[PATCH 2/5] serial: core: add sysfs attribute to suppress ready signalling on open

[Johan Hovold <johan@kernel.org>]

Add a nordy sysfs attribute to suppress raising the modem-control lines
on open to signal DTE readiness.

This can be use to prevent undesirable side-effects on open for
applications where the DTR and RTS lines are used for non-standard
purposes such as generating power-on and reset pulses.

Signed-off-by: Johan Hovold <johan@kernel.org>
---
 Documentation/ABI/testing/sysfs-tty |  7 +++++++
 drivers/tty/serial/serial_core.c    | 29 +++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-tty b/Documentation/ABI/testing/sysfs-tty
index e157130a6792..2634b4bf9c7f 100644
--- a/Documentation/ABI/testing/sysfs-tty
+++ b/Documentation/ABI/testing/sysfs-tty
@@ -161,3 +161,10 @@ Contact:	Andy Shevchenko <andriy.shevchenko@linux.intel.com>
 Description:
 		 Allows user to detach or attach back the given device as
 		 kernel console. It shows and accepts a boolean variable.
+
+What:		/sys/class/tty/ttyS0/nordy
+Date:		November 2020
+Contact:	Johan Hovold <johan@kernel.org>
+Description:
+		 Show and store the port NORDY flag which suppresses raising
+		 the modem-control lines on open to signal DTE readiness.
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index f41cba10b86b..063a617182ce 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2805,6 +2805,33 @@ static ssize_t console_store(struct device *dev,
 	return ret < 0 ? ret : count;
 }
 
+static ssize_t nordy_show(struct device *dev, struct device_attribute *attr,
+				char *buf)
+{
+	struct tty_port *port = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%d\n", tty_port_nordy(port));
+}
+
+static ssize_t nordy_store(struct device *dev, struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct tty_port *port = dev_get_drvdata(dev);
+	unsigned int val;
+	int ret;
+
+	ret = kstrtouint(buf, 0, &val);
+	if (ret)
+		return ret;
+
+	if (val > 1)
+		return -EINVAL;
+
+	tty_port_set_nordy(port, val);
+
+	return count;
+}
+
 static DEVICE_ATTR_RO(uartclk);
 static DEVICE_ATTR_RO(type);
 static DEVICE_ATTR_RO(line);
@@ -2819,6 +2846,7 @@ static DEVICE_ATTR_RO(io_type);
 static DEVICE_ATTR_RO(iomem_base);
 static DEVICE_ATTR_RO(iomem_reg_shift);
 static DEVICE_ATTR_RW(console);
+static DEVICE_ATTR_RW(nordy);
 
 static struct attribute *tty_dev_attrs[] = {
 	&dev_attr_uartclk.attr,
@@ -2835,6 +2863,7 @@ static struct attribute *tty_dev_attrs[] = {
 	&dev_attr_iomem_base.attr,
 	&dev_attr_iomem_reg_shift.attr,
 	&dev_attr_console.attr,
+	&dev_attr_nordy.attr,
 	NULL
 };
 
-- 
2.26.2