Previously, in the userspace, it was possible to use the "setterm" command
from util-linux to blank the VT console by default, using the following
command.
# setterm -blank force
According to the man page,
> The force option keeps the screen blank even if a key is pressed.
It was implemented by calling TIOCL_BLANKSCREEN.
case BLANKSCREEN:
ioctlarg = TIOCL_BLANKSCREEN;
if (ioctl(STDIN_FILENO, TIOCLINUX, &ioctlarg))
warn(_("cannot force blank"));
break;
However, after Linux 4.12, this command ceased to work anymore, which is
unexpected. By inspecting the kernel source, it shows that the issue was
triggered by the side-effect from commit a4199f5e ("tty: Disable default
console blanking interval").
The console blanking is implemented by function do_blank_screen(). It can
be called by the inactive timer "console_timer" with the time specified by
"blankinterval", or by the IOCTL call TIOCL_BLANKSCREEN. The variable
"blank_state" is used to indicate the current status of console blanking,
it can have three possible values, blank_off, blank_normal_wait, or
blank_vesa_wait.
When do_blank_screen() is called by "console_timer" or TIOCL_BLANKSCREEN:
1. If blank_state == vesa_off_interval, it means the console was already
soft-blanked previously, and now it's the time to use VESA to put the CRT
into a deeper sleep mode. When we're done, blank_state is set to blank_off,
means the screen is now off, return.
2. If blank_state == blank_normal_wait, it means the console was previously on,
waiting to be blanked by the inactive timer. It will set blank_state to
"blank_off", means the screen is now off, and it then blanks the console. The
exception is that, it also checks if VESA powersaving is on, if so, blank_state
will be set to "blank_vesa_wait" instead, and the timer "console_timer" is
reinitialized to call do_blank_screen() again after "vesa_off_interval", for
VESA blanking, see Step 1.
3. If blank_state != blank_normal_wait at this point, it means the screen is
neither on, or VESA blanked - it must be blanked already, so return.
The problem is, "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, per Step 3. Even if a forced blanking is required from the user
by calling TIOCL_BLANKSCREEN, the console won't be blanked.
In other words, the screen cannot be blanked if autoblanking is disabled,
unless we set a "blankinterval" beforehand, and the confusingly, the ioctl
will not even return an error code.
This behavior is unexpected from a user's point-of-view, since it's not
mentioned in any documentation. The setterm man page suggests it will always
work, and the kernel comments in uapi/linux/tiocl.h says
> #define TIOCL_BLANKSCREEN 14 /* keep screen blank even if a key is pressed */
The absence of an error is also misleading.
Personally, I consider it is either: (a) a bug of the implementation,
"blankinterval" shouldn't be a kill switch for manual console blanking, (b) or a
bug of inadequate documentation.
In the following patch, I suggest making a minor change to the console blanking
logic. We introduce a 4th "blank_state" - "blank_normal_notimer", it indicates
the console can be blanked, but not automatically by a timer. Then, we made a
change to "con_init()" - if (blankinterval == 0), "blank_state" will be set to
"blank_normal_notimer" (we have similar changes to "do_unblank_screen()" and
"poke_blanked_console()"). Finally, we change Step 3 in do_blank_screen() - now
it will return if "blank_state" is neither "blank_normal_wait" nor
"blank_normal_notimer", thus allowing the console to be blanked even if there's
no timed autoblanking.
Dear maintainers, please review if my proposed chance is appropriate.
Thanks,
Tom Li
Yifeng Li (1):
tty: vt.c: Fix TIOCL_BLANKSCREEN VT console blanking if blankinterval
== 0
drivers/tty/vt/vt.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
--
2.20.1
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.
In this commit, we introduce a 4th "blank_state" - "blank_normal_notimer",
it indicates the console can be blanked, but not automatically by a timer.
Then, we made a change to "con_init()" - if (blankinterval == 0),
"blank_state" will be set to "blank_normal_notimer" (we have similar
changes to "do_unblank_screen()" and "poke_blanked_console()"). Finally,
we change do_blank_screen() - now it will return if "blank_state" is
neither "blank_normal_wait" nor "blank_normal_notimer", thus allowing
the console to be blanked even if there's no timed autoblanking.
Signed-off-by: Yifeng Li <[email protected]>
---
drivers/tty/vt/vt.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 41ec8e5010f3..ec84bc2868e1 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -238,6 +238,7 @@ enum {
blank_off = 0,
blank_normal_wait,
blank_vesa_wait,
+ blank_normal_notimer, /* no auto blanking, only manual blanking */
};
/*
@@ -3334,6 +3335,8 @@ static int __init con_init(void)
if (blankinterval) {
blank_state = blank_normal_wait;
mod_timer(&console_timer, jiffies + (blankinterval * HZ));
+ } else {
+ blank_state = blank_normal_notimer;
}
for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
@@ -4168,7 +4171,8 @@ void do_blank_screen(int entering_gfx)
return;
}
- if (blank_state != blank_normal_wait)
+ if (blank_state != blank_normal_wait &&
+ blank_state != blank_normal_notimer)
return;
blank_state = blank_off;
@@ -4232,6 +4236,8 @@ void do_unblank_screen(int leaving_gfx)
if (blankinterval) {
mod_timer(&console_timer, jiffies + (blankinterval * HZ));
blank_state = blank_normal_wait;
+ } else {
+ blank_state = blank_normal_notimer;
}
console_blanked = 0;
@@ -4293,7 +4299,8 @@ void poke_blanked_console(void)
else if (blankinterval) {
mod_timer(&console_timer, jiffies + (blankinterval * HZ));
blank_state = blank_normal_wait;
- }
+ } else if (!blankinterval)
+ blank_state = blank_normal_notimer;
}
/*
--
2.20.1
On Tue, 26 Feb 2019, Yifeng Li wrote:
> 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.
>
> In this commit, we introduce a 4th "blank_state" - "blank_normal_notimer",
> it indicates the console can be blanked, but not automatically by a timer.
> Then, we made a change to "con_init()" - if (blankinterval == 0),
> "blank_state" will be set to "blank_normal_notimer" (we have similar
> changes to "do_unblank_screen()" and "poke_blanked_console()"). Finally,
> we change do_blank_screen() - now it will return if "blank_state" is
> neither "blank_normal_wait" nor "blank_normal_notimer", thus allowing
> the console to be blanked even if there's no timed autoblanking.
>
> Signed-off-by: Yifeng Li <[email protected]>
Wouldn't this problem be fixed simply by the following:
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 9646ff63e7..41b17c4b5a 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4151,8 +4151,6 @@ void do_blank_screen(int entering_gfx)
return;
}
- if (blank_state != blank_normal_wait)
- return;
blank_state = blank_off;
/* don't blank graphics */
I just can't find a logical reason for that conditional return.
Nicolas
> Wouldn't this problem be fixed simply by the following:
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 9646ff63e7..41b17c4b5a 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -4151,8 +4151,6 @@ void do_blank_screen(int entering_gfx)
> return;
> }
>
> - if (blank_state != blank_normal_wait)
> - return;
> blank_state = blank_off;
>
> /* don't blank graphics */
>
> I just can't find a logical reason for that conditional return.
>
>
> Nicolas
I see. I was concerned about the possibly unwanted effect if the function
is reentered, but it seems that there's no logic reason to worry at all.
I'll send a new patch according to your suggestion.
Thanks,
Tom Li