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Subject[PATCH 4.4 127/158] serial: sh-sci: Fix TX DMA buffer flushing and workqueue races
Date
[ Upstream commit 8493eab02608b0e82f67b892aa72882e510c31d0 ]

When uart_flush_buffer() is called, the .flush_buffer() callback zeroes
the tx_dma_len field. This may race with the work queue function
handling transmit DMA requests:

1. If the buffer is flushed before the first DMA API call,
dmaengine_prep_slave_single() may be called with a zero length,
causing the DMA request to never complete, leading to messages
like:

rcar-dmac e7300000.dma-controller: Channel Address Error happen

and, with debug enabled:

sh-sci e6e88000.serial: sci_dma_tx_work_fn: ffff800639b55000: 0...0, cookie 126

and DMA timeouts.

2. If the buffer is flushed after the first DMA API call, but before
the second, dma_sync_single_for_device() may be called with a zero
length, causing the transmit data not to be flushed to RAM, and
leading to stale data being output.

Fix this by:
1. Letting sci_dma_tx_work_fn() return immediately if the transmit
buffer is empty,
2. Extending the critical section to cover all DMA preparational work,
so tx_dma_len stays consistent for all of it,
3. Using local copies of circ_buf.head and circ_buf.tail, to make sure
they match the actual operation above.

Reported-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Suggested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20190624123540.20629-2-geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/tty/serial/sh-sci.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 669134e27ed9..c450e32c153d 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1203,6 +1203,7 @@ static void work_fn_tx(struct work_struct *work)
struct uart_port *port = &s->port;
struct circ_buf *xmit = &port->state->xmit;
dma_addr_t buf;
+ int head, tail;

/*
* DMA is idle now.
@@ -1212,16 +1213,23 @@ static void work_fn_tx(struct work_struct *work)
* consistent xmit buffer state.
*/
spin_lock_irq(&port->lock);
- buf = s->tx_dma_addr + (xmit->tail & (UART_XMIT_SIZE - 1));
+ head = xmit->head;
+ tail = xmit->tail;
+ buf = s->tx_dma_addr + (tail & (UART_XMIT_SIZE - 1));
s->tx_dma_len = min_t(unsigned int,
- CIRC_CNT(xmit->head, xmit->tail, UART_XMIT_SIZE),
- CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE));
- spin_unlock_irq(&port->lock);
+ CIRC_CNT(head, tail, UART_XMIT_SIZE),
+ CIRC_CNT_TO_END(head, tail, UART_XMIT_SIZE));
+ if (!s->tx_dma_len) {
+ /* Transmit buffer has been flushed */
+ spin_unlock_irq(&port->lock);
+ return;
+ }

desc = dmaengine_prep_slave_single(chan, buf, s->tx_dma_len,
DMA_MEM_TO_DEV,
DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
if (!desc) {
+ spin_unlock_irq(&port->lock);
dev_warn(port->dev, "Failed preparing Tx DMA descriptor\n");
/* switch to PIO */
sci_tx_dma_release(s, true);
@@ -1231,20 +1239,20 @@ static void work_fn_tx(struct work_struct *work)
dma_sync_single_for_device(chan->device->dev, buf, s->tx_dma_len,
DMA_TO_DEVICE);

- spin_lock_irq(&port->lock);
desc->callback = sci_dma_tx_complete;
desc->callback_param = s;
- spin_unlock_irq(&port->lock);
s->cookie_tx = dmaengine_submit(desc);
if (dma_submit_error(s->cookie_tx)) {
+ spin_unlock_irq(&port->lock);
dev_warn(port->dev, "Failed submitting Tx DMA descriptor\n");
/* switch to PIO */
sci_tx_dma_release(s, true);
return;
}

+ spin_unlock_irq(&port->lock);
dev_dbg(port->dev, "%s: %p: %d...%d, cookie %d\n",
- __func__, xmit->buf, xmit->tail, xmit->head, s->cookie_tx);
+ __func__, xmit->buf, tail, head, s->cookie_tx);

dma_async_issue_pending(chan);
}
--
2.20.1


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