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

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Message-ID: <CAJs94EaWQSqqUzqRoXO=vfBCf7CE+0Gt8C5-zGOB=t6ND0bSBg@mail.gmail.com>
Date:	Sun, 8 Nov 2015 13:52:19 +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

2015-11-07 19:03 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
> Hi Matwey,
>
> 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.
>
> Comments below.
>
>> 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);
>>  }
>
> Newline req'd here.
>
>> +static void serial8250_stop_rx(struct uart_port *port);
>
> You can eliminate this forward decl by relocating serial8250_stop_rx() to here
> (in a separate patch).
>
>> +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);
>> +             if (p->port.rs485.delay_rts_before_send > 0)
>> +                     mdelay(p->port.rs485.delay_rts_before_send);
>
> So irqs are off for x msecs, and this cpu can't be used for anything else now?
> I think this needs to be solved differently; maybe with a timer?

Call of serial8250_start_tx is wrapped with spin_lock_irq in serial_core.c:2154
I've tried to use msleep instead of mdelay but got "BUG: scheduling
while atomic".

>
>> +     }
>> +     if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX))
>> +             serial8250_stop_rx(&p->port);
>> +}
>
> Newline req'd here.
>
>> +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;
>>
>> +     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);
>
> Same issue with irqs off.
>
>> +             serial_port_out(&p->port, UART_MCR, UART_MCR_RTS);
>> +     }
>> +     /*
>> +     * 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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