linux-kernel - Re: [RFC] Input: introduce ABS

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Message-ID: <20131006231955.GA26952@yabbi.redhat.com>
Date:	Mon, 7 Oct 2013 10:19:55 +1100
From:	Peter Hutterer <peter.hutterer@...-t.net>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	David Herrmann <dh.herrmann@...il.com>,
	linux-input@...r.kernel.org, Henrik Rydberg <rydberg@...omail.se>,
	Benjamin Tissoires <benjamin.tissoires@...il.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Input: introduce ABS_MAX2/CNT2 and friends

On Sun, Oct 06, 2013 at 12:47:00AM -0700, Dmitry Torokhov wrote:
> On Fri, Oct 04, 2013 at 09:32:23AM +1000, Peter Hutterer wrote:
> > On Thu, Oct 03, 2013 at 12:10:36AM +0200, David Herrmann wrote:
> > > As we painfully noticed during the 3.12 merge-window our
> > > EVIOCGABS/EVIOCSABS API is limited to ABS_MAX<=0x3f. We tried several
> > > hacks to work around it but if we ever decide to increase ABS_MAX, the
> > > EVIOCSABS ioctl ABI might overflow into the next byte causing horrible
> > > misinterpretations in the kernel that we cannot catch.
> > > 
> > > Therefore, we decided to go with ABS_MAX2/CNT2 and introduce two new
> > > ioctls to get/set abs-params. They no longer encode the ABS code in the
> > > ioctl number and thus allow up to 4 billion ABS codes.
> > > 
> > > Unfortunately, the uinput API also hard-coded the ABS_CNT value in its
> > > ABI. To avoid any hacks in uinput, we simply introduce a new
> > > uinput_user_dev2 to replace the old one. The new API allows growing
> > > ABS_CNT2 values without any API changes.
> > > 
> > > Signed-off-by: David Herrmann <dh.herrmann@...il.com>
> > > ---
> > > Hi
> > > 
> > > This is only compile-tested but I wanted to get a first revision out to let
> > > people know what we're working on. Unfortunately, the ABS API has this horribly
> > > low ABS_MAX limit and we couldn't figure out a way to increase it while keeping
> > > ABI compatibility.
> > > 
> > > Any feedback and review is welcome. And if anyone spots ABI breakage by this
> > > patch, please let me know. If nothing comes up I will patch libevdev to use the
> > > new API, write some extensive test-cases and push this forward.
> > > 
> > > As a sidenote: I didn't modify joydev to use the new values. Fortunately, the
> > > joydev API would allow switching to ABS_CNT2 without breaking API, but it would
> > > limit the new ABS_CNT2 to 16k. This is quite high but nothing compared to the
> > >  2^32 that we can theoretically support now. If you think 16k ought to be enough
> > > (probably?) I can adjust the joydev API, too.
> > > All other kernel users were converted to the new values. Nothing left behind..
> > 
> > 
> > just a comment from skimming the patch:
> > if you need a new uinput abi anyway, can we add the resolution here? it's
> > sorely needed for some tests. see also the patch Benjamin sent a while ago
> > ("input/uinput: support abs resolution", July 15 2013)
> 
> Indeed. Also, while we are at it, would it make sense to allow
> requesting a range of ABS infos at once?

yes, but what API did you have in mind?

Cheers,
   Peter
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