From: Arnd Bergmann <arnd@arndb.de>
To: Kay Sievers <kay.sievers@vrfy.org>
Subject: Re: [RFC][PATCH] input: Introduce device information ioctl
Date: Tue, 7 Dec 2010 13:44:00 +0100
User-Agent: KMail/1.12.2 (Linux/2.6.31-22-generic; KDE/4.3.2; x86_64; ; )
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>
References: <1291706726-8835-1-git-send-email-rydberg@euromail.se> <20101207105620.GB23729@core.coreip.homeip.net> <AANLkTinmcKgUJfs+0_E6Zke6Y0+nfMU71Z8sY_-7-0iE@mail.gmail.com>
In-Reply-To: <AANLkTinmcKgUJfs+0_E6Zke6Y0+nfMU71Z8sY_-7-0iE@mail.gmail.com>
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="utf-8"
Content-Transfer-Encoding: 7bit
Message-Id: <201012071344.01173.arnd@arndb.de>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1284
Lines: 30

On Tuesday 07 December 2010, Kay Sievers wrote:
> On Tue, Dec 7, 2010 at 11:56, Dmitry Torokhov <dmitry.torokhov@gmail.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/