vt: perform safe console erase in the right order

Commit 4b4ecd9 ("vt: Perform safe console erase only once") removed
what appeared to be an extra call to scr_memsetw(). This missed the fact
that set_origin() must be called before clearing the screen otherwise
old screen content gets restored on the screen when using vgacon. Let's
fix that by moving all the scrollback handling to flush_scrollback()
where it logically belongs, and invoking it before the actual screen
clearing in csi_J(), making the code simpler in the end.

Reported-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Matthew Whitehead <tedheadster@gmail.com>
Fixes: 4b4ecd9 ("vt: Perform safe console erase only once")
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Nicolas Pitre

drivers/tty/vt/vt.c

@@ -935,8 +935,11 @@ static void flush_scrollback(struct vc_data *vc)
 {
 	WARN_CONSOLE_UNLOCKED();
 
+	set_origin(vc);
 	if (vc->vc_sw->con_flush_scrollback)
 		vc->vc_sw->con_flush_scrollback(vc);
+	else
+		vc->vc_sw->con_switch(vc);
 }
 
 /*
@@ -1505,8 +1508,10 @@ static void csi_J(struct vc_data *vc, int vpar)
 			count = ((vc->vc_pos - vc->vc_origin) >> 1) + 1;
 			start = (unsigned short *)vc->vc_origin;
 			break;
+		case 3: /* include scrollback */
+			flush_scrollback(vc);
+			/* fallthrough */
 		case 2: /* erase whole display */
-		case 3: /* (and scrollback buffer later) */
 			vc_uniscr_clear_lines(vc, 0, vc->vc_rows);
 			count = vc->vc_cols * vc->vc_rows;
 			start = (unsigned short *)vc->vc_origin;
@@ -1515,13 +1520,7 @@ static void csi_J(struct vc_data *vc, int vpar)
 			return;
 	}
 	scr_memsetw(start, vc->vc_video_erase_char, 2 * count);
-	if (vpar == 3) {
-		set_origin(vc);
-		flush_scrollback(vc);
-		if (con_is_visible(vc))
-			update_screen(vc);
-	} else if (con_should_update(vc))
-		do_update_region(vc, (unsigned long) start, count);
+	update_region(vc, (unsigned long) start, count);
 	vc->vc_need_wrap = 0;
 }