vcs: restore and document initial POLLPRI event

Restore and document the forced initial POLLPRI event reporting when
poll() is used for the first time. This used to be the implemented
behavior before recent changes. Because of the way poll() is implemented,
this prevents losing an event happening between the last read() and the
first poll() invocation.

Since poll() for /dev/vcs* was not always supported, user space probes
for its availability as follows:

	int fd = open("/dev/vcsa", O_RDONLY);
	struct pollfd p = { .fd = fd, .events = POLLPRI };
	available = (poll(&p, 1, 0) == 1);

Semantically, it makes sense to signal the first event as such even if
it might be spurious. The screen could be modified, and modified back
to its initial state before we get to read it, so users must be prepared
for that anyway.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vc_screen.c

@@ -140,6 +140,15 @@ vcs_poll_data_get(struct file *file)
 	poll->cons_num = console(file_inode(file));
 	init_waitqueue_head(&poll->waitq);
 	poll->notifier.notifier_call = vcs_notifier;
+	/*
+	 * In order not to lose any update event, we must pretend one might
+	 * have occurred before we have a chance to register our notifier.
+	 * This is also how user space has come to detect which kernels
+	 * support POLLPRI on /dev/vcs* devices i.e. using poll() with
+	 * POLLPRI and a zero timeout.
+	 */
+	poll->event = VT_UPDATE;
+
 	if (register_vt_notifier(&poll->notifier) != 0) {
 		kfree(poll);
 		return NULL;