Date:	Mon, 7 Oct 2013 12:30:58 +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 05:04:36PM -0700, Dmitry Torokhov wrote:
> Peter Hutterer <peter.hutterer@...-t.net> wrote:
> >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?
> >
> 
> I was thinking about specifying the start ABS but and the count and array of absinfo structures to be filled.

yeah, that works for me (I suspect 90% of users will use ABS_MAX anyway :)

Cheers,
   Peter

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