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patch "tty/serial: atmel: RS485 HD w/DMA: enable RX after TX is stopped" added to tty-linus

PATCH [email protected] 1 participant 1 message Mar 27, 2019

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Stable branch TTY layer and serial drivers Arm/Microchip (AT91) SoC support Microchip AT91 serial driver
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This is a note to let you know that I've just added the patch titled

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

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 69646d7a3689fbe1a65ae90397d22ac3f1b8d40f Mon Sep 17 00:00:00 2001
From: Razvan Stefanescu <[email protected]>
Date: Tue, 19 Mar 2019 15:20:35 +0200
Subject: 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 <[email protected]>
Acked-by Richard Genoud <[email protected]>
Cc stable <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 drivers/tty/serial/atmel_serial.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)
drivers/tty/serial/atmel_serial.c +21 -3
index 55ca3416113f..0b4f36905321 100644
@@ -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);
 	}
 }
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2.21.0