[PATCH V4 1/4] dt-bindings: serial: rs485: Add GPIO controlling RX enable during TX

[PATCH V4 1/4] dt-bindings: serial: rs485: Add GPIO controlling RX enable during TX

[Christoph Niedermaier]

Add a binding for a generic definition of an output GPIO that sets the
state of rs485-rx-during-tx. The idea is that the hardware already controls
the option receiving during sending before it gets to the signal receiving
hardware. The standard RS485 is a half-duplex bus that in most cases is
driven by an UART controller. The advantage of using this GPIO is that it
is independent of the capabilities of the UART core and the UART driver.
On the hardware side the interface to the bus is controlled by a transceiver,
that has a pin called RE (RX Enable) or similar, which connects the bus to
the RX signal of the UART controller. The GPIO can switch between two states
to control the RE pin via an electrical circuit:
- Active:
  The RE pin is always active. The UART RX see everything on the bus and
  therefore also what happens with the TX signal on the bus.
- Inactive:
  The RE pin is always active, but during sending on the bus the pin RE is
  inactive. So basically the receiving during sending is suppressed.

A possible circuit diagram could look like this:
                                  ┌──────────────────┐
                                  │       RS485      │
                TX ───────────────┤D                 │
                                  │    Transceiver   │
               RTS ────┬──────────┤DE                │
                       │          │                  │
                       │ ┌─────┐  │                  │
                       └─┤&    │  │                  │
                         │     ├──┤!RE               │
!rx_during_tx_gpio ──────┤     │  │                  │
                         └─────┘  │                  │
                                  │                  │
                RX ───────────────┤R                 │
                                  │                  │
                                  └──────────────────┘

Here the RTS pin of the UART core is used to control TX via the transceiver
pin DE (Drive Enable). RE and rx_during_tx_gpio are active low.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Reviewed-by: Rob Herring <robh@kernel.org>
---
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
Cc: Alexander Dahl <ada@thorsis.com>
Cc: Marek Vasut <marex@denx.de>
Cc: kernel@dh-electronics.com
Cc: devicetree@vger.kernel.org
To: linux-serial@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
---
V2: - Rework of the commit message
    - Rework GPIO property comment
V3: - Rework the binding description
    - Rework message title
    - Rework of the commit message
V4: - Add Reviewed-by tag
---
 Documentation/devicetree/bindings/serial/rs485.yaml | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/devicetree/bindings/serial/rs485.yaml b/Documentation/devicetree/bindings/serial/rs485.yaml
index 90a1bab40f05..a973ab7d7e46 100644
--- a/Documentation/devicetree/bindings/serial/rs485.yaml
+++ b/Documentation/devicetree/bindings/serial/rs485.yaml
@@ -51,6 +51,12 @@ properties:
     description: GPIO pin to enable RS485 bus termination.
     maxItems: 1
 
+  rs485-rx-during-tx-gpios:
+    description: Output GPIO pin that sets the state of rs485-rx-during-tx. This
+      signal can be used to control the RX part of an RS485 transceiver. Thereby
+      the active state enables RX during TX.
+    maxItems: 1
+
 additionalProperties: true
 
 ...
-- 
2.11.0


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[PATCH V4 2/4] serial: core: Add option to output RS485 RX_DURING_TX state via GPIO

[Christoph Niedermaier]

This patch provides a generic GPIO variable for outputting the state
of RS485 RX_DURING_TX. The GPIO is defined by the devicetree property
"rs485-rx-during-tx-gpios". To use it in a low level serial driver,
the evaluation of this variable must be implemented there accordingly.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
---
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Alexander Dahl <ada@thorsis.com>
Cc: Marek Vasut <marex@denx.de>
Cc: kernel@dh-electronics.com
Cc: Linux Team <linux-imx@nxp.com>
To: linux-serial@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
---
V2: - Rework of the commit message
    - Rework GPIO property comment
V3: - Rework GPIO property comment
V4: - No changes
---
 drivers/tty/serial/serial_core.c | 12 ++++++++++++
 include/linux/serial_core.h      |  1 +
 2 files changed, 13 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index b9fbbee598b8..867f2675caca 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3410,6 +3410,7 @@ int uart_get_rs485_mode(struct uart_port *port)
 	struct device *dev = port->dev;
 	u32 rs485_delay[2];
 	int ret;
+	int rx_during_tx_gpio_flag;
 
 	ret = device_property_read_u32_array(dev, "rs485-rts-delay",
 					     rs485_delay, 2);
@@ -3458,6 +3459,17 @@ int uart_get_rs485_mode(struct uart_port *port)
 	if (port->rs485_term_gpio)
 		port->rs485_supported.flags |= SER_RS485_TERMINATE_BUS;
 
