dummycon: Trigger redraw when switching consoles with deferred takeover

Signal vt subsystem to redraw console when switching to dummycon
with deferred takeover enabled. Makes the console switch to fbcon
and displays the available output.

With deferred takeover enabled, dummycon acts as the placeholder
until the first output to the console happens. At that point, fbcon
takes over. If the output happens while dummycon is not active, it
cannot inform fbcon. This is the case if the vt subsystem runs in
graphics mode.

A typical graphical boot starts plymouth, a display manager and a
compositor; all while leaving out dummycon. Switching to a text-mode
console leaves the console with dummycon even if a getty terminal
has been started.

Returning true from dummycon's con_switch helper signals the vt
subsystem to redraw the screen. If there's output available dummycon's
con_putc{s} helpers trigger deferred takeover of fbcon, which sets a
display mode and displays the output. If no output is available,
dummycon remains active.

v2:
- make the comment slightly more verbose (Javier)

Signed-off-by: Thomas Zimmermann <[e-mail]>
Reported-by: Andrei Borzenkov <[e-mail]>
Closes: https://bugzilla.suse.com/show_bug.cgi?id=1242191
Tested-by: Andrei Borzenkov <[e-mail]>
Acked-by: Javier Martinez Canillas <[e-mail]>
Fixes: 83d83bebf401 ("console/fbcon: Add support for deferred console takeover")
Cc: Hans de Goede <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: <[e-mail]> # v4.19+
Link: https://lore.kernel.org/r/[e-mail]

drivers/video/console/dummycon.c

···
 	/* Redraw, so that we get putc(s) for output done while blanked */
 	return true;
 }
+
+static bool dummycon_switch(struct vc_data *vc)
+{
+	/*
+	 * Redraw, so that we get putc(s) for output done while switched
+	 * away. Informs deferred consoles to take over the display.
+	 */
+	return true;
+}
 #else
 static void dummycon_putc(struct vc_data *vc, u16 c, unsigned int y,
 			  unsigned int x) { }
···
 			   unsigned int ypos, unsigned int xpos) { }
 static bool dummycon_blank(struct vc_data *vc, enum vesa_blank_mode blank,
 			   bool mode_switch)
+{
+	return false;
+}
+static bool dummycon_switch(struct vc_data *vc)
 {
 	return false;
 }
···
 static bool dummycon_scroll(struct vc_data *vc, unsigned int top,
 			    unsigned int bottom, enum con_scroll dir,
 			    unsigned int lines)
-{
-	return false;
-}
-
-static bool dummycon_switch(struct vc_data *vc)
 {
 	return false;
 }