tty: serial: fsl_lpuart: avoid TX FIFO overflow

In some rare cases, the TX DMA gets started while there are still
data in the TX FIFO and the Transmit Data Register Empty Flag
(TDRE) is asserted. In this case, the DMA writes data and overflows
the TX FIFO. It is not quite clear why the TDRE is asserted in this
case, reading the status register which should clear the flag did
not alleviate the problem. It seems that the switching between
DMA and interrupt mode leads to this glitches.

Avoid TX FIFO overrun by using a burst size of 1, which allows to
transfer all data using DMA and avoid switching between DMA and
interrupt mode entirely.

1 files changed, 13 insertions, 53 deletions

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 921f68a85b5c..a4531a159a52 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -342,38 +342,19 @@ static void lpuart_copy_rx_to_tty(struct lpuart_port *sport,
 			FSL_UART_RX_DMA_BUFFER_SIZE, DMA_TO_DEVICE);
 }
 
-static void lpuart_pio_tx(struct lpuart_port *sport)
-{
-	struct circ_buf *xmit = &sport->port.state->xmit;
-	unsigned long flags;
-
-	spin_lock_irqsave(&sport->port.lock, flags);
-
-	while (!uart_circ_empty(xmit) &&
-		readb(sport->port.membase + UARTTCFIFO) < sport->txfifo_size) {
-		writeb(xmit->buf[xmit->tail], sport->port.membase + UARTDR);
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		sport->port.icount.tx++;
-	}
-
-	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
-		uart_write_wakeup(&sport->port);
-
-	if (uart_circ_empty(xmit))
-		writeb(readb(sport->port.membase + UARTCR5) | UARTCR5_TDMAS,
-			sport->port.membase + UARTCR5);
-
-	spin_unlock_irqrestore(&sport->port.lock, flags);
-}
-
-static int lpuart_dma_tx(struct lpuart_port *sport, unsigned long count)
+static int lpuart_dma_tx(struct lpuart_port *sport)
 {
 	struct circ_buf *xmit = &sport->port.state->xmit;
+	unsigned long count = CIRC_CNT_TO_END(xmit->head,
+					xmit->tail, UART_XMIT_SIZE);
 	dma_addr_t tx_bus_addr;
 
+	if (!count)
+		return 0;
+
 	dma_sync_single_for_device(sport->port.dev, sport->dma_tx_buf_bus,
 				UART_XMIT_SIZE, DMA_TO_DEVICE);
-	sport->dma_tx_bytes = count & ~(sport->txfifo_size - 1);
+	sport->dma_tx_bytes = count;
 	tx_bus_addr = sport->dma_tx_buf_bus + xmit->tail;
 	sport->dma_tx_desc = dmaengine_prep_slave_single(sport->dma_tx_chan,
 					tx_bus_addr, sport->dma_tx_bytes,
@@ -393,25 +374,6 @@ static int lpuart_dma_tx(struct lpuart_port *sport, unsigned long count)
 	return 0;
 }
 
-static void lpuart_prepare_tx(struct lpuart_port *sport)
-{
-	struct circ_buf *xmit = &sport->port.state->xmit;
-	unsigned long count =  CIRC_CNT_TO_END(xmit->head,
-					xmit->tail, UART_XMIT_SIZE);
-
-	if (!count)
-		return;
-
-	if (count < sport->txfifo_size)
-		writeb(readb(sport->port.membase + UARTCR5) & ~UARTCR5_TDMAS,
-				sport->port.membase + UARTCR5);
-	else {
-		writeb(readb(sport->port.membase + UARTCR5) | UARTCR5_TDMAS,
-				sport->port.membase + UARTCR5);
-		lpuart_dma_tx(sport, count);
-	}
-}
-
 static void lpuart_dma_tx_complete(void *arg)
 {
 	struct lpuart_port *sport = arg;
@@ -428,7 +390,7 @@ static void lpuart_dma_tx_complete(void *arg)
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
 		uart_write_wakeup(&sport->port);
 
-	lpuart_prepare_tx(sport);
+	lpuart_dma_tx(sport);
 
 	spin_unlock_irqrestore(&sport->port.lock, flags);
 }
@@ -585,7 +547,7 @@ static void lpuart_start_tx(struct uart_port *port)
 
 	if (sport->lpuart_dma_tx_use) {
 		if (!uart_circ_empty(xmit) && !sport->dma_tx_in_progress)
-			lpuart_prepare_tx(sport);
+			lpuart_dma_tx(sport);
 	} else {
 		if (readb(port->membase + UARTSR1) & UARTSR1_TDRE)
 			lpuart_transmit_buffer(sport);
@@ -782,10 +744,8 @@ static irqreturn_t lpuart_int(int irq, void *dev_id)
 			lpuart_rxint(irq, dev_id);
 	}
 	if (sts & UARTSR1_TDRE && !(crdma & UARTCR5_TDMAS)) {
-		if (sport->lpuart_dma_tx_use)
-			lpuart_pio_tx(sport);
-		else
-			lpuart_txint(irq, dev_id);
+		BUG_ON(sport->lpuart_dma_tx_use);
+		lpuart_txint(irq, dev_id);
 	}
 
 	return IRQ_HANDLED;
@@ -935,7 +895,7 @@ static void lpuart_setup_watermark(struct lpuart_port *sport)
 		writeb(UARTSFIFO_RXUF, sport->port.membase + UARTSFIFO);
 	}
 
-	writeb(0, sport->port.membase + UARTTWFIFO);
+	writeb(sport->txfifo_size / 2, sport->port.membase + UARTTWFIFO);
 	writeb(1, sport->port.membase + UARTRWFIFO);
 
 	/* Restore cr2 */
@@ -988,7 +948,7 @@ static int lpuart_dma_tx_request(struct uart_port *port)
 	dma_buf = sport->port.state->xmit.buf;
 	dma_tx_sconfig.dst_addr = sport->port.mapbase + UARTDR;
 	dma_tx_sconfig.dst_addr_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
-	dma_tx_sconfig.dst_maxburst = sport->txfifo_size;
+	dma_tx_sconfig.dst_maxburst = 1;
 	dma_tx_sconfig.direction = DMA_MEM_TO_DEV;
 	ret = dmaengine_slave_config(sport->dma_tx_chan, &dma_tx_sconfig);