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bug#30505: Starting console/terminal Unicode support

From: Ludovic Courtès
Subject: bug#30505: Starting console/terminal Unicode support
Date: Wed, 21 Feb 2018 23:21:34 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux)

Hi Danny,

Danny Milosavljevic <address@hidden> skribis:

> On Mon, 19 Feb 2018 21:35:05 +0100
> address@hidden (Ludovic Courtès) wrote:
>> > It was a bad idea to do the "\x1b%G" in the first place.  
>> Because it’s redundant with IUTF8?
> I meant because the Linux kernel does it already and it's better not to
> have random multi-byte racy writes onto the tty while the mingetty
> is starting up (and possibly buffering and pending half of another sequence).

That ‘unicode-start’ code run when the ‘console-font-ttyN’ service is
started, right after ‘term-ttyN’ (mingetty) is up.  You’d hope mingetty
is really up and running once ‘term-ttyN’ is up, no?

> As far as I understand it's not redundant to do both.
> According to src/Linux/linux-4.12-rc2/drivers/tty/vt/vt.c, the 'G' controls
> the conversion utf-8->unicode that happens before the virtual terminal
> displays a corresponding character on the screen.
> On the other hand, the termios iutf8 is meant for the program running in the 
> session.

OK, I see.

> Say you have bash on vt1, then bash can check termios for the settings and 
> find
> out whether vt1 is UTF-8-capable (also has other settings like whether the
> terminal already supports line editing etc - old-school terminals were quite
> cool; a friend of mine salvaged a real one ^^).
> Note that drivers/tty/vt/vt.c only copies ONE way, from the 'G' flag to the
> termios (and that seldomly).
> Makes sense since the programs shouldn't have a say in what the terminal can 
> do :)
> So I'd say guix services fiddling with termios is ... weird and the 'G' 
> slightly
> less weird.


>> This ‘unicode-start’ procedure is essentially a port of the
>> ‘unicode_start’ script from ‘kbd’.  I suppose the justification is to
>> make sure we’re using UTF-8 input regardless of what the kernel defaults
>> or command-line options are.
> Yeah, but it's asking for trouble.
> I just checked Linux, it defaults to utf8 (IUTF8 in termios, and 
> 'G').
> I'd suggest to remove both.

When we’re using the defaults, I understand both are unnecessary.  In
other cases (custom kernel build with different defaults, different
kernel command-line settings, etc.), we’d just get it wrong with things
breaking down the path, no?

Also, FWIW, systemd’s vconsole-setup.c does exactly that:


And again that corresponds to the ‘unicode_start’ script.



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