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

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Message-ID: <d0fc5b47-26fd-e6d3-b50d-012e739b87c6@microchip.com>
Date:   Wed, 8 Jan 2020 16:03:01 +0000
From:   <Codrin.Ciubotariu@...rochip.com>
To:     <richard.genoud@...il.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

On 08.01.2020 15:45, Richard Genoud wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> 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.

We are struggling with some e-mail changes, sorry for that. Thanks for 
letting me know about this.

>> ---
>>   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;
>>
>