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

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Message-Id: <201012071344.01173.arnd@arndb.de>
Date:	Tue, 7 Dec 2010 13:44:00 +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:
> On Tue, Dec 7, 2010 at 11:56, Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> > On Tue, Dec 07, 2010 at 11:48:28AM +0100, Kay Sievers wrote:
> 
> >> Please don't add new ioctls which are not extensible. The ioctl should
> >> carry the length or the version of the structure it asks for, so it
> >> can be extended in the future.
> >
> > Size of ioctl data is encoded in ioctl, it can be extended easily. For
> > examples take a look at how EVIOCGKEYCODE and EVIOCGSKEYCODE are handled
> > in recent kernels.
> 
> 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.

	Arnd
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