[RFC] OMAP: allow GPIO pin to enable/disable external UART driver chip.

[RFC] OMAP: allow GPIO pin to enable/disable external UART driver chip.

[Mark Jackson]

When a UART transmitter is connected to (eg) a RS485 driver, it is
necessary to turn the driver on/off as quickly as possible.  This is
best achieved in the serial driver itself (rather than in userspace
where the latency can be quite large).

This patch allows the GPIO pin to be defined (via DT) that controls
the enabling of the driver at the start of a message, and disables
the driver when the message has been completed.

Still to do:-
Allow userspace to turn this feature on/off
Do the same for the receiver (useful for 2 wire RS485)

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
---
 drivers/tty/serial/omap-serial.c |   57 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/drivers/tty/serial/omap-serial.c b/drivers/tty/serial/omap-serial.c
index b6d1728..1d3d117 100644
--- a/drivers/tty/serial/omap-serial.c
+++ b/drivers/tty/serial/omap-serial.c
@@ -40,9 +40,12 @@
 #include <linux/pm_runtime.h>
 #include <linux/of.h>
 #include <linux/gpio.h>
+#include <linux/of_gpio.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/platform_data/serial-omap.h>
 
+#include <dt-bindings/gpio/gpio.h>
+
 #define OMAP_MAX_HSUART_PORTS	6
 
 #define UART_BUILD_REVISION(x, y)	(((x) << 8) | (y))
@@ -156,6 +159,10 @@ struct uart_omap_port {
 	int			DTR_inverted;
 	int			DTR_active;
 
+	int			txen_gpio;
+	int			txen_inverted;
+	int			txen_active;
+
 	struct pm_qos_request	pm_qos_request;
 	u32			latency;
 	u32			calc_latency;
@@ -272,8 +279,25 @@ static void serial_omap_enable_ms(struct uart_port *port)
 static void serial_omap_stop_tx(struct uart_port *port)
 {
 	struct uart_omap_port *up = to_uart_omap_port(port);
+	struct circ_buf *xmit = &up->port.state->xmit;
 
 	pm_runtime_get_sync(up->dev);
+
+	// if txen currently active
+	if (up->txen_active) {
+		// do nothing if current tx not yet completed
+		int res = serial_in(up, UART_LSR) & UART_LSR_TEMT;
+		if (!res)
+			return;
+
+		// if there's no more data to send, turn off txen
+		if (uart_circ_empty(xmit)) {
+			gpio_set_value(up->txen_gpio,
+					   up->txen_active == up->txen_inverted);
+			up->txen_active = 0;
+		}
+	}
+
 	if (up->ier & UART_IER_THRI) {
 		up->ier &= ~UART_IER_THRI;
 		serial_out(up, UART_IER, up->ier);
@@ -300,6 +324,16 @@ static void transmit_chars(struct uart_omap_port *up, unsigned int lsr)
 	struct circ_buf *xmit = &up->port.state->xmit;
 	int count;
 
+	// if txen in use
+	if (gpio_is_valid(up->txen_gpio)) {
+		// turn on txen ?
+		if (!uart_circ_empty(xmit) && (!up->txen_active)) {
+			gpio_set_value(up->txen_gpio,
+				       up->txen_active == up->txen_inverted);
+			up->txen_active = 1;
+		}
+	}
+
 	if (up->port.x_char) {
 		serial_out(up, UART_TX, up->port.x_char);
 		up->port.icount.tx++;
@@ -1468,6 +1502,28 @@ static int serial_omap_probe(struct platform_device *pdev)
 		goto err_port_line;
 	}
 
+	// init tx enable signal
+	up->txen_gpio = -EINVAL;
+	up->txen_inverted = 0;
+	up->txen_active = 0;
+	if (pdev->dev.of_node)
+	{
+		enum of_gpio_flags flags;
+		up->txen_gpio = of_get_named_gpio_flags(pdev->dev.of_node,
+							"txen-gpio", 0, &flags);
+		up->txen_inverted = (flags == GPIO_ACTIVE_LOW);
+		if (gpio_is_valid(up->txen_gpio)) {
+			ret = gpio_request(up->txen_gpio, "omap-serial");
+			if (ret < 0)
+				goto err_txen;
+			ret = gpio_direction_output(up->txen_gpio,
+						    up->txen_inverted);
+			if (ret < 0)
+				goto err_txen;
+		} else
+			up->txen_gpio = -EINVAL;
+	}
+
 	up->pins = devm_pinctrl_get_select_default(&pdev->dev);
 	if (IS_ERR(up->pins)) {
 		dev_warn(&pdev->dev, "did not get pins for uart%i error: %li\n",
@@ -1529,6 +1585,7 @@ err_add_port:
 	pm_runtime_put(&pdev->dev);
 	pm_runtime_disable(&pdev->dev);
 err_ioremap:
+err_txen:
 err_port_line:
 	dev_err(&pdev->dev, "[UART%d]: failure [%s]: %d\n",
 				pdev->id, __func__, ret);
-- 
1.7.9.5

