patch "serial: samsung: fix DMA mode enter condition for small FIFO sizes" added to tty-next

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

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

to my tty git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-next branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will also be merged in the next major kernel release
during the merge window.

If you have any questions about this process, please let me know.


>From 81ccb2a69f76b88295a1da9fc9484df715fe3bfa Mon Sep 17 00:00:00 2001
From: Marek Szyprowski <m.szyprowski@samsung.com>
Date: Fri, 31 Jul 2015 10:58:27 +0200
Subject: 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>
---
 drivers/tty/serial/samsung.c | 13 +++++++++++--
 drivers/tty/serial/samsung.h |  1 +
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/samsung.c b/drivers/tty/serial/samsung.c
index 8c963d6d9074..faee021c27b0 100644
--- 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 d275032aa68d..fc5deaa4f382 100644
--- 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;
-- 
2.5.0