tty/serial: atmel: RS485 HD w/DMA: enable RX after TX is stopped

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>

diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
index 55ca3416113ffab41fa4c08704190f73e8a7c3cd..0b4f3690532145da4228b8b8255f0d5da928a31f 100644 (file)
--- 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 *arg)
        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);
@@ -1382,9 +1389,20 @@ atmel_handle_transmit(struct uart_port *port, unsigned int pending)
        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);
        }
 }