serial: Refactor uart_flush_buffer() from uart_close()

In the context of the final tty & port close, flushing the tx
ring buffer after the hardware has already been shutdown and
the ring buffer freed is neither required nor desirable.

uart_flush_buffer() performs 3 operations:
1. Resets tx ring buffer indices, but the tx ring buffer has
already been freed and the indices are reset if the port is
re-opened.
2. Calls uart driver's flush_buffer() method
5 in-tree uart drivers define flush_buffer() methods:
amba-pl011, atmel-serial, imx, serial-tegra, timbuart
These have been refactored into the shutdown() method, if
required.
3. Kicks the ldisc for more writing, but this is undesirable.
The file handle is being released; any waiting writer will
will be kicked out by tty_release() with a warning. Further,
the N_TTY ldisc may generate SIGIO for a file handle which
is no longer valid.

Cc: Nicolas Ferre <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Laxman Dewangan <[e-mail]>
Signed-off-by: Peter Hurley <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/amba-pl011.c

···
 			plat->exit();
 	}
 
+	pl011_dma_flush_buffer(port);
 }
 
 static void

drivers/tty/serial/atmel_serial.c

···
 }
 
 /*
+ * Flush any TX data submitted for DMA. Called when the TX circular
+ * buffer is reset.
+ */
+static void atmel_flush_buffer(struct uart_port *port)
+{
+	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
+
+	if (atmel_use_pdc_tx(port)) {
+		UART_PUT_TCR(port, 0);
+		atmel_port->pdc_tx.ofs = 0;
+	}
+}
+
+/*
  * Disable the port
  */
 static void atmel_shutdown(struct uart_port *port)
···
 	atmel_free_gpio_irq(port);
 
 	atmel_port->ms_irq_enabled = false;
-}
 
-/*
- * Flush any TX data submitted for DMA. Called when the TX circular
- * buffer is reset.
- */
-static void atmel_flush_buffer(struct uart_port *port)
-{
-	struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
-
-	if (atmel_use_pdc_tx(port)) {
-		UART_PUT_TCR(port, 0);
-		atmel_port->pdc_tx.ofs = 0;
-	}
+	atmel_flush_buffer(port);
 }
 
 /*

drivers/tty/serial/serial-tegra.c

···
 	return ret;
 }
 
+/*
+ * Flush any TX data submitted for DMA and PIO. Called when the
+ * TX circular buffer is reset.
+ */
+static void tegra_uart_flush_buffer(struct uart_port *u)
+{
+	struct tegra_uart_port *tup = to_tegra_uport(u);
+
+	tup->tx_bytes = 0;
+	if (tup->tx_dma_chan)
+		dmaengine_terminate_all(tup->tx_dma_chan);
+	return;
+}
+
 static void tegra_uart_shutdown(struct uart_port *u)
 {
 	struct tegra_uart_port *tup = to_tegra_uport(u);
···
 	tegra_uart_dma_channel_free(tup, true);
 	tegra_uart_dma_channel_free(tup, false);
 	free_irq(u->irq, tup);
+
+	tegra_uart_flush_buffer(u);
 }
 
 static void tegra_uart_enable_ms(struct uart_port *u)
···
 	tegra_uart_read(tup, UART_IER);
 
 	spin_unlock_irqrestore(&u->lock, flags);
-	return;
-}
-
-/*
- * Flush any TX data submitted for DMA and PIO. Called when the
- * TX circular buffer is reset.
- */
-static void tegra_uart_flush_buffer(struct uart_port *u)
-{
-	struct tegra_uart_port *tup = to_tegra_uport(u);
-
-	tup->tx_bytes = 0;
-	if (tup->tx_dma_chan)
-		dmaengine_terminate_all(tup->tx_dma_chan);
 	return;
 }
 

drivers/tty/serial/serial_core.c

···
 
 	mutex_lock(&port->mutex);
 	uart_shutdown(tty, state);
-	uart_flush_buffer(tty);
 
 	tty_ldisc_flush(tty);
 

drivers/tty/serial/timbuart.c

···
 	dev_dbg(port->dev, "%s\n", __func__);
 	free_irq(port->irq, uart);
 	iowrite32(0, port->membase + TIMBUART_IER);
+
+	timbuart_flush_buffer(port);
 }
 
 static int get_bindex(int baud)