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serial: 8250: Support rs485 bus termination GPIO

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Commit e8759ad17d ("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>
This commit is contained in:
Lukas Wunner 2020-05-18 16:45:02 +02:00 committed by Greg Kroah-Hartman
parent f40a6be4a4
commit d58a2df3d8
3 changed files with 21 additions and 0 deletions
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3
drivers/tty/serial/8250/8250_port.c
View file

@ -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.

16
drivers/tty/serial/serial_core.c
View file

@ -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);

2
include/linux/serial_core.h
View file

@ -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 */
};