Re: [PATCH] vt: Add VC_MUTE (v2)

[Arthur Taylor]

> The "don't enqueue stuff" semantics of K_OFF shouldn't be a function of
> the keyboard map state; we should be able to switch among cooked/raw/
> unicode without changing whether events are delivered.  Otherwise - if
> changing to K_UNICODE undoes K_OFF - then suddenly Alt-F2 under
> Gnome will switch VT instead of summoning the "run command" dialog.

A patch to disable the muting should be added to the SysRq handler for
the "unRaw" action.

I do not think that KBMODE is only about the keyboard map. The
behaviours that KBMODE influences are:
-Whether input is generated.
-Whether input is buffered by line.
-Whether key events are put through the keymap and translated by
modifiers or sent as raw keyup/down events.
-Whether the keymap is unicode.
-Whether special keys are interpreted by the kernel.

The meaning of KBMODE is a bit overloaded It does make sense to split
the generate / don't generate input into its own ioctl. However, IMO
this change is part of an incomplete fix to a user space problem.

The issue creating a need for moving the generate / don't generate
input to its own ioctl was that through some plumbing, when changing
the keymap setting in an X graphical environment session systemd is
told to switch the console keymap. As part of switching the keymap, if
unicode is selected, systemd blindly calls KDSKBMODE with K_UNICODE.
This currently messes up X, which this change attempts to address.

I'm not sure if letting systemd freely meddle with KBMODE and moving
the important-for-X behaviour to another ioctl is such a wise idea.
systemd already changes the kernel default KBMODE to/from unicode by a
module parameter. Further, this will only work with newer or patched X
servers.

Depending on the input configuration and drivers used by X, either the
input system is used directly and the virtual console keyboard is
ignored, or X uses the virtual console keyboard for input and switches
it into RAW mode.

In both modes X does not want the kernel to interpret special keys. In
the former mode X doesn't want the kernel to generate input (as
otherwise it needs to periodically flush a buffer it never reads to
stop it from overflowing and causing a kworker to go nuts), in the
later mode X wants raw key up/down events which are not line buffered.

Setting the console to K_UNICODE is none of these things and causes
one of many possible problems.

KBMODE (like the console keymap) is a virtual console setting which is
stored in the virtual terminal and saved/restored from the console
upon a VT switch.

X is usually run on a VT it allocates and then closes on exit, but you
can also specify a VT for X to run on which could already have been
open by another process (say getty.) In either case, X saves the
KBMODE of the VT during initialization and restores it upon exit.

systemd calling KDSKBMODE on a VT which X is running either has no
affect or a negative affect on X. In the rare case that X is started
on a VT with another text mode cooked input process, KBMODE is
unaffected after X exits.

-Art
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