+	rx_during_tx_gpio_flag = (rs485conf->flags & SER_RS485_RX_DURING_TX) ?
+				 GPIOD_OUT_HIGH : GPIOD_OUT_LOW;
+	port->rs485_rx_during_tx_gpio = devm_gpiod_get_optional(dev,
+								"rs485-rx-during-tx",
+								rx_during_tx_gpio_flag);
+	if (IS_ERR(port->rs485_rx_during_tx_gpio)) {
+		ret = PTR_ERR(port->rs485_rx_during_tx_gpio);
+		port->rs485_rx_during_tx_gpio = NULL;
+		return dev_err_probe(dev, ret, "Cannot get rs485-rx-during-tx-gpios\n");
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(uart_get_rs485_mode);
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index fd59f600094a..7d3784b18a6f 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -579,6 +579,7 @@ struct uart_port {
 	struct serial_rs485     rs485;
 	struct serial_rs485	rs485_supported;	/* Supported mask for serial_rs485 */
 	struct gpio_desc	*rs485_term_gpio;	/* enable RS485 bus termination */
+	struct gpio_desc	*rs485_rx_during_tx_gpio; /* Output GPIO that sets the state of RS485 RX during TX */
 	struct serial_iso7816   iso7816;
 	void			*private_data;		/* generic platform data pointer */
 };
-- 
2.11.0


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[PATCH V4 3/4] serial: imx: Add support for RS485 RX_DURING_TX output GPIO

[Christoph Niedermaier]

If a RX_DURING_TX GPIO is defined by the DT property "rs485-rx-during-tx-gpios"
this patch switches this GPIO accordingly to the RS485 flag RX_DURING_TX in user
space. In addition, the i.MX UART receiver is no longer turned on and off during
sending, because now the hardware is responsible for connecting or disconnecting
RX during TX controlled by this GPIO.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
---
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Alexander Dahl <ada@thorsis.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: kernel@dh-electronics.com
Cc: Linux Team <linux-imx@nxp.com>
To: linux-serial@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
---
V2: - Rework of the commit message
V3: - No changes
V4: - Direct call of gpiod_set_value_cansleep() without if statement
---
 drivers/tty/serial/imx.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 757825edb0cd..a2aea88128af 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -476,7 +476,8 @@ static void imx_uart_stop_tx(struct uart_port *port)
 				imx_uart_rts_inactive(sport, &ucr2);
 			imx_uart_writel(sport, ucr2, UCR2);
 
-			imx_uart_start_rx(port);
+			if (!port->rs485_rx_during_tx_gpio)
+				imx_uart_start_rx(port);
 
 			sport->tx_state = OFF;
 		}
@@ -705,7 +706,8 @@ static void imx_uart_start_tx(struct uart_port *port)
 				imx_uart_rts_inactive(sport, &ucr2);
 			imx_uart_writel(sport, ucr2, UCR2);
 
-			if (!(port->rs485.flags & SER_RS485_RX_DURING_TX))
+			if (!(port->rs485.flags & SER_RS485_RX_DURING_TX) &&
+			    !port->rs485_rx_during_tx_gpio)
 				imx_uart_stop_rx(port);
 
 			sport->tx_state = WAIT_AFTER_RTS;
@@ -1955,6 +1957,10 @@ static int imx_uart_rs485_config(struct uart_port *port, struct ktermios *termio
 	    rs485conf->flags & SER_RS485_RX_DURING_TX)
 		imx_uart_start_rx(port);
 
+	if (port->rs485_rx_during_tx_gpio)
+		gpiod_set_value_cansleep(port->rs485_rx_during_tx_gpio,
+					 !!(rs485conf->flags & SER_RS485_RX_DURING_TX));
+
 	return 0;
 }
 
-- 
2.11.0


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[PATCH V4 4/4] serial: stm32: Add support for rs485 RX_DURING_TX output GPIO

[Christoph Niedermaier]

If a RX_DURING_TX GPIO is defined by the DT property "rs485-rx-during-tx-gpios"
this patch switches this GPIO accordingly to the RS485 flag RX_DURING_TX in user
space. Controlled by this GPIO, now the hardware is responsible for connecting
or disconnecting RX during TX.

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
---
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Alexander Dahl <ada@thorsis.com>
Cc: Marek Vasut <marex@denx.de>
Cc: kernel@dh-electronics.com
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-serial@vger.kernel.org
To: linux-arm-kernel@lists.infradead.org
---
V2: - Rework of the commit message
V3: - No changes
V4: - Direct call of gpiod_set_value_cansleep() without if statement
---
 drivers/tty/serial/stm32-usart.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/stm32-usart.c b/drivers/tty/serial/stm32-usart.c
index a1490033aa16..f1b1df78118f 100644
--- a/drivers/tty/serial/stm32-usart.c
+++ b/drivers/tty/serial/stm32-usart.c
@@ -226,7 +226,11 @@ static int stm32_usart_config_rs485(struct uart_port *port, struct ktermios *ter
 
 	stm32_usart_clr_bits(port, ofs->cr1, BIT(cfg->uart_enable_bit));
 
-	rs485conf->flags |= SER_RS485_RX_DURING_TX;
+	if (port->rs485_rx_during_tx_gpio)
+		gpiod_set_value_cansleep(port->rs485_rx_during_tx_gpio,
+					 !!(rs485conf->flags & SER_RS485_RX_DURING_TX));
+	else
+		rs485conf->flags |= SER_RS485_RX_DURING_TX;
 
 	if (rs485conf->flags & SER_RS485_ENABLED) {
 		cr1 = readl_relaxed(port->membase + ofs->cr1);
-- 
2.11.0


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