serial: tegra: check the count and read if any from dma

It is only necessary to read data from the dma buffer when the count
value is non-zero and hence, tegra_uart_copy_rx_to_tty() so only be
called when this is the case.

Although, this was being tested for in two places, there is a third
place where this was not tested. However, instead of adding another
if-statement prior to calling tegra_uart_copy_rx_to_tty(), move the test
inside the function.

Signed-off-by: Shardar Shariff Md <smohammed@nvidia.com>
[jonathanh@nvidia.com: Re-worked patch to move the check for the count
 value inside the function tegra_uart_copy_rx_to_tty(). Updated
 changelog with more commentary.]
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial-tegra.c

@@ -554,6 +554,10 @@ static void tegra_uart_copy_rx_to_tty(struct tegra_uart_port *tup,
 {
 	int copied;
 
+	/* If count is zero, then there is no data to be copied */
+	if (!count)
+		return;
+
 	tup->uport.icount.rx += count;
 	if (!tty) {
 		dev_err(tup->uport.dev, "No tty port\n");
@@ -588,8 +592,7 @@ static void tegra_uart_rx_dma_complete(void *args)
 		set_rts(tup, false);
 
 	/* If we are here, DMA is stopped */
-	if (count)
-		tegra_uart_copy_rx_to_tty(tup, port, count);
+	tegra_uart_copy_rx_to_tty(tup, port, count);
 
 	tegra_uart_handle_rx_pio(tup, port);
 	if (tty) {
@@ -626,8 +629,7 @@ static void tegra_uart_handle_rx_dma(struct tegra_uart_port *tup,
 	count = tup->rx_bytes_requested - state.residue;
 
 	/* If we are here, DMA is stopped */
-	if (count)
-		tegra_uart_copy_rx_to_tty(tup, port, count);
+	tegra_uart_copy_rx_to_tty(tup, port, count);
 
 	tegra_uart_handle_rx_pio(tup, port);
 	if (tty) {