[PATCH] serial: 8250_dma: sync regions for device before transfer

[PATCH] serial: 8250_dma: sync regions for device before transfer

[Andy Shevchenko]

The documentation says that we have to sync DMA regions after CPU access and
before any DMA transfer is going on. The actual DMA for buffer happens when we
call dmaengine_submit() (*). The original code syncs data after this call which
might lead to the corrupted data in actual stream.

(*) Accordingly to the Documentation/dmaengine.txt the dmaengine_submit call
shouldn't do the transfer, but some DMA controller drivers abuse this rule.
Thus, we would like to have this patch until all DMA controller drivers will be
fixed. Even after that it will be harmless.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/tty/serial/8250/8250_dma.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index 148ffe4..f70ee29 100644
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -76,6 +76,8 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 
 	dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
 
+	dma_sync_single_for_device(dma->txchan->device->dev, dma->tx_addr,
+				   UART_XMIT_SIZE, DMA_TO_DEVICE);
 	desc = dmaengine_prep_slave_single(dma->txchan,
 					   dma->tx_addr + xmit->tail,
 					   dma->tx_size, DMA_MEM_TO_DEV,
@@ -90,9 +92,6 @@ int serial8250_tx_dma(struct uart_8250_port *p)
 
 	dma->tx_cookie = dmaengine_submit(desc);
 
-	dma_sync_single_for_device(dma->txchan->device->dev, dma->tx_addr,
-				   UART_XMIT_SIZE, DMA_TO_DEVICE);
-
 	dma_async_issue_pending(dma->txchan);
 
 	return 0;
@@ -129,6 +128,8 @@ int serial8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
 	if (dma_status)
 		return 0;
 
+	dma_sync_single_for_device(dma->rxchan->device->dev, dma->rx_addr,
+				   dma->rx_size, DMA_FROM_DEVICE);
 	desc = dmaengine_prep_slave_single(dma->rxchan, dma->rx_addr,
 					   dma->rx_size, DMA_DEV_TO_MEM,
 					   DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
@@ -140,9 +141,6 @@ int serial8250_rx_dma(struct uart_8250_port *p, unsigned int iir)
 
 	dma->rx_cookie = dmaengine_submit(desc);
 
-	dma_sync_single_for_device(dma->rxchan->device->dev, dma->rx_addr,
-				   dma->rx_size, DMA_FROM_DEVICE);
-
 	dma_async_issue_pending(dma->rxchan);
 
 	return 0;
-- 
2.0.0

Re: [PATCH] serial: 8250_dma: sync regions for device before transfer

[Heikki Krogerus]

Hi Andy,

On Thu, Jun 12, 2014 at 06:51:18PM +0300, Andy Shevchenko wrote:
> The documentation says that we have to sync DMA regions after CPU access and
> before any DMA transfer is going on. The actual DMA for buffer happens when we
> call dmaengine_submit() (*). The original code syncs data after this call which
> might lead to the corrupted data in actual stream.
> 
> (*) Accordingly to the Documentation/dmaengine.txt the dmaengine_submit call
> shouldn't do the transfer, but some DMA controller drivers abuse this rule.
> Thus, we would like to have this patch until all DMA controller drivers will be
> fixed. Even after that it will be harmless.

It's probable better that we directly fix the DMA engine drivers
instead of working around the issue here.


Thanks,

-- 
heikki