linux-kernel - [PATCH 4.4 28/48] tty/serial: atmel_serial: BUG: stop DMA from transmitting in stop

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Date:   Wed, 18 Jan 2017 11:46:37 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Richard Genoud <richard.genoud@...il.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>
Subject: [PATCH 4.4 28/48] tty/serial: atmel_serial: BUG: stop DMA from transmitting in stop_tx

4.4-stable review patch.  If anyone has any objections, please let me know.

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

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

commit 89d8232411a85b9a6b12fd5da4d07d8a138a8e0c upstream.

If we don't disable the transmitter in atmel_stop_tx, the DMA buffer
continues to send data until it is emptied.
This cause problems with the flow control (CTS is asserted and data are
still sent).

So, disabling the transmitter in atmel_stop_tx is a sane thing to do.

Tested on at91sam9g35-cm(DMA)
Tested for regressions on sama5d2-xplained(Fifo) and at91sam9g20ek(PDC)

Signed-off-by: Richard Genoud <richard.genoud@...il.com>
Acked-by: Nicolas Ferre <nicolas.ferre@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/tty/serial/atmel_serial.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -470,6 +470,14 @@ static void atmel_stop_tx(struct uart_po
 		/* disable PDC transmit */
 		atmel_uart_writel(port, ATMEL_PDC_PTCR, ATMEL_PDC_TXTDIS);
 	}
+
+	/*
+	 * Disable the transmitter.
+	 * This is mandatory when DMA is used, otherwise the DMA buffer
+	 * is fully transmitted.
+	 */
+	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXDIS);
+
 	/* Disable interrupts */
 	atmel_uart_writel(port, ATMEL_US_IDR, atmel_port->tx_done_mask);
 
@@ -502,6 +510,9 @@ static void atmel_start_tx(struct uart_p
 
 	/* Enable interrupts */
 	atmel_uart_writel(port, ATMEL_US_IER, atmel_port->tx_done_mask);
+
+	/* re-enable the transmitter */
+	atmel_uart_writel(port, ATMEL_US_CR, ATMEL_US_TXEN);
 }
 
 /*