linux-kernel - [PATCH 3/5] USB: serial: add sysfs attribute to suppress ready signalling on open

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Date:   Mon, 30 Nov 2020 16:37:40 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        "Mychaela N . Falconia" <falcon@...ecalypso.org>,
        linux-serial@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>
Subject: [PATCH 3/5] USB: serial: add sysfs attribute to suppress ready signalling on open

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@...nel.org>
---
 drivers/usb/serial/bus.c | 38 ++++++++++++++++++++++++++++++++++++--
 1 file changed, 36 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/serial/bus.c b/drivers/usb/serial/bus.c
index eb0195cf37dd..37bbeab6666e 100644
--- a/drivers/usb/serial/bus.c
+++ b/drivers/usb/serial/bus.c
@@ -35,6 +35,40 @@ static int usb_serial_device_match(struct device *dev,
 	return 0;
 }
 
+static ssize_t nordy_show(struct device *dev, struct device_attribute *attr,
+				char *buf)
+{
+	struct usb_serial_port *port = dev_get_drvdata(dev);
+
+	return sprintf(buf, "%d\n", tty_port_nordy(&port->port));
+}
+
+static ssize_t nordy_store(struct device *dev, struct device_attribute *attr,
+				const char *buf, size_t count)
+{
+	struct usb_serial_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->port, val);
+
+	return count;
+}
+static DEVICE_ATTR_RW(nordy);
+
+static struct attribute *tty_attrs[] = {
+	&dev_attr_nordy.attr,
+	NULL
+};
+ATTRIBUTE_GROUPS(tty);
+
 static int usb_serial_device_probe(struct device *dev)
 {
 	struct usb_serial_driver *driver;
@@ -60,8 +94,8 @@ static int usb_serial_device_probe(struct device *dev)
 	}
 
 	minor = port->minor;
-	tty_dev = tty_port_register_device(&port->port, usb_serial_tty_driver,
-					   minor, dev);
+	tty_dev = tty_port_register_device_attr(&port->port,
+			usb_serial_tty_driver, minor, dev, port, tty_groups);
 	if (IS_ERR(tty_dev)) {
 		retval = PTR_ERR(tty_dev);
 		goto err_port_remove;
-- 
2.26.2