Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 66f37aaf authored by Cyrille Pitchen's avatar Cyrille Pitchen Committed by Greg Kroah-Hartman
Browse files

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: default avatarCyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: default avatarNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent a7ae7f82

drivers/tty/serial/atmel_serial.c

+55 −39
Original line number Diff line number Diff line
@@ -880,32 +880,6 @@ static int atmel_prepare_tx_dma(struct uart_port *port) 
	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);



	/*

	 * 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.

	 * 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.

	 */

	if (pending > ring->head) {

		count = pending - ring->head;

	ring->head = sg_dma_len(&atmel_port->sg_rx) - state.residue;

	BUG_ON(ring->head > sg_dma_len(&atmel_port->sg_rx));

	/*

	 * 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 (ring->head < ring->tail) {

		count = sg_dma_len(&atmel_port->sg_rx) - ring->tail;



		tty_insert_flip_string(tport, ring->buf + ring->tail, count);

		ring->tail = 0;

		port->icount.rx += count;

	}



		atmel_flip_buffer_rx_dma(port, ring->buf + ring->head, count);

	/* Finally we read data from tail to head */

	if (ring->tail < ring->head) {

		count = ring->head - ring->tail;



		ring->head += count;

		if (ring->head == sg_dma_len(&atmel_port->sg_rx))

		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);

}