Discussion:
[PATCH 1/1] tty/serial: at91: fix rx ring buffer management
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Cyrille Pitchen
2014-10-20 17:12:20 UTC
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This patch swaps the use "tail" and "head" to fit the semantic of the linux
circular buffer documentation:
- head: the point at which the producer (the DMA controller) inserts items.
- tail: the point at which the consumer (the serial framework) finds the next
item.

Besides the former code of the rx ring buffer didn't manage the case where
head < tail, which might lead to loss of data. To fix this bug the data are now
sent from the DMA buffer to the serial framework in two steps:
1 - First, we test if head < tail. If so, we copy the data from tail to the end
of the DMA buffer then reset tail to zero.
2 - Finally, we copy data from tail to head then set tail to head.

In addition, since tty_insert_flip_string() may now be called twice,
atmel_flip_buffer_rx_dma() becomes less efficient than moving the calls
dma_sync_sg_for_cpu(), dma_sync_sg_for_device(), tty_insert_flip_string() and
tty_flip_buffer_push() directly into atmel_rx_from_dma().

Signed-off-by: Cyrille Pitchen <***@atmel.com>
Acked-by: Nicolas Ferre <***@atmel.com>
---
drivers/tty/serial/atmel_serial.c | 94 +++++++++++++++++++++++----------------
1 file changed, 55 insertions(+), 39 deletions(-)

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index b0603e1..c2a1d4a 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -773,32 +773,6 @@ chan_err:
return -EINVAL;
}

-static void atmel_flip_buffer_rx_dma(struct uart_port *port,
- char *buf, size_t count)
-{
- struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
- struct tty_port *tport = &port->state->port;
-
- dma_sync_sg_for_cpu(port->dev,
- &atmel_port->sg_rx,
- 1,
- DMA_DEV_TO_MEM);
-
- tty_insert_flip_string(tport, buf, count);
-
- dma_sync_sg_for_device(port->dev,
- &atmel_port->sg_rx,
- 1,
- DMA_DEV_TO_MEM);
- /*
- * Drop the lock here since it might end up calling
- * uart_start(), which takes the lock.
- */
- spin_unlock(&port->lock);
- tty_flip_buffer_push(tport);
- spin_lock(&port->lock);
-}
-
static void atmel_complete_rx_dma(void *arg)
{
struct uart_port *port = arg;
@@ -827,11 +801,12 @@ static void atmel_release_rx_dma(struct uart_port *port)
static void atmel_rx_from_dma(struct uart_port *port)
{
struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);
+ struct tty_port *tport = &port->state->port;
struct circ_buf *ring = &atmel_port->rx_ring;
struct dma_chan *chan = atmel_port->chan_rx;
struct dma_tx_state state;
enum dma_status dmastat;
- size_t pending, count;
+ size_t count;


/* Reset the UART timeout early so that we don't miss one */
@@ -846,27 +821,68 @@ static void atmel_rx_from_dma(struct uart_port *port)
tasklet_schedule(&atmel_port->tasklet);
return;
}
- /* current transfer size should no larger than dma buffer */
- pending = sg_dma_len(&atmel_port->sg_rx) - state.residue;
- BUG_ON(pending > sg_dma_len(&atmel_port->sg_rx));

+ /* CPU claims ownership of RX DMA buffer */
+ dma_sync_sg_for_cpu(port->dev,
+ &atmel_port->sg_rx,
+ 1,
+ DMA_DEV_TO_MEM);
+
+ /*
+ * ring->head points to the end of data already written by the DMA.
+ * ring->tail points to the beginning of data to be read by the
+ * framework.
+ * The current transfer size should not be larger than the dma buffer
+ * length.
+ */
+ ring->head = sg_dma_len(&atmel_port->sg_rx) - state.residue;
+ BUG_ON(ring->head > sg_dma_len(&atmel_port->sg_rx));
/*
- * This will take the chars we have so far,
- * ring->head will record the transfer size, only new bytes come
- * will insert into the framework.
+ * At this point ring->head may point to the first byte right after the
+ * last byte of the dma buffer:
+ * 0 <= ring->head <= sg_dma_len(&atmel_port->sg_rx)
+ *
+ * However ring->tail must always points inside the dma buffer:
+ * 0 <= ring->tail <= sg_dma_len(&atmel_port->sg_rx) - 1
+ *
+ * Since we use a ring buffer, we have to handle the case
+ * where head is lower than tail. In such a case, we first read from
+ * tail to the end of the buffer then reset tail.
*/
- if (pending > ring->head) {
- count = pending - ring->head;
+ if (ring->head < ring->tail) {
+ count = sg_dma_len(&atmel_port->sg_rx) - ring->tail;

- atmel_flip_buffer_rx_dma(port, ring->buf + ring->head, count);
+ tty_insert_flip_string(tport, ring->buf + ring->tail, count);
+ ring->tail = 0;
+ port->icount.rx += count;
+ }

- ring->head += count;
- if (ring->head == sg_dma_len(&atmel_port->sg_rx))
- ring->head = 0;
+ /* Finally we read data from tail to head */
+ if (ring->tail < ring->head) {
+ count = ring->head - ring->tail;

+ tty_insert_flip_string(tport, ring->buf + ring->tail, count);
+ /* Wrap ring->head if needed */
+ if (ring->head >= sg_dma_len(&atmel_port->sg_rx))
+ ring->head = 0;
+ ring->tail = ring->head;
port->icount.rx += count;
}

+ /* USART retreives ownership of RX DMA buffer */
+ dma_sync_sg_for_device(port->dev,
+ &atmel_port->sg_rx,
+ 1,
+ DMA_DEV_TO_MEM);
+
+ /*
+ * Drop the lock here since it might end up calling
+ * uart_start(), which takes the lock.
+ */
+ spin_unlock(&port->lock);
+ tty_flip_buffer_push(tport);
+ spin_lock(&port->lock);
+
UART_PUT_IER(port, ATMEL_US_TIMEOUT);
}
--
1.8.2.2
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