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Subject[PATCH 5.0 078/146] tty/serial: atmel: RS485 HD w/DMA: enable RX after TX is stopped
Date
5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Razvan Stefanescu <razvan.stefanescu@microchip.com>

commit 69646d7a3689fbe1a65ae90397d22ac3f1b8d40f upstream.

In half-duplex operation, RX should be started after TX completes.

If DMA is used, there is a case when the DMA transfer completes but the
TX FIFO is not emptied, so the RX cannot be restarted just yet.

Use a boolean variable to store this state and rearm TX interrupt mask
to be signaled again that the transfer finished. In interrupt transmit
handler this variable is used to start RX. A warning message is generated
if RX is activated before TX fifo is cleared.

Fixes: b389f173aaa1 ("tty/serial: atmel: RS485 half duplex w/DMA: enable
RX after TX is done")
Signed-off-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/serial/atmel_serial.c | 24 +++++++++++++++++++++---
1 file changed, 21 insertions(+), 3 deletions(-)

--- a/drivers/tty/serial/atmel_serial.c
+++ b/drivers/tty/serial/atmel_serial.c
@@ -166,6 +166,8 @@ struct atmel_uart_port {
unsigned int pending_status;
spinlock_t lock_suspended;

+ bool hd_start_rx; /* can start RX during half-duplex operation */
+
/* ISO7816 */
unsigned int fidi_min;
unsigned int fidi_max;
@@ -933,8 +935,13 @@ static void atmel_complete_tx_dma(void *
if (!uart_circ_empty(xmit))
atmel_tasklet_schedule(atmel_port, &atmel_port->tasklet_tx);
else if (atmel_uart_is_half_duplex(port)) {
- /* DMA done, stop TX, start RX for RS485 */
- atmel_start_rx(port);
+ /*
+ * DMA done, re-enable TXEMPTY and signal that we can stop
+ * TX and start RX for RS485
+ */
+ atmel_port->hd_start_rx = true;
+ atmel_uart_writel(port, ATMEL_US_IER,
+ atmel_port->tx_done_mask);
}

spin_unlock_irqrestore(&port->lock, flags);
@@ -1378,9 +1385,20 @@ atmel_handle_transmit(struct uart_port *
struct atmel_uart_port *atmel_port = to_atmel_uart_port(port);

if (pending & atmel_port->tx_done_mask) {
- /* Either PDC or interrupt transmission */
atmel_uart_writel(port, ATMEL_US_IDR,
atmel_port->tx_done_mask);
+
+ /* Start RX if flag was set and FIFO is empty */
+ if (atmel_port->hd_start_rx) {
+ if (!(atmel_uart_readl(port, ATMEL_US_CSR)
+ & ATMEL_US_TXEMPTY))
+ dev_warn(port->dev, "Should start RX, but TX fifo is not empty\n");
+
+ atmel_port->hd_start_rx = false;
+ atmel_start_rx(port);
+ return;
+ }
+
atmel_tasklet_schedule(atmel_port, &atmel_port->tasklet_tx);
}
}

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