linux-kernel - Re: tty: add 'active' sysfs attribute to tty0 and console device

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Message-ID: <AANLkTimn7zyEjLhmOmUVQGnLDfM-A_uEgWETsM3nsyuY@mail.gmail.com>
Date:	Tue, 16 Nov 2010 17:13:07 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg KH <greg@...ah.com>,
	Lennart Poettering <lennart@...ttering.net>,
	Werner Fink <werner@...e.de>, Jiri Slaby <jslaby@...e.cz>
Subject: Re: tty: add 'active' sysfs attribute to tty0 and console device

On Tue, Nov 16, 2010 at 16:57, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> On Tue, 16 Nov 2010 16:46:40 +0100
> Kay Sievers <kay.sievers@...y.org> wrote:
>
>> commit be0d5f02c9194fe41c1aad11d7282db117bda938
>> Author: Kay Sievers <kay.sievers@...y.org>
>> Date:   Tue Nov 9 18:53:59 2010 +0100
>>
>>     tty: add 'active' sysfs attribute to tty0 and console device
>
> This is all somewhat weird.
>
>>     Userspace can query the actual virtual console, and the configured
>>     console devices behind /dev/tt0 and /dev/console.
>
> All the other vt interface code is in the vt driver, the ioctls for it
> are in the vt driver and a query about what is the active vt only has
> meaning within that context as you need to post a waitevent first to
> track changes during the query. So if you need a VT_GETACTIVE interface
> put it in the tty ioctls where it can be properly locked and used.
>
>> +What:                /sys/class/tty/tty0/active
>> +Date:                Nov 2010
>> +Contact:     Kay Sievers <kay.sievers@...y.org>
>> +Description:
>> +              Shows the currently active virtual console
>> +              device, like 'tty1'.
>> +              The file supports poll() to detect virtual
>> +              console switches.
>
> NAK this, its a nonsense interface
>
> Seriously what use is an interface that tells you "what the console might
> have been", this is why we have a proper event tracking interface instead.

You mean the VT_WAITEVENT? Sleeping ioctls() can't be used. The time
between the check for the current, and you go to sleep in the ioctl()
for teh next is a window which isn't covered with such interface.

And 'might have been' does not matter, it's fine. We just need change
notification, and can query the actual vt again.

Kay
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