Commit 2bed8a8e authored Dec 12, 2016 by Daniel Jedrychowski Committed by Greg Kroah-Hartman Jan 11, 2017

Clearing FIFOs in RS485 emulation mode causes subsequent transmits to break



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.

Signed-off-by: Daniel Jedrychowski <avistel@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent c130b666

drivers/tty/serial/8250/8250_port.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -1413,7 +1413,7 @@ static void __do_stop_tx_rs485(struct uart_8250_port *p)
		* 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);