linux-kernel - [PATCH 3.16 210/370] tty/serial: atmel: RS485 half duplex w/DMA: enable RX after TX is done

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Message-ID: <lsq.1489146383.228358725@decadent.org.uk>
Date:   Fri, 10 Mar 2017 11:46:23 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Alexandre Belloni" <alexandre.belloni@...e-electrons.com>,
        "Richard Genoud" <richard.genoud@...il.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Gil Weber" <webergil@...il.com>
Subject: [PATCH 3.16 210/370] tty/serial: atmel: RS485 half duplex w/DMA:
 enable RX after TX is done

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Richard Genoud <richard.genoud@...il.com>

commit b389f173aaa1204d6dc1f299082a162eb0491545 upstream.

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.

Reported-by: Gil Weber <webergil@...il.com>
Fixes: 0058f0871efe7b01c6
Tested-by: Gil Weber <webergil@...il.com>
Signed-off-by: Richard Genoud <richard.genoud@...il.com>
Acked-by: Alexandre Belloni <alexandre.belloni@...e-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[bwh: Backported to 3.16:
 - s/port->rs485/atmel_port->rs485/
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/tty/serial/atmel_serial.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -734,6 +734,11 @@ static void atmel_complete_tx_dma(void *
 	/* Do we really need this? */
 	if (!uart_circ_empty(xmit))
 		tasklet_schedule(&atmel_port->tasklet);
+	else if ((atmel_port->rs485.flags & SER_RS485_ENABLED) &&
+		 !(atmel_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);
 }
@@ -806,12 +811,6 @@ static void atmel_tx_dma(struct uart_por
 		desc->callback = atmel_complete_tx_dma;
 		desc->callback_param = atmel_port;
 		atmel_port->cookie_tx = dmaengine_submit(desc);
-
-	} else {
-		if (atmel_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)