Patch "Clearing FIFOs in RS485 emulation mode causes subsequent transmits to break" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

Clearing FIFOs in RS485 emulation mode causes subsequent transmits to break

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
clearing-fifos-in-rs485-emulation-mode-causes-subsequent-transmits-to-break.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Daniel Jedrychowski <[email protected]>
Date: Mon, 12 Dec 2016 09:18:28 +1100
Subject: Clearing FIFOs in RS485 emulation mode causes subsequent transmits to break

From: Daniel Jedrychowski <[email protected]>

commit 2bed8a8e70729f996af92042d3ad0f11870acc1f upstream.

When in RS485 emulation mode, __do_stop_tx_rs485() calls serial8250_clear_fifos(). This not only clears the FIFOs, but also sets all bits in their control register (UART_FCR) to 0.

One of the effects of this is the disabling of the FIFOs, which turns them into single-byte holding registers. The rest of the driver doesn't know this, which results in the lions share of characters passed into a write call to be dropped.

(I can supply logic analyzer screenshots if necessary)

This fix replaces the serial8250_clear_fifos() call to serial8250_clear_and_reinit_fifos() - this prevents the "dropped characters" issue from manifesting again while retaining the requirement of clearing the RX FIFO after transmission if the SER_RS485_RX_DURING_TX flag is disabled.

---
 drivers/tty/serial/8250/8250_port.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1411,7 +1411,7 @@ static void __do_stop_tx_rs485(struct ua
 	 * Enable previously disabled RX interrupts.
 	 */
 	if (!(p->port.rs485.flags & SER_RS485_RX_DURING_TX)) {
-		serial8250_clear_fifos(p);
+		serial8250_clear_and_reinit_fifos(p);
 
 		p->ier |= UART_IER_RLSI | UART_IER_RDI;
 		serial_port_out(&p->port, UART_IER, p->ier);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/clearing-fifos-in-rs485-emulation-mode-causes-subsequent-transmits-to-break.patch