linux-kernel - Re: [PATCH] um: change defconfig to stop spawning xterm

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Message-ID: <20130722223238.GC13191@tango.0pointer.de>
Date:	Tue, 23 Jul 2013 00:32:39 +0200
From:	Lennart Poettering <mzxreary@...inter.de>
To:	Ramkumar Ramachandra <artagnon@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Richard Weinberger <richard@....at>
Subject: Re: [PATCH] um: change defconfig to stop spawning xterm

On Mon, 22.07.13 16:13, Ramkumar Ramachandra (artagnon@...il.com) wrote:

> 
> [Corrected Lennart's email ID]
> 
> Richard Weinberger wrote:
> > CC'ing Lennart.
> >
> > Am 22.07.2013 11:45, schrieb Ramkumar Ramachandra:
> >> Ramkumar Ramachandra wrote:
> >>> [1]: http://lists.freedesktop.org/archives/systemd-devel/2013-July/012152.html
> >>
> >> ... and the patches were rejected.  Lennart says that UML providing
> >> /dev/tty* is wrong, and that UML should call them /dev/hvc* (or
> >> something).  Can we do something about the situation?  Can we remove
> >> /dev/tty*, and provide /dev/hvc*?  Will we be breaking existing users?
> >>
> >> Thanks.
> >>
> >> Lennart Poettering wrote:
> >>> UML shouldn't be penalized for not implementing some terminal emulation,
> >>> but it should be penalized for doing so under the label of "VT support",
> >>> which it simply is not providing.
> >>>
> >>> They can call their ttys any way they want. If the call them
> >>> /dev/tty[1..64] however, then they need to implement the VC
> >>> interfaces. All of them.
> >
> > Lennart, can you please explain us why /dev/tty[1..64] is forced to
> > have virtual console support?

/dev/tty[1..64] is the userspace API to the kernel VT subsystem. If you
support it you need to match up all /dev/tty[1..64] with a
/dev/vcs[1..64] + /dev/vcsa[1..64]. You need to expose a tty that
understands TERM=linux and the ioctls listed on console_ioctl(4). You
need /dev/tty0 as something that behaves like a symlink to the fg
VT. You should also support files like /sys/class/tty/tty0/active with
its POLLHUP iface.

If you expose a very different terminal than a VT as /dev/tty[1..64]
this will confuse a lot of userspace, since userspace, be it
systemd/logind, gpm, X11, openvt, ... all expect that /dev/tty[1..64] is
the VT subsystem where all that functionality is available. And you just
broke the userspace API quite badly.

It's totally fine to register ttys with a different feature set under
some other name you like, but if you name it /dev/tty[1..64] then
userspace expects this to be the real deal.

The various hypervisors understood this and provide their ttys under the
name of /dev/hvc0 and suchlike. UML should probably do something
similar. If you pick a name of your own, you have complete freedom what
you actually implement...

Lennart

-- 
Lennart Poettering - Red Hat, Inc.
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