tty: serial: fsl_lpuart: Fix Tx DMA edge case

In the case where head == 0 on the circular buffer, there should be one
DMA buffer, not two.  The second zero-length buffer would break the
lpuart driver, transfer would never complete.

Signed-off-by: Aaron Brice <aaron.brice@datasoft.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Tested-by: Stefan Agner <stefan@agner.ch>
Tested-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index de9d5107c00a..76103f2c4a80 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -328,7 +328,7 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
 
         sport->dma_tx_bytes = uart_circ_chars_pending(xmit);
 
-        if (xmit->tail < xmit->head) {
+        if (xmit->tail < xmit->head || xmit->head == 0) {
                 sport->dma_tx_nents = 1;
                 sg_init_one(sgl, xmit->buf + xmit->tail, sport->dma_tx_bytes);
         } else {
@@ -359,7 +359,6 @@ static void lpuart_dma_tx(struct lpuart_port *sport)
         sport->dma_tx_in_progress = true;
         sport->dma_tx_cookie = dmaengine_submit(sport->dma_tx_desc);
         dma_async_issue_pending(sport->dma_tx_chan);
-
 }
 
 static void lpuart_dma_tx_complete(void *arg)