linux-kernel - [PATCH] tty/serial: atmel: RS485 half duplex w/DMA: enable RX after TX is done

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Date:   Tue,  6 Dec 2016 13:05:33 +0100
From:   Richard Genoud <richard.genoud@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Gil Weber <webergil@...il.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Richard Genoud <richard.genoud@...il.com>,
        stable@...r.kernel.org
Subject: [PATCH] tty/serial: atmel: RS485 half duplex w/DMA: enable RX after TX is done

When using RS485 in half duplex, RX should be enabled when TX is
finished, and stopped when TX starts.

Before commit 0058f0871efe7b01c6 ("tty/serial: atmel: fix RS485 half
duplex with DMA"), RX was not disabled in atmel_start_tx() if the DMA
was used. So, collisions could happened.

But disabling RX in atmel_start_tx() uncovered another bug:
RX was enabled again in the wrong place (in atmel_tx_dma) instead of
being enabled when TX is finished (in atmel_complete_tx_dma), so the
transmission simply stopped.

This bug was not triggered before commit 0058f0871efe7b01c6
("tty/serial: atmel: fix RS485 half duplex with DMA") because RX was
never disabled before.

Moving atmel_start_rx() in atmel_complete_tx_dma() corrects the problem.

Cc: stable@...r.kernel.org
Reported-by: Gil Weber <webergil@...il.com>
Tested-by: Gil Weber <webergil@...il.com>
Signed-off-by: Richard Genoud <richard.genoud@...il.com>
---
 drivers/tty/serial/atmel_serial.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 168b10cad47b..11c0117af80b 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -798,6 +798,11 @@ static void atmel_complete_tx_dma(void *arg)
 	 */
 	if (!uart_circ_empty(xmit))
 		atmel_tasklet_schedule(atmel_port, &atmel_port->tasklet_tx);
+	else if ((port->rs485.flags & SER_RS485_ENABLED) &&
+		 !(port->rs485.flags & SER_RS485_RX_DURING_TX)) {
+		/* DMA done, stop TX, start RX for RS485 */
+		atmel_start_rx(port);
+	}
 
 	spin_unlock_irqrestore(&port->lock, flags);
 }
@@ -900,12 +905,6 @@ static void atmel_tx_dma(struct uart_port *port)
 		desc->callback = atmel_complete_tx_dma;
 		desc->callback_param = atmel_port;
 		atmel_port->cookie_tx = dmaengine_submit(desc);
-
-	} else {
-		if (port->rs485.flags & SER_RS485_ENABLED) {
-			/* DMA done, stop TX, start RX for RS485 */
-			atmel_start_rx(port);
-		}
 	}
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)