linux-kernel - Re: [PATCH] tty/serial: atmel: RS485 & ISO7816: wait for TXRDY before sending data

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Date:   Wed, 8 Jan 2020 14:45:05 +0100
From:   Richard Genoud <richard.genoud@...il.com>
To:     Codrin.Ciubotariu@...rochip.com, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, jslaby@...e.com,
        Nicolas.Ferre@...rochip.com, alexandre.belloni@...tlin.com,
        Ludovic.Desroches@...rochip.com
Subject: Re: [PATCH] tty/serial: atmel: RS485 & ISO7816: wait for TXRDY before
 sending data

Le 07/01/2020 à 12:17, Codrin.Ciubotariu@...rochip.com a écrit :
> At this moment, TXEMPTY is checked before sending data on RS485 and ISO7816
> modes. However, TXEMPTY is risen when FIFO (if used) or the Transmit Shift
> Register are empty, even though TXRDY might be up and controller is able to
> receive data. Since the controller sends data only when TXEMPTY is ready,
> on RS485, when DMA is not used, the RTS pin is driven low after each byte.
> With this patch, the characters will be transmitted when TXRDY is up and
> so, RTS pin will remain high between bytes.
> The performance improvement on RS485 is about 8% with a baudrate of 300.
> 
> Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>
seems ok to me
Acked-by: Richard Genoud <richard.genoud@...il.com>

NB: MS exchange has added some =3D and =20 here and there, but git am
doesn't seems to be bothered by them.
> ---
>  drivers/tty/serial/atmel_serial.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index a8dc8af83f39..19c8fb9faa36 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -313,7 +313,11 @@ static int atmel_config_rs485(struct uart_port *port,
>  
>  	if (rs485conf->flags & SER_RS485_ENABLED) {
>  		dev_dbg(port->dev, "Setting UART to RS485\n");
> -		atmel_port->tx_done_mask = ATMEL_US_TXEMPTY;
> +		if (port->rs485.flags & SER_RS485_RX_DURING_TX)
> +			atmel_port->tx_done_mask = ATMEL_US_TXRDY;
> +		else
> +			atmel_port->tx_done_mask = ATMEL_US_TXEMPTY;
> +
>  		atmel_uart_writel(port, ATMEL_US_TTGR,
>  				  rs485conf->delay_rts_after_send);
>  		mode |= ATMEL_US_USMODE_RS485;
> @@ -831,7 +835,7 @@ static void atmel_tx_chars(struct uart_port *port)
>  	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
>  
>  	if (port->x_char &&
> -	    (atmel_uart_readl(port, ATMEL_US_CSR) & atmel_port->tx_done_mask)) {
> +	    (atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_TXRDY)) {
>  		atmel_uart_write_char(port, port->x_char);
>  		port->icount.tx++;
>  		port->x_char = 0;
> @@ -839,8 +843,7 @@ static void atmel_tx_chars(struct uart_port *port)
>  	if (uart_circ_empty(xmit) || uart_tx_stopped(port))
>  		return;
>  
> -	while (atmel_uart_readl(port, ATMEL_US_CSR) &
> -	       atmel_port->tx_done_mask) {
> +	while (atmel_uart_readl(port, ATMEL_US_CSR) & ATMEL_US_TXRDY) {
>  		atmel_uart_write_char(port, xmit->buf[xmit->tail]);
>  		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
>  		port->icount.tx++;
> @@ -851,10 +854,20 @@ static void atmel_tx_chars(struct uart_port *port)
>  	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
>  		uart_write_wakeup(port);
>  
> -	if (!uart_circ_empty(xmit))
> +	if (!uart_circ_empty(xmit)) {
> +		/* we still have characters to transmit, so we should continue
> +		 * transmitting them when TX is ready, regardless of
> +		 * mode or duplexity
> +		 */
> +		atmel_port->tx_done_mask |= ATMEL_US_TXRDY;
> +
>  		/* Enable interrupts */
>  		atmel_uart_writel(port, ATMEL_US_IER,
>  				  atmel_port->tx_done_mask);
> +	} else {
> +		if (atmel_uart_is_half_duplex(port))
> +			atmel_port->tx_done_mask &= ~ATMEL_US_TXRDY;
> +	}
>  }
>  
>  static void atmel_complete_tx_dma(void *arg)
> @@ -2525,8 +2538,7 @@ static int atmel_init_port(struct atmel_uart_port *atmel_port,
>  	 * Use TXEMPTY for interrupt when rs485 or ISO7816 else TXRDY or
>  	 * ENDTX|TXBUFE
>  	 */
> -	if (port->rs485.flags & SER_RS485_ENABLED ||
> -	    port->iso7816.flags & SER_ISO7816_ENABLED)
> +	if (atmel_uart_is_half_duplex(port))
>  		atmel_port->tx_done_mask = ATMEL_US_TXEMPTY;
>  	else if (atmel_use_pdc_tx(port)) {
>  		port->fifosize = PDC_BUFFER_SIZE;
>