linux-kernel - [PATCH 4.14 008/190] serial: imx: Fix handling of TC irq in combination with DMA

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Message-Id: <20200619141633.883957087@linuxfoundation.org>
Date:   Fri, 19 Jun 2020 16:30:53 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Frieder Schrempf <frieder.schrempf@...tron.de>
Subject: [PATCH 4.14 008/190] serial: imx: Fix handling of TC irq in combination with DMA

From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

commit 1866541492641c02874bf51f9d8712b5510f2c64 upstream.

When using RS485 half duplex the Transmitter Complete irq is needed to
determine the moment when the transmitter can be disabled. When using
DMA this irq must only be enabled when DMA has completed to transfer all
data. Otherwise the CPU might busily trigger this irq which is not
properly handled and so the also pending irq for the DMA transfer cannot
trigger.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
[Backport to v4.14]
Signed-off-by: Frieder Schrempf <frieder.schrempf@...tron.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/tty/serial/imx.c |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -538,6 +538,11 @@ static void dma_tx_callback(void *data)
 
 	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&sport->port))
 		imx_dma_tx(sport);
+	else if (sport->port.rs485.flags & SER_RS485_ENABLED) {
+		temp = readl(sport->port.membase + UCR4);
+		temp |= UCR4_TCEN;
+		writel(temp, sport->port.membase + UCR4);
+	}
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 }
@@ -555,6 +560,10 @@ static void imx_dma_tx(struct imx_port *
 	if (sport->dma_is_txing)
 		return;
 
+	temp = readl(sport->port.membase + UCR4);
+	temp &= ~UCR4_TCEN;
+	writel(temp, sport->port.membase + UCR4);
+
 	sport->tx_bytes = uart_circ_chars_pending(xmit);
 
 	if (xmit->tail < xmit->head || xmit->head == 0) {
@@ -617,10 +626,15 @@ static void imx_start_tx(struct uart_por
 		if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
 			imx_stop_rx(port);
 
-		/* enable transmitter and shifter empty irq */
-		temp = readl(port->membase + UCR4);
-		temp |= UCR4_TCEN;
-		writel(temp, port->membase + UCR4);
+		/*
+		 * Enable transmitter and shifter empty irq only if DMA is off.
+		 * In the DMA case this is done in the tx-callback.
+		 */
+		if (!sport->dma_is_enabled) {
+			temp = readl(port->membase + UCR4);
+			temp |= UCR4_TCEN;
+			writel(temp, port->membase + UCR4);
+		}
 	}
 
 	if (!sport->dma_is_enabled) {