serial: 8250_dma: don't bother DMA with small transfers
authorAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Thu, 22 Jan 2015 12:40:27 +0000 (14:40 +0200)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Mon, 2 Feb 2015 18:11:26 +0000 (10:11 -0800)
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>
drivers/tty/serial/8250/8250_dma.c

index e508939daea3f3128a2ada1e696a5f751d212077..21d01a491405a2c52cd1ec02f3198fa4c6b8f10c 100644 (file)
@@ -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,