcr;
    --- a/drivers/tty/serial/serial_core.c
    +++ b/drivers/tty/serial/serial_core.c
    @@ -1075,6 +1075,11 @@ uart_tiocmset(struct tty_struct *tty, un
    goto out;

    if (!tty_io_error(tty)) {
    + if (uport->rs485.flags & SER_RS485_ENABLED) {
    + set &= ~TIOCM_RTS;
    + clear &= ~TIOCM_RTS;
    + }
    +
    uart_update_mctrl(uport, set, clear);
    ret = 0;
    }

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