serial: 8250_dma: use linear buffer for transmit

8250_dma used the circular xmit->buf as DMA output buffer. This causes messages that wrap around
in the circular buffer to be transmitted using 2 DMA transfers. Depending on baud rate and processor
load this can cause an interchar gap in the middle of the message. On the receiving end the gap
may cause a short receive timeout, possibly long enough to terminate a DMA transfer, but too short
to restart a receive DMA transfer in time thus causing a receive buffer overrun.
Fix this but creating a linear tx_buffer and copying all of xmit->buf into it.

Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>

drivers/tty/serial/8250/8250.h

@@ -41,6 +41,7 @@ struct uart_8250_dma {
  dma_cookie_t tx_cookie;
 
  void *rx_buf;
+ void *tx_buf;
 
  size_t rx_size;
  size_t tx_size;

drivers/tty/serial/8250/8250_dma.c

@@ -8,6 +8,7 @@
 #include <linux/tty_flip.h>
 #include <linux/serial_reg.h>
 #include <linux/dma-mapping.h>
+#include <linux/slab.h>
 
 #include "8250.h"
 
@@ -65,6 +66,9 @@ int serial8250_tx_dma(struct uart_8250_port *p)
  struct circ_buf *xmit = &p->port.state->xmit;
  struct dma_async_tx_descriptor *desc;
  int ret;
+ size_t chunk1;
+ size_t chunk2;
+ int head;
 
  if (dma->tx_running)
  return 0;
@@ -75,10 +79,18 @@ int serial8250_tx_dma(struct uart_8250_port *p)
  return 0;
  }
 
- dma->tx_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
+ head = xmit->head;
+ chunk1 = CIRC_CNT_TO_END(head, xmit->tail, UART_XMIT_SIZE);
+ memcpy(dma->tx_buf, xmit->buf + xmit->tail, chunk1);
+
+ chunk2 = CIRC_CNT(head, xmit->tail, UART_XMIT_SIZE) - chunk1;
+ if (chunk2 > 0) {
+ memcpy(dma->tx_buf + chunk1, xmit->buf, chunk2);
+ }
+ dma->tx_size = chunk1 + chunk2;
 
  desc = dmaengine_prep_slave_single(dma->txchan,
- dma->tx_addr + xmit->tail,
+ dma->tx_addr,
  dma->tx_size, DMA_MEM_TO_DEV,
  DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
  if (!desc) {
@@ -217,20 +229,27 @@ int serial8250_request_dma(struct uart_8250_port *p)
  }
 
  /* TX buffer */
+ dma->tx_buf = kmalloc(UART_XMIT_SIZE, GFP_KERNEL);
+ if (!dma->tx_buf) {
+ ret = -ENOMEM;
+ goto free_coherent_rx;
+ }
+
  dma->tx_addr = dma_map_single(dma->txchan->device->dev,
- p->port.state->xmit.buf,
+ dma->tx_buf,
  UART_XMIT_SIZE,
  DMA_TO_DEVICE);
  if (dma_mapping_error(dma->txchan->device->dev, dma->tx_addr)) {
- dma_free_coherent(dma->rxchan->device->dev, dma->rx_size,
- dma->rx_buf, dma->rx_addr);
  ret = -ENOMEM;
- goto err;
+ goto free_coherent_rx;
  }
 
  dev_dbg_ratelimited(p->port.dev, "got both dma channels\n");
 
  return 0;
+free_coherent_rx:
+ dma_free_coherent(dma->rxchan->device->dev, dma->rx_size,
+ dma->rx_buf, dma->rx_addr);
 err:
  dma_release_channel(dma->txchan);
 release_rx:
@@ -257,6 +276,8 @@ void serial8250_release_dma(struct uart_8250_port *p)
  dmaengine_terminate_sync(dma->txchan);
  dma_unmap_single(dma->txchan->device->dev, dma->tx_addr,
  UART_XMIT_SIZE, DMA_TO_DEVICE);
+ kfree(dma->tx_buf);
+ dma->tx_buf = NULL;
  dma_release_channel(dma->txchan);
  dma->txchan = NULL;
  dma->tx_running = 0;