Re: [PATCH v2 4/4] serial: 8250: Support rs485 bus termination GPIO

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, May 12, 2020 at 02:40:04PM +0200, Lukas Wunner wrote:
> Commit e8759ad17d41 ("serial: uapi: Add support for bus termination")
> introduced the ability to enable rs485 bus termination from user space.
> So far the feature is only used by a single driver, 8250_exar.c, using a
> hardcoded GPIO pin specific to Siemens IOT2040 products.

Perhaps you need to Cc this to Jan Kiszka.

> Provide for a more generic solution by allowing specification of an
> rs485 bus termination GPIO pin in the device tree:  Amend the serial
> core to retrieve the GPIO from the device tree (or ACPI table) and amend
> the default ->rs485_config() callback for 8250 drivers to change the
> GPIO on request from user space.

Perhaps the followup can be turning 8250_exar to request those GPIOs it
provides (as hogs or regular ones, I don't know).

FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

> Signed-off-by: Lukas Wunner <lukas@wunner.de>
> ---
> Changes v1 -> v2:
> * Drop unnecessary devm_gpiod_put(). (Andy)
> * Use GPIOD_OUT_LOW macro for brevity. (Andy)
> * Document the rationale for disabling termination by default.
> * Drop nonsensical read of GPIO after setting its direction to out.
> 
>  drivers/tty/serial/8250/8250_port.c |  4 ++++
>  drivers/tty/serial/serial_core.c    | 16 ++++++++++++++++
>  include/linux/serial_core.h         |  2 ++
>  3 files changed, 22 insertions(+)
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index f77bf820b7a3..b5b630d02110 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -681,6 +681,10 @@ int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485)
>  	memset(rs485->padding, 0, sizeof(rs485->padding));
>  	port->rs485 = *rs485;
>  
> +	if (port->rs485_term_gpio)
> +		gpiod_set_value(port->rs485_term_gpio,
> +				rs485->flags & SER_RS485_TERMINATE_BUS);
> +
>  	/*
>  	 * Both serial8250_em485_init() and serial8250_em485_destroy()
>  	 * are idempotent.
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 43b6682877d5..57840cf90388 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -3317,6 +3317,7 @@ int uart_get_rs485_mode(struct uart_port *port)
>  	 * to get to a defined state with the following properties:
>  	 */
>  	rs485conf->flags &= ~(SER_RS485_RX_DURING_TX | SER_RS485_ENABLED |
> +			      SER_RS485_TERMINATE_BUS |
>  			      SER_RS485_RTS_AFTER_SEND);
>  	rs485conf->flags |= SER_RS485_RTS_ON_SEND;
>  
> @@ -3331,6 +3332,21 @@ int uart_get_rs485_mode(struct uart_port *port)
>  		rs485conf->flags |= SER_RS485_RTS_AFTER_SEND;
>  	}
>  
> +	/*
> +	 * Disabling termination by default is the safe choice:  Else if many
> +	 * bus participants enable it, no communication is possible at all.
> +	 * Works fine for short cables and users may enable for longer cables.
> +	 */
> +	port->rs485_term_gpio = devm_gpiod_get_optional(dev, "rs485-term",
> +							GPIOD_OUT_LOW);
> +	if (IS_ERR(port->rs485_term_gpio)) {
> +		ret = PTR_ERR(port->rs485_term_gpio);
> +		port->rs485_term_gpio = NULL;
> +		if (ret != -EPROBE_DEFER)
> +			dev_err(dev, "Cannot get rs485-term-gpios\n");
> +		return ret;
> +	}
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(uart_get_rs485_mode);
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index b649a2b894e7..9fd550e7946a 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -10,6 +10,7 @@
>  #include <linux/bitops.h>
>  #include <linux/compiler.h>
>  #include <linux/console.h>
> +#include <linux/gpio/consumer.h>
>  #include <linux/interrupt.h>
>  #include <linux/circ_buf.h>
>  #include <linux/spinlock.h>
> @@ -251,6 +252,7 @@ struct uart_port {
>  	struct attribute_group	*attr_group;		/* port specific attributes */
>  	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
>  	struct serial_rs485     rs485;
> +	struct gpio_desc	*rs485_term_gpio;	/* enable RS485 bus termination */
>  	struct serial_iso7816   iso7816;
>  	void			*private_data;		/* generic platform data pointer */
>  };
> -- 
> 2.26.2
> 

-- 
With Best Regards,
Andy Shevchenko