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

Matwey V. Kornilov
2015-11-09T15:45:30Z

2015-11-09 17:40 GMT+03:00 Peter Hurley <peter@hurleysoftware.com>:
> On 11/08/2015 05:52 AM, Matwey V. Kornilov wrote:
>> 2015-11-07 19:03 GMT+03:00 Peter Hurley <peter@hurleysoftware.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@sai.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.

I am not sure that understand you correctly. Do you think that the
following would be ok?

wait_queue_head_t wait;
init_waitqueue_head(&wait);
wait_event_timeout(wait, 0, p->port.rs485.delay_rts_before_send * HZ / 1000);

>
> 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;
>>>>
>>>
>>
>>
>>
>

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