From mboxrd@z Thu Jan  1 00:00:00 1970
From: Arnd Bergmann <arnd@arndb.de>
Subject: Re: [RFC][PATCH] input: Introduce device information ioctl
Date: Tue, 7 Dec 2010 13:55:08 +0100
Message-ID: <201012071355.08714.arnd@arndb.de>
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To: Kay Sievers <kay.sievers@vrfy.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>, Henrik Rydberg <rydberg@euromail.se>, Jiri Kosina <jkosina@suse.cz>, linux-input@vger.kernel.org, linux-kernel@vger.kernel.org, Ping Cheng <pingc@wacom.com>, Chris Bagwell <chris@cnpbagwell.com>

On Tuesday 07 December 2010, Kay Sievers wrote:
> >> Oh, how does that work? With the ioctl call, userspace has to supply
> >> the size it expects to be returned from the kernel. How does the
> >> kernel otherwise know how much it is allowed to copy to the user?
> >
> > The ioctl command number itself is calculated from the size of the
> > data that gets passed:
> >
> > #define EVIOCGDEVINFO          _IOR('E', 0x09, struct input_devinfo)
> >
> > If struct input_devinfo ever changes (which it can, but should not),
> > the command changes as well.
> 
> So unlike statet, it's not extensible, and this struct and this ioctl
> can never change?

Exactly. We have plenty of unused ioctl numbers free though.

	Arnd