linux-kernel - Re: [RFC][PATCH] input: Introduce device information ioctl

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Message-Id: <201012071355.08714.arnd@arndb.de>
Date:	Tue, 7 Dec 2010 13:55:08 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Henrik Rydberg <rydberg@...omail.se>,
	Jiri Kosina <jkosina@...e.cz>, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ping Cheng <pingc@...om.com>,
	Chris Bagwell <chris@...bagwell.com>
Subject: Re: [RFC][PATCH] input: Introduce device information ioctl

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
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