[PATCH] tty: serial: pch_uart: correct usage of dma_unmap_sg

[Peng Fan]

Per Documentation/DMA-API-HOWTO.txt,
To unmap a scatterlist, just call:
	dma_unmap_sg(dev, sglist, nents, direction);

.. note::

	The 'nents' argument to the dma_unmap_sg call must be
	the _same_ one you passed into the dma_map_sg call,
	it should _NOT_ be the 'count' value _returned_ from the
	dma_map_sg call.

However in the driver, priv->nent is directly assigned with value
returned from dma_map_sg, and dma_unmap_sg use priv->nent for unmap,
this breaks the API usage.

So introduce a new entry orig_nent to remember 'nents'.

Fixes: da3564ee027e("pch_uart: add multi-scatter processing")
Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
---

V1:
 Code inspection, not tested.

 drivers/tty/serial/pch_uart.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/pch_uart.c b/drivers/tty/serial/pch_uart.c
index 6157213a8359..c16234bca78f 100644
--- a/drivers/tty/serial/pch_uart.c
+++ b/drivers/tty/serial/pch_uart.c
@@ -233,6 +233,7 @@ struct eg20t_port {
 	struct dma_chan			*chan_rx;
 	struct scatterlist		*sg_tx_p;
 	int				nent;
+	int				orig_nent;
 	struct scatterlist		sg_rx;
 	int				tx_dma_use;
 	void				*rx_buf_virt;
@@ -787,9 +788,10 @@ static void pch_dma_tx_complete(void *arg)
 	}
 	xmit->tail &= UART_XMIT_SIZE - 1;
 	async_tx_ack(priv->desc_tx);
-	dma_unmap_sg(port->dev, sg, priv->nent, DMA_TO_DEVICE);
+	dma_unmap_sg(port->dev, sg, priv->orig_nent, DMA_TO_DEVICE);
 	priv->tx_dma_use = 0;
 	priv->nent = 0;
+	priv->orig_nent = 0;
 	kfree(priv->sg_tx_p);
 	pch_uart_hal_enable_interrupt(priv, PCH_UART_HAL_TX_INT);
 }
@@ -1010,6 +1012,7 @@ static unsigned int dma_handle_tx(struct eg20t_port *priv)
 		dev_err(priv->port.dev, "%s:dma_map_sg Failed\n", __func__);
 		return 0;
 	}
+	priv->orig_nent = num;
 	priv->nent = nent;
 
 	for (i = 0; i < nent; i++, sg++) {
-- 
2.16.4