usb: cdc-acm: return correct error code on unsupported break

In ACM support for sending breaks to devices is optional.
If a device says that it doenot support sending breaks,
the host must respect that.
Given the number of optional features providing tty operations
for each combination is not practical and errors need to be
returned dynamically if unsupported features are requested.

In case a device does not support break, we want the tty layer
to treat that like it treats drivers that statically cannot
support sending a break. It ignores the inability and does nothing.
This patch uses EOPNOTSUPP to indicate that.

Signed-off-by: Oliver Neukum <[e-mail]>
Fixes: 9e98966c7bb94 ("tty: rework break handling")
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_io.c

···
 	if (!retval) {
 		msleep_interruptible(duration);
 		retval = tty->ops->break_ctl(tty, 0);
+	} else if (retval == -EOPNOTSUPP) {
+		/* some drivers can tell only dynamically */
+		retval = 0;
 	}
 	tty_write_unlock(tty);
 

drivers/usb/class/cdc-acm.c

···
 	struct acm *acm = tty->driver_data;
 	int retval;
 
+	if (!(acm->ctrl_caps & USB_CDC_CAP_BRK))
+		return -EOPNOTSUPP;
+
 	retval = acm_send_break(acm, state ? 0xffff : 0);
 	if (retval < 0)
 		dev_dbg(&acm->control->dev,