Re: [RFC] OMAP: allow GPIO pin to enable/disable external UART driver chip.

[Andrew Ruder]

Sorry for the late reply, I've been thinking about this for some time
and was sad to see it didn't really evoke any sort of discussion :(.

On Sat, Aug 10, 2013 at 02:58:08PM +0100, Mark Jackson wrote:
> When a UART transmitter is connected to (eg) a RS485 driver, it is
> necessary to turn the driver on/off as quickly as possible.  This is
> best achieved in the serial driver itself (rather than in userspace
> where the latency can be quite large).
> 
> This patch allows the GPIO pin to be defined (via DT) that controls
> the enabling of the driver at the start of a message, and disables
> the driver when the message has been completed.
> 
> Still to do:-
> Allow userspace to turn this feature on/off
> Do the same for the receiver (useful for 2 wire RS485)

I've been wondering about this as well but I have a slightly different
situation.  On my board the RTS line controls the RS485 transmit/receive
direction.

I don't know if you've found the Documentation/serial/serial-rs485.txt
file but it describes, at the very least, a standard API For controlling
several parameters related to RS485 including configurable delay between
rts->start of data/end of data->rts.  Unfortunately it seems like only
one driver really implements the full API (Atmel AT91) and I guess it
needs to be bolted onto each serial driver individually (although it
seems like a fairly general concept that could be handled at another
level).

That being said, maybe this patch would better be rethought as a way to
specify a GPIO for the RTS line (I don't know enough about OMAP and
whether or not it already provides for hardware flow control in its
builtin UARTs and you just aren't using it for RS485 flow control?) and
then in a separate patch implement this already documented user->kernel
API?

- Andy

Re: [RFC] OMAP: allow GPIO pin to enable/disable external UART driver chip.

[Mark Jackson]

On 13/08/13 21:12, Andrew Ruder wrote:
> Sorry for the late reply, I've been thinking about this for some time
> and was sad to see it didn't really evoke any sort of discussion :(.
> 
> On Sat, Aug 10, 2013 at 02:58:08PM +0100, Mark Jackson wrote:
>> When a UART transmitter is connected to (eg) a RS485 driver, it is
>> necessary to turn the driver on/off as quickly as possible.  This is
>> best achieved in the serial driver itself (rather than in userspace
>> where the latency can be quite large).
>>
>> This patch allows the GPIO pin to be defined (via DT) that controls
>> the enabling of the driver at the start of a message, and disables
>> the driver when the message has been completed.
>>
>> Still to do:-
>> Allow userspace to turn this feature on/off
>> Do the same for the receiver (useful for 2 wire RS485)
> 
> I've been wondering about this as well but I have a slightly different
> situation.  On my board the RTS line controls the RS485 transmit/receive
> direction.
> 
> I don't know if you've found the Documentation/serial/serial-rs485.txt
> file but it describes, at the very least, a standard API For controlling
> several parameters related to RS485 including configurable delay between
> rts->start of data/end of data->rts.  Unfortunately it seems like only
> one driver really implements the full API (Atmel AT91) and I guess it
> needs to be bolted onto each serial driver individually (although it
> seems like a fairly general concept that could be handled at another
> level).
> 
> That being said, maybe this patch would better be rethought as a way to
> specify a GPIO for the RTS line (I don't know enough about OMAP and
> whether or not it already provides for hardware flow control in its
> builtin UARTs and you just aren't using it for RS485 flow control?) and
> then in a separate patch implement this already documented user->kernel
> API?

I've actually submitted a newer version that does support the documented
API.  See http://permalink.gmane.org/gmane.linux.ports.arm.omap/102765

Does this address some of your questions ?