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Subject: Re: [PATCH v2 2/2] tty/serial: atmel: RS485 HD w/DMA: enable RX after
 TX is stopped
To:     Razvan Stefanescu <razvan.stefanescu@microchip.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>
Cc:     Gil Weber <webergil@gmail.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Ludovic Desroches <ludovic.desroches@microchip.com>,
        linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org
References: <20190319132035.18481-1-razvan.stefanescu@microchip.com>
 <20190319132035.18481-3-razvan.stefanescu@microchip.com>
From:   Richard Genoud <richard.genoud@gmail.com>
Message-ID: <58640e9d-5570-3649-31cd-3ffcd0d6c3af@sorico.fr>
Date:   Tue, 26 Mar 2019 16:12:05 +0100
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Le 19/03/2019 à 14:20, Razvan Stefanescu a écrit :
> In half-duplex operation, RX should be started after TX completes.
> 
> If DMA is used, there is a case when the DMA transfer completes but the
> TX FIFO is not emptied, so the RX cannot be restarted just yet.
> 
> Use a boolean variable to store this state and rearm TX interrupt mask
> to be signaled again that the transfer finished. In interrupt transmit
> handler this variable is used to start RX. A warning message is generated
> if RX is activated before TX fifo is cleared.
> 
> Fixes: b389f173aaa1 ("tty/serial: atmel: RS485 half duplex w/DMA: enable
> RX after TX is done")
> Signed-off-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>

NB: backport on kernel older than 4.20 will fail because of the iso7816
variables fidi_min/fidi_max.
> ---
> Changelog:
> v2:
>   - start RX and display warning in case of error
>   - add fix info
> 
>  drivers/tty/serial/atmel_serial.c | 25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index b4b89a16a41b..5b2f859c327c 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -166,6 +166,8 @@ struct atmel_uart_port {
>  	unsigned int		pending_status;
>  	spinlock_t		lock_suspended;
>  
> +	bool			hd_start_rx;	/* can start RX during half-duplex operation */
> +
>  	/* ISO7816 */
>  	unsigned int		fidi_min;
>  	unsigned int		fidi_max;
> @@ -933,8 +935,13 @@ static void atmel_complete_tx_dma(void *arg)
>  	if (!uart_circ_empty(xmit))
>  		atmel_tasklet_schedule(atmel_port, &atmel_port->tasklet_tx);
>  	else if (atmel_uart_is_half_duplex(port)) {
> -		/* DMA done, stop TX, start RX for RS485 */
> -		atmel_start_rx(port);
> +		/*
> +		 * DMA done, re-enable TXEMPTY and signal that we can stop
> +		 * TX and start RX for RS485
> +		 */
> +		atmel_port->hd_start_rx = true;
> +		atmel_uart_writel(port, ATMEL_US_IER,
> +				  atmel_port->tx_done_mask);
>  	}
>  
>  	spin_unlock_irqrestore(&port->lock, flags);
> @@ -1378,9 +1385,20 @@ atmel_handle_transmit(struct uart_port *port, unsigned int pending)
>  	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
>  
>  	if (pending & atmel_port->tx_done_mask) {
> -		/* Either PDC or interrupt transmission */
>  		atmel_uart_writel(port, ATMEL_US_IDR,
>  				  atmel_port->tx_done_mask);
> +
> +		/* Start RX if flag was set and FIFO is empty */
> +		if (atmel_port->hd_start_rx) {
> +			if (!(atmel_uart_readl(port, ATMEL_US_CSR)
> +					& ATMEL_US_TXEMPTY))
> +				dev_warn(port->dev, "Should start RX, but TX fifo is not empty\n");
> +
> +			atmel_port->hd_start_rx = false;
> +			atmel_start_rx(port);
> +			return;
> +		}
> +
>  		atmel_tasklet_schedule(atmel_port, &atmel_port->tasklet_tx);
>  	}
>  }
>