serial: 8250_dma: don't bother DMA with small transfers

If we would like to send amount of data less than FIFO size we better would do
this via PIO mode. Otherwise the overhead could be significant.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index e508939daea3f3128a2ada1e696a5f751d212077..21d01a491405a2c52cd1ec02f3198fa4c6b8f10c 100644 (file)
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -80,6 +80,10 @@ int serial8250_tx_dma(struct uart_8250_port *p)
                return 0;
 
        dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
+       if (dma->tx_size < p->port.fifosize) {
+               ret = -EINVAL;
+               goto err;
+       }
 
        desc = dmaengine_prep_slave_single(dma->txchan,
                                           dma->tx_addr + xmit->tail,