Re: [PATCH 2/6] Input: elantech - use firmware provided x, y ranges

[Chase Douglas <chase.douglas@canonical.com>]

On 09/06/2011 11:47 AM, Henrik Rydberg wrote:
>>>> I see that the current code scales the coordinates for v2, but it's only
>>>> half-resolution. That's not a huge deal since the resolution of modern
>>>> touchpads is very high. We could leave it scaled to not break abi, if
>>>> that was a concern. However, with new devices it makes sense to state
>>>> the ranges in terms of what the device actually supports. Otherwise,
>>>> we're just masking out useful data that userspace could be using.
>>>>
>>>
>>> I disagree. I believe that ST and MT ranges reported for the same
>>> working surface should match, especially since many devices derive ST
>>> data from MT.
>>>
>>> As far as devices that have ranges 0-15 in MT mode - I am not sure how
>>> useful such MT steam anyway and if we are better of just ignore them
>>> (maybe just use the data to report number of fingers on the surface but
>>> otherwise use standard ST protocol).
>>
>> The MT data could still be useful for pinch to zoom or potentially
>> rotate (though most low res devices probably are only semi-mt). I don't
>> want to forsake pinch to zoom just because we can't pass on the
>> resolution of MT data properly.
> 
> We are not forsaking anything. At best, the detail missed out by using
> the same scaling for ST and MT is the ability to distinguish the axis
> with the best or poorest resolution. However, some devices report
> higher MT than ST resolution, not because it is better (it is indeed
> worse), but simply to not lose bits in the filtering process. In other
> words: the logic is not without corner cases, and it only makes a
> difference to the transitional devices that are not fully MT
> capable. If this really was a concern, userspace could use the ST
> coordinates for some transformations instead.

So what do we want userspace to do? In the absence of supplying this
information we need to provide guidelines. We could say:

* Always use ST coords + finger count for drag gestures (i.e. scrolling)
* Use MT coords for pinch and rotate

If this is acceptable, we should add it to the documentation. Thoughts?

-- Chase