Patch "serial: samsung: fix DMA for FIFO smaller than cache line size" has been added to the 4.1-stable tree

PATCH [email protected] 1 participant 1 message Sep 17, 2015

Subsystems (3)

Stable branch Arm Samsung Exynos TTY layer and serial drivers

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2015-09-17 05:24 UTC #0 lore raw

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

serial: samsung: fix DMA for FIFO smaller than cache line size

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
serial-samsung-fix-dma-for-fifo-smaller-than-cache-line-size.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 736cd79f483fd7a1e0b71e6eaddf01d8d87fbbbb Mon Sep 17 00:00:00 2001

From: Robert Baldyga <[email protected]>
Date: Fri, 31 Jul 2015 10:58:28 +0200
Subject: serial: samsung: fix DMA for FIFO smaller than cache line size

From: Robert Baldyga <[email protected]>

commit 736cd79f483fd7a1e0b71e6eaddf01d8d87fbbbb upstream.

So far DMA mode were activated when only number of bytes to send was equal or greater than min_dma_size. Due to requirement that DMA transaction buffer should be aligned to cache line size, the excessive bytes were written to FIFO before starting DMA transaction. The problem occurred when FIFO size were smaller than cache alignment, because writing all excessive bytes to FIFO would fail. It happened in DMA mode when PIO interrupts disabled, which caused driver hung.

The solution is to test if buffer is alligned to cache line size before activating DMA mode, and if it's not, running PIO mode to align buffer and then starting DMA transaction. In PIO mode, when interrupts are enabled, lack of space in FIFO isn't the problem, so buffer aligning will always finish with success.

Reported-by Krzysztof Kozlowski <[email protected]>

Signed-off-by Robert Baldyga <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 drivers/tty/serial/samsung.c |   36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

drivers/tty/serial/samsung.c +21 ~~-15~~

@@ -295,15 +295,6 @@ static int s3c24xx_serial_start_tx_dma(s
 	if (ourport->tx_mode != S3C24XX_TX_DMA)
 		enable_tx_dma(ourport);
 
-	while (xmit->tail & (dma_get_cache_alignment() - 1)) {
-		if (rd_regl(port, S3C2410_UFSTAT) & ourport->info->tx_fifofull)
-			return 0;
-		wr_regb(port, S3C2410_UTXH, xmit->buf[xmit->tail]);
-		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
-		port->icount.tx++;
-		count--;
-	}
-
 	dma->tx_size = count & ~(dma_get_cache_alignment() - 1);
 	dma->tx_transfer_addr = dma->tx_addr + xmit->tail;

@@ -343,7 +334,8 @@ static void s3c24xx_serial_start_next_tx
 	}
 
 	if (!ourport->dma || !ourport->dma->tx_chan ||
-	    count < ourport->min_dma_size)
+	    count < ourport->min_dma_size ||
+	    xmit->tail & (dma_get_cache_alignment() - 1))
 		s3c24xx_serial_start_tx_pio(ourport);
 	else
 		s3c24xx_serial_start_tx_dma(ourport, count);

@@ -737,7 +729,7 @@ static irqreturn_t s3c24xx_serial_tx_cha
 	struct uart_port *port = &ourport->port;
 	struct circ_buf *xmit = &port->state->xmit;
 	unsigned long flags;
-	int count;
+	int count, dma_count = 0;
 
 	spin_lock_irqsave(&port->lock, flags);

@@ -745,8 +737,12 @@ static irqreturn_t s3c24xx_serial_tx_cha
 
 	if (ourport->dma && ourport->dma->tx_chan &&
 	    count >= ourport->min_dma_size) {
-		s3c24xx_serial_start_tx_dma(ourport, count);
-		goto out;
+		int align = dma_get_cache_alignment() -
+			(xmit->tail & (dma_get_cache_alignment() - 1));
+		if (count-align >= ourport->min_dma_size) {
+			dma_count = count-align;
+			count = align;
+		}
 	}
 
 	if (port->x_char) {

@@ -767,14 +763,24 @@ static irqreturn_t s3c24xx_serial_tx_cha
 
 	/* try and drain the buffer... */
 
-	count = port->fifosize;
-	while (!uart_circ_empty(xmit) && count-- > 0) {
+	if (count > port->fifosize) {
+		count = port->fifosize;
+		dma_count = 0;
+	}
+
+	while (!uart_circ_empty(xmit) && count > 0) {
 		if (rd_regl(port, S3C2410_UFSTAT) & ourport->info->tx_fifofull)
 			break;
 
 		wr_regb(port, S3C2410_UTXH, xmit->buf[xmit->tail]);
 		xmit->tail = (xmit->tail + 1) & (UART_XMIT_SIZE - 1);
 		port->icount.tx++;
+		count--;
+	}
+
+	if (!count && dma_count) {
+		s3c24xx_serial_start_tx_dma(ourport, dma_count);
+		goto out;
 	}
 
 	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/serial-samsung-fix-dma-for-fifo-smaller-than-cache-line-size.patch queue-4.1/serial-samsung-fix-dma-mode-enter-condition-for-small-fifo-sizes.patch