[PATCH 4.2 083/120] serial: samsung: fix DMA mode enter condition for small FIFO sizes

Greg Kroah-Hartman
2015-09-19T18:05:05Z

From: Marek Szyprowski <m.szyprowski@samsung.com>

commit 81ccb2a69f76b88295a1da9fc9484df715fe3bfa upstream.

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.

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>

---
drivers/tty/serial/samsung.c | 13 +++++++++++--
drivers/tty/serial/samsung.h | 1 +
2 files changed, 12 insertions(+), 2 deletions(-)

--- a/drivers/tty/serial/samsung.c
+++ b/drivers/tty/serial/samsung.c
@@ -342,7 +342,8 @@ static void s3c24xx_serial_start_next_tx
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);
@@ -742,7 +743,8 @@ static irqreturn_t s3c24xx_serial_tx_cha

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;
}
@@ -1838,6 +1840,13 @@ static int s3c24xx_serial_probe(struct p
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);
--- 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;