serial: 8250: Support rs485 bus termination GPIO

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.

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 8250_exar.c can be converted to the generic approach in a
follow-up patch.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Link: https://lore.kernel.org/r/94c6c800d1ca9fa04766dd1d43a8272c5ad4bedd.1589811297.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index bf2722e..1632f7d 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -681,6 +681,9 @@ int serial8250_em485_config(struct uart_port *port, struct serial_rs485 *rs485)
        memset(rs485->padding, 0, sizeof(rs485->padding));
        port->rs485 = *rs485;
 
+       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 43b6682..57840cf 100644 (file)
--- 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 b649a2b..9fd550e 100644 (file)
--- 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 */
 };