Re: [RFC] Input: introduce ABS_MAX2/CNT2 and friends

[Dmitry Torokhov]

Peter Hutterer <peter.hutterer@xxxxxxxxx> 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@xxxxxxxxx>
>> > > ---
>> > > 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.


Thanks.

-- 
Dmitry
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