serial: samsung: fix DMA mode enter condition for small FIFO sizes

Due to some of serial ports can have FIFO size smaller than cache line
size, and because of need to align DMA buffer address to cache line size,
it's necessary to calculate minimum number of bytes for which we want
to start DMA transaction to be at least cache line size. The simplest
way to meet this requirement is to get maximum of cache line size and
FIFO size.

Cc: <stable@vger.kernel.org> # v3.18+
Reported-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
index 8c963d6d90747f8b615c4c9a3696dbdb4e474f7b..faee021c27b033a1d3ccb9516459e88cc45899e7 100644 (file)
--- a/drivers/tty/serial/samsung.c
+++ b/drivers/tty/serial/samsung.c
@@ -341,7 +341,8 @@ static void s3c24xx_serial_start_next_tx(struct s3c24xx_uart_port *ourport)
                return;
        }
 
-       if (!ourport->dma || !ourport->dma->tx_chan || count < port->fifosize)
+       if (!ourport->dma || !ourport->dma->tx_chan ||
+           count < ourport->min_dma_size)
                s3c24xx_serial_start_tx_pio(ourport);
        else
                s3c24xx_serial_start_tx_dma(ourport, count);
@@ -741,7 +742,8 @@ static irqreturn_t s3c24xx_serial_tx_chars(int irq, void *id)
 
        count = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
 
-       if (ourport->dma && ourport->dma->tx_chan && count >= port->fifosize) {
+       if (ourport->dma && ourport->dma->tx_chan &&
+           count >= ourport->min_dma_size) {
                s3c24xx_serial_start_tx_dma(ourport, count);
                goto out;
        }
@@ -1837,6 +1839,13 @@ static int s3c24xx_serial_probe(struct platform_device *pdev)
        else if (ourport->info->fifosize)
                ourport->port.fifosize = ourport->info->fifosize;
 
+       /*
+        * DMA transfers must be aligned at least to cache line size,
+        * so find minimal transfer size suitable for DMA mode
+        */
+       ourport->min_dma_size = max_t(int, ourport->port.fifosize,
+                                   dma_get_cache_alignment());
+
        probe_index++;
 
        dbg("%s: initialising port %p...\n", __func__, ourport);

diff --git a/drivers/tty/serial/samsung.h b/drivers/tty/serial/samsung.h
index d275032aa68d47ee68c42a0370ff7eecc42fb192..fc5deaa4f382def3e5d1bfe69497302a6c3a4abb 100644 (file)
--- a/drivers/tty/serial/samsung.h
+++ b/drivers/tty/serial/samsung.h
@@ -82,6 +82,7 @@ struct s3c24xx_uart_port {
        unsigned char                   tx_claimed;
        unsigned int                    pm_level;
        unsigned long                   baudclk_rate;
+       unsigned int                    min_dma_size;
 
        unsigned int                    rx_irq;
        unsigned int                    tx_irq;