[PATCH] tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0

[Yifeng Li]

In vt.c, "blank_state" will be initialized to "blank_normal_wait" in
con_init() if AND ONLY IF ("blankinterval" > 0). If "blankinterval" is 0,
"blank_state" will be "blank_off" (== 0), and a call to do_blank_screen()
will always abort. Even if a forced blanking is required from the user by
calling TIOCL_BLANKSCREEN, the console won't be blanked.

After Linux 4.12, blankinterval is 0 by default, which caused an unexpected
breakage of previously working "setterm -blank force". "blankinterval"
should not act like a kill switch for TIOCL_BLANKSCREEN.

My original proposal was adding a new "blank_normal_notimer" to indicate
manual console blanking via ioctl, however, it has been pointed out that
that fix was unnecessary, since do_blank_screen() is safe to reentry. To
fix it, we simply remove the "blank_state != blank_off" check. See Ref
for a detailed discussion.

Suggested-by: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
Fixes: a4199f5e ("tty: Disable default console blanking interval")
Reference: https://lore.kernel.org/lkml/20190225164346.1359-1-tomli@xxxxxxxx
Signed-off-by: Yifeng Li <tomli@xxxxxxxx>
---
 drivers/tty/vt/vt.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 41ec8e5010f3..279f5b2ac44a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4168,8 +4168,6 @@ void do_blank_screen(int entering_gfx)
 		return;
 	}
 
-	if (blank_state != blank_normal_wait)
-		return;
 	blank_state = blank_off;
 
 	/* don't blank graphics */
-- 
2.20.1