From http://permalink.gmane.org/gmane.linux.usb.general/9236

attempt to diagnose https://bugzilla.redhat.com/show_bug.cgi?id=787607

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 9543b19..6dcc3a3 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -39,6 +39,7 @@
 #include <linux/serial.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>
+#include <linux/serial.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/uaccess.h>
@@ -734,15 +734,40 @@ static int acm_tty_tiocmset(struct tty_s
 	return acm_set_control(acm, acm->ctrlout = newctrl);
 }
 
+static int get_serial_info(struct acm *acm, struct serial_struct __user *info)
+{
+	struct serial_struct tmp;
+
+	if (!info)
+		return -EINVAL;
+
+	memset(&tmp, 0, sizeof(tmp));
+	tmp.flags = ASYNC_LOW_LATENCY;
+	tmp.xmit_fifo_size = acm->writesize;
+	tmp.baud_base = le32_to_cpu(acm->line.dwDTERate);
+
+	if (copy_to_user(info, &tmp, sizeof(tmp)))
+		return -EFAULT;
+	else
+		return 0;
+}
+
 static int acm_tty_ioctl(struct tty_struct *tty,
 					unsigned int cmd, unsigned long arg)
 {
 	struct acm *acm = tty->driver_data;
+	int rv = -ENOIOCTLCMD;
 
 	if (!ACM_READY(acm))
 		return -EINVAL;
 
-	return -ENOIOCTLCMD;
+	switch (cmd) {
+	case TIOCGSERIAL: /* gets serial port data */
+		rv = get_serial_info(acm, (struct serial_struct __user *) arg);
+		break;
+	}
+
+	return rv;
 }
 
 static const __u32 acm_tty_speed[] = {