atmel_serial: avoid stopping pdc during transmission

I found a problem related to losing data during pdc transmission in
atmel_serial: connect ttyS1 with ttyS2 using a loopback cable, send 30
byte of packet from one to the other and waiting for 30 byte.  On the
other side just read and echo the data received.

We always call atmel_tx_dma() from the tasklet regardless of what interrupt
triggered it.
Signed-off-by: Nmichael <trimarchi@gandalf.sssup.it>
Acked-by: NHaavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/serial/atmel_serial.c

@@ -96,6 +96,7 @@
 
  /* PDC registers */
 #define UART_PUT_PTCR(port,v)	__raw_writel(v, (port)->membase + ATMEL_PDC_PTCR)
+#define UART_GET_TCR(port)      __raw_readl((port)->membase + ATMEL_PDC_TCR)
 #define UART_GET_PTSR(port)	__raw_readl((port)->membase + ATMEL_PDC_PTSR)
 
 #define UART_PUT_RPR(port,v)	__raw_writel(v, (port)->membase + ATMEL_PDC_RPR)
@@ -562,17 +563,22 @@ static void atmel_tx_dma(struct uart_port *port)
 	struct atmel_dma_buffer *pdc = &atmel_port->pdc_tx;
 	int count;
 
+	/* nothing left to transmit? */
+	if (UART_GET_TCR(port))
+		return;
+
 	xmit->tail += pdc->ofs;
 	xmit->tail &= UART_XMIT_SIZE - 1;
 
 	port->icount.tx += pdc->ofs;
 	pdc->ofs = 0;
 
-	if (!uart_circ_empty(xmit)) {
 	/* more to transmit - setup next transfer */
 
 	/* disable PDC transmit */
 	UART_PUT_PTCR(port, ATMEL_PDC_TXTDIS);
+
+	if (!uart_circ_empty(xmit)) {
 		dma_sync_single_for_device(port->dev,
 					   pdc->dma_addr,
 					   pdc->dma_size,
@@ -586,11 +592,6 @@ static void atmel_tx_dma(struct uart_port *port)
 		/* re-enable PDC transmit and interrupts */
 		UART_PUT_PTCR(port, ATMEL_PDC_TXTEN);
 		UART_PUT_IER(port, ATMEL_US_ENDTX | ATMEL_US_TXBUFE);
-	} else {
-		/* nothing left to transmit - disable the transmitter */
-
-		/* disable PDC transmit */
-		UART_PUT_PTCR(port, ATMEL_PDC_TXTDIS);
 	}
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)