imx-serial: Reduce RX DMA startup latency when opening for reading

Reduce RX DMA start latency for the first reception when port is opened
for reading. Instead of waiting for an interrupt signaling data on RX
FIFO or data too old on RX FIFO, start RX DMA immediately when the
serial port is opened for reading.

Before this patch, the average RX DMA latency for the first reception
was 42489 microseconds with a standard deviation of 25721 microseconds
in 36 samples.

After the patch the average RX DMA latency for the first reception, when
the serial port is opened for reading, is 653 microseconds with a
standard deviation of 294 microseconds in 36 samples.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -719,6 +719,27 @@ out:
 	return IRQ_HANDLED;
 }
 
+static void imx_disable_rx_int(struct imx_port *sport)
+{
+	unsigned long temp;
+
+	sport->dma_is_rxing = 1;
+
+	/* disable the receiver ready and aging timer interrupts */
+	temp = readl(sport->port.membase + UCR1);
+	temp &= ~(UCR1_RRDYEN);
+	writel(temp, sport->port.membase + UCR1);
+
+	temp = readl(sport->port.membase + UCR2);
+	temp &= ~(UCR2_ATEN);
+	writel(temp, sport->port.membase + UCR2);
+
+	/* disable the rx errors interrupts */
+	temp = readl(sport->port.membase + UCR4);
+	temp &= ~UCR4_OREN;
+	writel(temp, sport->port.membase + UCR4);
+}
+
 static void clear_rx_errors(struct imx_port *sport);
 static int start_rx_dma(struct imx_port *sport);
 /*
@@ -734,21 +755,8 @@ static void imx_dma_rxint(struct imx_port *sport)
 
 	temp = readl(sport->port.membase + USR2);
 	if ((temp & USR2_RDR) && !sport->dma_is_rxing) {
-		sport->dma_is_rxing = 1;
 
-		/* disable the receiver ready and aging timer interrupts */
-		temp = readl(sport->port.membase + UCR1);
-		temp &= ~(UCR1_RRDYEN);
-		writel(temp, sport->port.membase + UCR1);
-
-		temp = readl(sport->port.membase + UCR2);
-		temp &= ~(UCR2_ATEN);
-		writel(temp, sport->port.membase + UCR2);
-
-		/* disable the rx errors interrupts */
-		temp = readl(sport->port.membase + UCR4);
-		temp &= ~UCR4_OREN;
-		writel(temp, sport->port.membase + UCR4);
+		imx_disable_rx_int(sport);
 
 		/* tell the DMA to receive the data. */
 		start_rx_dma(sport);
@@ -1330,6 +1338,33 @@ static int imx_startup(struct uart_port *port)
 	 * Enable modem status interrupts
 	 */
 	imx_enable_ms(&sport->port);
+
+	/*
+	 * If the serial port is opened for reading start RX DMA immediately
+	 * instead of waiting for RX FIFO interrupts. In our iMX53 the average
+	 * delay for the first reception dropped from approximately 35000
+	 * microseconds to 1000 microseconds.
+	 */
+	if (sport->dma_is_enabled) {
+		struct tty_struct *tty = sport->port.state->port.tty;
+		struct tty_file_private *file_priv;
+		int readcnt = 0;
+
+		spin_lock(&tty->files_lock);
+
+		if (!list_empty(&tty->tty_files))
+			list_for_each_entry(file_priv, &tty->tty_files, list)
+				if (!(file_priv->file->f_flags & O_WRONLY))
+					readcnt++;
+
+		spin_unlock(&tty->files_lock);
+
+		if (readcnt > 0) {
+			imx_disable_rx_int(sport);
+			start_rx_dma(sport);
+		}
+	}
+
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 
 	return 0;