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

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Date:	Mon, 9 Nov 2015 09:40:49 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Matwey V. Kornilov" <matwey@....msu.ru>
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

On 11/08/2015 05:52 AM, Matwey V. Kornilov wrote:
> 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

Yep, which is why I pointed out "irqs are off for x msecs".

> I've tried to use msleep instead of mdelay but got "BUG: scheduling
> while atomic".

Right, can't sleep while irqs are off, which is why I suggested something
like a timer.

Regards,
Peter Hurley

>>> +     }
>>> +     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;
>>>
>>
> 
> 
> 

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