tty/serial: at91: fix rx ring buffer management

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 <cyrille.pitchen@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 55 insertions, 39 deletions

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 8a84034d130c..9eb624a22431 100644
--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -880,32 +880,6 @@ 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;
@@ -934,11 +908,12 @@ 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 */
@@ -953,27 +928,68 @@ 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);
 }