Re: [PATCH] Revert "serial: 8250: Don't touch RTS modem control while in rs485 mode"

[Su Bao Cheng <baocheng_su@163.com>]

On 11/22/21 1:43 AM, Lukas Wunner wrote:
> On Sun, Nov 21, 2021 at 10:00:51AM +0100, Jan Kiszka wrote:
>> Meanwhile reproduced myself, and now I believe your patch is broken in
>> ignoring the internal call path to serial8250_set_mctrl, coming from
>> uart_port_dtr_rts:
> [...]
>> This case is not triggered by userspace setting a custom RTS value but
>> by the uart-internal machinery selecting it based on the rs485 mode,
>> among other things. That path must not be intercepted and made
>> conditional using the current MCR state but has to write the request
>> value *as is*.
> 
> Thanks for the analysis and sorry for the breakage.  I'm proposing the
> fix below.  Let me know if that works for you.
> 
> However I believe that omap_8250_startup() should be amended to not set
> up->mcr = 0 unconditionally.  Rather, it should set the RTS bit if rs485
> is enabled and RTS polarity is inverted (as seems to be the case on your
> product).  Right now, even with the fix below you'll see a brief glitch
> wherein RTS is asserted (so the transceiver's driver is enabled) and
> immediately deasserted when opening the port.  This may disturb the
> communication of other devices on the bus.  Do you agree?  If so, I can
> prepare a separate fix for that.  Note that we may have never noticed
> that without f45709df7731, so... ;)
> 
> Thanks,
> 
> Lukas
> 

The new patch works on our setup.

Thanks,

Baocheng Su

> -- >8 --
> Subject: [PATCH] serial: 8250: Fix RTS modem control while in rs485 mode
> 
> 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.
> 
> Fixes: f45709df7731 ("serial: 8250: Don't touch RTS modem control while in rs485 mode")
> Reported-by: Su Bao Cheng <baocheng.su@siemens.com>
> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> Cc: Chao Zeng <chao.zeng@siemens.com>
> Cc: stable@vger.kernel.org # v5.7+
> ---
>  drivers/tty/serial/8250/8250_port.c | 7 -------
>  drivers/tty/serial/serial_core.c    | 5 +++++
>  2 files changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 5775cbff8f6e..46e2079ad1aa 100644
> --- 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_port *port, unsigned int mctrl)
>  	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;
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 1e738f265eea..6a38e9d7b87a 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1075,6 +1075,11 @@ uart_tiocmset(struct tty_struct *tty, unsigned int set, unsigned int clear)
>  		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;
>  	}
>