[PATCH AUTOSEL 5.10 4/6] vt: add missing notification when switching back to text mode

[Sasha Levin <sashal@kernel.org>]

From: Nicolas Pitre <npitre@baylibre.com>

[ Upstream commit ff78538e07fa284ce08cbbcb0730daa91ed16722 ]

Programs using poll() on /dev/vcsa to be notified when VT changes occur
were missing one case: the switch from gfx to text mode.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Link: https://lore.kernel.org/r/9o5ro928-0pp4-05rq-70p4-ro385n21n723@onlyvoer.pbz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES**

This commit should be backported to stable kernel trees for the
following reasons:

1. **It fixes a real bug affecting userspace programs**: The commit
   addresses a missing notification that prevents programs using
   `poll()` on `/dev/vcsa` from being properly notified when the VT
   switches from graphics mode back to text mode. This is a functional
   regression that breaks userspace applications monitoring VT state
   changes.

2. **The fix is minimal and low-risk**: The change adds only a single
   line - `notify_update(vc);` - in the `do_unblank_screen()` function.
   This follows an established pattern where `notify_update()` is called
   after VT state changes to notify userspace watchers.

3. **Consistent with existing code patterns**: Looking at the codebase,
   `notify_update()` is already called in similar contexts throughout
   vt.c. For example, in `vc_do_resize()`, we see:
  ```c
  vt_event_post(VT_EVENT_RESIZE, vc->vc_num, vc->vc_num);
  notify_update(vc);
  ```
  The same pattern should apply when unblanking the screen.

4. **Similar commits were backported**: The historical commits provided
   show that previous fixes adding or correcting `notify_update()` calls
   were marked for stable backporting (all three YES examples had `Cc:
   stable@vger.kernel.org`). These commits fixed similar issues where
   userspace poll() notifications were missing.

5. **Clear symptom and fix**: The bug has a clear symptom (missing
   notifications when switching from graphics to text mode) and a
   straightforward fix that directly addresses the root cause. The
   `leaving_gfx` parameter in `do_unblank_screen()` specifically
   indicates this transition scenario.

6. **No architectural changes**: This is purely a bug fix that restores
   expected behavior without introducing new features or changing the
   architecture of the VT subsystem.

The commit meets all the criteria for stable backporting: it fixes an
important bug affecting userspace, the fix is minimal and contained, and
it follows established patterns for similar fixes that were previously
backported.

 drivers/tty/vt/vt.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 5d9de3a53548b..98ca54330d771 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4452,6 +4452,7 @@ void do_unblank_screen(int leaving_gfx)
 	set_palette(vc);
 	set_cursor(vc);
 	vt_event_post(VT_EVENT_UNBLANK, vc->vc_num, vc->vc_num);
+	notify_update(vc);
 }
 EXPORT_SYMBOL(do_unblank_screen);
 
-- 
2.39.5