USB: allow users to run setserial with cdc-acm

We had a user report that running setserial on /dev/ttyACM0 didn't work.
He pointed at an old patch by Oliver Neukum from 2008 that never went anywhere..
http://permalink.gmane.org/gmane.linux.usb.general/9236

I made some minor changes to get it to apply again, and got the user to retest on 3.1,
and he reported it worked for him. https://bugzilla.redhat.com/show_bug.cgi?id=787607

The diff below is against 3.3rc. The only difference between this and
the version the user tested is the removal of the if (!ACM_READY) test
Havard removed ACM_READY in 99823f45
I'm unclear if there's need for a different test in its place.

From: Oliver Neukum <oliver@neukum.org>
Signed-off-by: NDave Jones <davej@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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>
@@ -773,10 +774,37 @@ static int acm_tty_tiocmset(struct tty_struct *tty,
 	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)
 {
-	return -ENOIOCTLCMD;
+	struct acm *acm = tty->driver_data;
+	int rv = -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[] = {