[RFC v2 libinput 0/2] Buttonset interface - numbered axes

[Hans de Goede]

Hi,

On 20-03-15 01:38, Peter Hutterer wrote:
> On Fri, Mar 20, 2015 at 10:19:06AM +1000, Peter Hutterer wrote:
> [...]
>>> 5) If a device has multiple axes of the same type, is the order
>>> supposed to be ABI? Example: gamepads tend to have two analog-sticks.
>>> How do applications reliably find the left and right stick?
>>
>> well, our promise is that the same device will come up in the same order of
>> axes. so once you figure out that the left stick is the first, you can rely
>> on it always being the first on that device.
>>
>> if the kernel driver changes out from underneath us that could be
>> interesting but then again that'd be an argument for a kernel regression :)
>
> one clarification here:
> the intention was that the axis order is fixed within the type, but may
> otherwise change over time.
> i.e. if you have two RING axes, they'll stay the same order but they do
> _not_ always have the same axis number.
>
> The device may have X, Y, Z, RING, RING but change to X, Y, Z, WHEEL,
> RING, RING in the future. Or even X, RING, Y, RING, Z.
>
> We only guarantee that the first ring is the same ring as before. in other
> words, the axis number is _not_ ABI, it's just an index.
>
> (This was prompted by Jason's reply re:accelerometers in the 27QHD, we may
> not support them now but if they are added in the future they may slot in at
> any position.)

This is somewhat unfortunate, but as I think about a way around it, this seems
unavoidable. So I think this is fine, but it needs to be very clearly documented
somewhere else then in this mailinglist post.

Regards,

Hans