usb: cdc-acm: add PPS support

This patch adds support for PPS to CDC devices. Changes to the DCD pin
are monitored and passed to the ldisc system, which is used by
pps-ldisc.

Signed-off-by: Dan Drown <dan-netdev@drown.org>
Link: https://lore.kernel.org/r/ZM8ExV6bAvJtIA1d@vps3.drown.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -28,6 +28,7 @@
 #include <linux/serial.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>
+#include <linux/tty_ldisc.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
 #include <linux/uaccess.h>
@@ -324,8 +325,17 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 
 		if (difference & USB_CDC_SERIAL_STATE_DSR)
 			acm->iocount.dsr++;
-		if (difference & USB_CDC_SERIAL_STATE_DCD)
+		if (difference & USB_CDC_SERIAL_STATE_DCD) {
+			if (acm->port.tty) {
+				struct tty_ldisc *ld = tty_ldisc_ref(acm->port.tty);
+				if (ld) {
+					if (ld->ops->dcd_change)
+						ld->ops->dcd_change(acm->port.tty, newctrl & USB_CDC_SERIAL_STATE_DCD);
+					tty_ldisc_deref(ld);
+				}
+			}
 			acm->iocount.dcd++;
+		}
 		if (newctrl & USB_CDC_SERIAL_STATE_BREAK) {
 			acm->iocount.brk++;
 			tty_insert_flip_char(&acm->port, 0, TTY_BREAK);