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

Peter Hurley
2015-11-10T16:12:16Z

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).

> + 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;
>