RS485 uses a single differential signal pair for both sending and
receiving. So there is an additional signal necessary to switch between
sending and receiving. Usually this signal is RTS, there are a few
pieces of evidence:
- the rs485 device tree binding calls this signal RTS;
- all drivers I checked use RTS for direction switching
(atmel_serial, crisv10, imx, fsl_lpuart, omap-serial, sc16is7xx.c);
- nearly all machines I saw called this signal RTS in their schematic;
and
- the members of struct serial_rs485 and related defines use RTS in
their names.
With such a setup using the imx driver I have the problem that opening a
serial device that is in RS485 mode for reading enables RTS via:
tty_port_open() -> tty_port_block_til_ready() ->
tty_port_raise_dtr_rts() -> uart_dtr_rts() ->
uart_update_mctrl()
and so disables receiving. The imx driver supports two different ways to
drive RTS: a) a native mode using UCR2_CTS and UCR2_CTSC (Freescale
named the UART signals from the modem POV, so what we call RTS is CTS in
the registers) and b) a gpio.
For case a) imx_set_mctrl already ignores TIOCM_RTS if the uart is in
RS485 mode. For case b) however it doesn't and so data may be missed to
read.
As I expect all drivers have the same problem it makes sense to fix this
in serial_core instead of teaching the imx driver to also ignore RTS for
the gpio case.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
drivers/tty/serial/serial_core.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 2b74ad9bfc64..8a52f28cf1ce 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1662,15 +1662,23 @@ static void uart_dtr_rts(struct tty_port *port, int onoff)
{
struct uart_state *state = container_of(port, struct uart_state, port);
struct uart_port *uport;
+ unsigned int dtrrts = TIOCM_DTR | TIOCM_RTS;
uport = uart_port_ref(state);
if (!uport)
return;
+ /*
+ * If RS485 is active, don't mess with RTS because that is supposed to
+ * enable the transmitter
+ */
+ if (uport->rs485.flags & SER_RS485_ENABLED)
+ dtrrts &= ~TIOCM_RTS;
+
if (onoff)
- uart_set_mctrl(uport, TIOCM_DTR | TIOCM_RTS);
+ uart_set_mctrl(uport, dtrrts);
else
- uart_clear_mctrl(uport, TIOCM_DTR | TIOCM_RTS);
+ uart_clear_mctrl(uport, dtrrts);
uart_port_deref(uport);
}
--
2.11.0
--
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