linux-kernel - Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250

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Date:	Tue, 10 Nov 2015 19:25:25 +0300
From:	"Matwey V. Kornilov" <matwey@....msu.ru>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
	linux-serial@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250

2015-11-10 19:12 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
> Hi Matwey,
>
> I noticed 3 other issues here; see below.
>
> On 11/07/2015 05:09 AM, Matwey V. Kornilov wrote:
>> Implementation of software emulation of RS485 direction handling is based
>> on omap-serial driver.  It is acts as the following. At transmission start,
>> RTS is set (if required) and receiver is off (if required). At transmission
>> stop, RTS is set (if required) and fifo is flushed.
>>
>> Signed-off-by: Matwey V. Kornilov <matwey@....msu.ru>
>> ---
>>  drivers/tty/serial/8250/8250_port.c | 32 ++++++++++++++++++++++++++++++++
>>  1 file changed, 32 insertions(+)
>>
>> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
>> index 52d82d2..a9291f7 100644
>> --- a/drivers/tty/serial/8250/8250_port.c
>> +++ b/drivers/tty/serial/8250/8250_port.c
>> @@ -559,7 +559,37 @@ static void serial8250_rpm_put_tx(struct uart_8250_port *p)
>>       pm_runtime_mark_last_busy(p->port.dev);
>>       pm_runtime_put_autosuspend(p->port.dev);
>>  }
>> +static void serial8250_stop_rx(struct uart_port *port);
>> +static void serial8250_rs485_start_tx(struct uart_8250_port *p)
>> +{
>> +     if (p->capabilities & UART_CAP_HW485 || !(p->port.rs485.flags & SER_RS485_ENABLED))
>> +             return;
>> +
>> +     if (p->port.rs485.flags & SER_RS485_RTS_ON_SEND) {
>> +             serial_port_out(&p->port, UART_MCR, UART_MCR_RTS);
>
> The SER_RS485_RTS_ON_SEND bit is supposed to be the logic level of RTS,
> so RTS should be driven to either 0 or 1 here (not just to 1).
>
>> +             if (p->port.rs485.delay_rts_before_send > 0)
>> +                     mdelay(p->port.rs485.delay_rts_before_send);
>> +     }
>> +     if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
>> +             serial8250_stop_rx(&p->port);
>> +}
>> +static void serial8250_rs485_stop_tx(struct uart_8250_port *p)
>> +{
>> +     if (p->capabilities & UART_CAP_HW485 || !(p->port.rs485.flags & SER_RS485_ENABLED))
>> +             return;
>
> Unlike omap-serial, the 8250 stop_tx() will trigger _before_ the transmitter
> is drained. Some mechanism is required to defer until the transmitter
> is empty.
>

In serial8250_stop_tx() I see the following:

        if (port->type == PORT_16C950) {
                up->acr |= UART_ACR_TXDIS;
                serial_icr_write(up, UART_ACR, up->acr);
        }

AFAIU this will disable transmission unconditionally, or not? What
will happen here with data in FIFO?

I think that parts of my rs485 stuff have to be moved to
serial8250_tx_chars where It will be simpler to handle things without
involving mdelays.

>> +     if (p->port.rs485.flags & SER_RS485_RTS_AFTER_SEND) {
>> +             if (p->port.rs485.delay_rts_after_send > 0)
>> +                     mdelay(p->port.rs485.delay_rts_after_send);
>> +             serial_port_out(&p->port, UART_MCR, UART_MCR_RTS);
>
> As with the SER_RS485_RTS_ON_SEND, RTS should be driven to either 0 or 1 here
> as well (not just to 1).
>
> Regards,
> Peter Hurley
>
>> +     }
>> +     /*
>> +     * Empty the RX FIFO, we are not interested in anything
>> +     * received during the half-duplex transmission.
>> +     */
>> +     if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
>> +             serial8250_clear_fifos(p);
>> +}
>>  /*
>>   * IER sleep support.  UARTs which have EFRs need the "extended
>>   * capability" bit enabled.  Note that on XR16C850s, we need to
>> @@ -1309,6 +1339,7 @@ static void serial8250_stop_tx(struct uart_port *port)
>>               up->acr |= UART_ACR_TXDIS;
>>               serial_icr_write(up, UART_ACR, up->acr);
>>       }
>> +     serial8250_rs485_stop_tx(up);
>>       serial8250_rpm_put(up);
>>  }
>>
>> @@ -1317,6 +1348,7 @@ static void serial8250_start_tx(struct uart_port *port)
>>       struct uart_8250_port *up = up_to_u8250p(port);
>>
>>       serial8250_rpm_get_tx(up);
>> +     serial8250_rs485_start_tx(up);
>>
>>       if (up->dma && !up->dma->tx_dma(up))
>>               return;
>>
>



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382
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