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

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Date:	Tue, 7 Dec 2010 13:40:26 +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 10:16, Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> > Hi Henrik,
> >
> > On Tue, Dec 07, 2010 at 08:25:26AM +0100, Henrik Rydberg wrote:
> >> Today, userspace sets up an input device based on the data it emits.
> >> This is not always enough; a tablet and a touchscreen may emit exactly
> >> the same data, for instance, but the former should be set up with a
> >> pointer whereas the latter does not need to. Recently, a new type of
> >> touchpad has emerged where the buttons are under the pad, which changes
> >> handling logic without changing the emitted data. This patch introduces
> >> a new ioctl, EVIOCGDEVINFO, which allows userspace to extract information
> >> about the device resulting in proper setup.
> >
> > If we agree that the new ioctl is suitable we'llalso need to wireit up
> > through sysfs. Also, can we keep all definitions to INPUT_ namespace?
> 
> 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. Sysfs should be good enough for such
> interface though.

Please never add any ioctls that have a version or length field!

Ideally ioctls should have only scalar arguments, not structures,
so I'd recommend splitting it into two, so you can read type and
capability parameters separately.

	Arnd
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