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

Matwey V. Kornilov
2015-11-12T12:35:14Z

2015-11-10 19:12 GMT+03:00 Peter Hurley <peter@hurleysoftware.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@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);
>> }
>> +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).

By the way, I've found that p->mcr caches MCR inconsistently. Is it
supposed to be so? Or p->mcr is not for caching?

>
>> + 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.
>
>> + 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