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

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

On 2021/10/27 下午7:39, Lukas Wunner wrote:
> On Wed, Oct 27, 2021 at 07:16:44PM +0800, Su Bao Cheng wrote:
>> This reverts commit f45709df7731ad36306a28a3e1af7309d55c35f5.
>>
>> The `serial8250_do_set_mctrl` not only used by userspace ioctl but
>> also the kernel itself.
>>
>> During tty_open, the uart_port_startup sets the MCR to 0, and then use
>> set_mctrl to restore the MCR, so at this time, the MCR read does not
>> reflect the desired value.
> 
> I don't quite follow.  Where is uart_port_startup() setting the MCR to 0?
> Are you referring to the call to uart_port_dtr_rts()?  That function should
> set RTS correctly according to RS485 state, so I don't see where any
> breakage may occur.
>
> What is the user-visible issue that you seek to fix with the revert?
> 

Sorry for the late response, the company exchange server does not work
for me at this moment, so I have to use the private email instead.

The issue is observed on omap8250 hardware (CPU: AM6548). the use case
is RS485 half-duplex (2 wire mode), in this mode the RTS pin is used to
control the direction and is software controller via the MCR[1]
register. The problem is that the RS485 transmitting is OK, but the
receiving is not working. Similar issue also exists for the RS422, i.e.
the 4-wire full-duplex mode of RS485, but this time the TX does not
work, RX is fine.

The MCR is set to 0 at this line within uart_port_startup():
	retval = uport->ops->startup(uport);

On omap8250, the startup() points to omap_8250_startup(), within it:
	up->mcr = 0;

For software controlled RTS pin of RS485 half-duplex, when not in the
transmitting, the MCR[1] should be constant to indicate the current
direction is receiving. This is set in serial8250_em485_stop_tx().

So after this point of setting the MCR to 0, this up->mcr register
mirror does not reflect the actual desired value anymore. Further
checking against it leads to false result.

Another possible fix could be, instead of setting the mcr to 0 blindly,
one could check if the current operating mode is RS485 half-duplex, and
if so, mask the MCR[1], so that this bit is not changed. Because the
MCR[1] will be changed to the correct value before TX in
serial8250_em485_start_tx(), this change would not impact the transmitting.

> Thanks,
> 
> Lukas
> 
>> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
>> index 66374704747e..40736e460956 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;
>> -- 
>> 2.25.1