Messages in this thread Patch in this message | | | From | Yifeng Li <> | Subject | [PATCH RESEND] tty: vt.c: Fix TIOCL_BLANKSCREEN console blanking if blankinterval == 0 | Date | Tue, 5 Mar 2019 07:02:49 +0800 |
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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.
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 a4199f5eb809 ("tty: Disable default console blanking interval").
The console blanking is implemented by function do_blank_screen() 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.
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
> /* keep screen blank even if a key is pressed */ > #define TIOCL_BLANKSCREEN 14
To fix it, we simply remove the "blank_state != blank_off" check, as pointed out by Nicolas Pitre, this check doesn't logically make sense and it's safe to remove.
Suggested-by: Nicolas Pitre <nicolas.pitre@linaro.org> Fixes: a4199f5eb809 ("tty: Disable default console blanking interval") Signed-off-by: Yifeng Li <tomli@tomli.me> --- 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
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