Re: [PATCH v3] bcm5974: Set BUTTONPAD property

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

On 01/17/2012 07:06 PM, Dmitry Torokhov wrote:
> On Thu, Jan 12, 2012 at 11:19:40AM +0100, Chase Douglas wrote:
>> On 01/12/2012 01:22 AM, Henrik Rydberg wrote:
>>>> Here's what I believe the meanings should be:
>>>>
>>>> Touchpad: pointer, !direct
>>>> Touchscreen: !pointer, direct
>>>> Drawing tablet: pointer, direct
>>>> Magic mouse-like devices: !pointer, !direct
>>>
>>> Yes, this is what everyone is saying, except !pointer && !direct means
>>> "default" or "figure it out some other way".
>>>
>>>> However, there is a further problem in that we can't easily support
>>>> multiple tools with different behavior on the same evdev device. What
>>>> would you say a bamboo touch+pen is, which I believe is used as an
>>>> indirect device for touch but a direct device for tools. Thus, in the
>>>> thread I linked from back in September, Henrik and I agreed that direct
>>>> should only apply when the tool is touch, and pointer should apply for
>>>> all other tools. This would result in the following:
>>>
>>> To try to move back to a sane track, try this, where the word "apply"
>>> in the previous paragraph has been changed to "care" instead:
>>
>> I am still having trouble understanding what you are saying. If I
>> literally try to insert "care" into the paragraph, I am confused because
>> it's not quite correct grammar. I'm really trying to understand though.
>> Also, maybe a better term than "don't care" is "not applicable"?
>>
>> It would help me most if you could explicitly provide your own
>> definition of the properties.
>>
>>>> Touchpad: !pointer, !direct
>>>
>>> pointer && !direct, since pointer is "dont care".
>>
>> Here you say !direct if "don't care".
>>
>>>> Touchscreen: !pointer, direct
>>>
>>> Yes, !pointer && direct.
>>>
>>>> Drawing tablet (no touch): pointer, !direct
>>>
>>> pointer && direct, but the tool is not touch, so direct is "dont care".
>>
>> Here you say direct if "don't care".
>>
>> Why the difference?
>>
>>>> Pen+touch tablet: pointer, direct
>>>
>>> Yes, pointer && direct
>>>
>>>> Magic mouse-like devices: !pointer, !direct
>>>
>>> Both pointer and direct are "dont care", and the device needs to be
>>> detected some other way. If there ever will be a special driver for
>>> magic-mouse-like devices, using both relative pointer and touch data,
>>> it will make sense to add a special property for such devices.
>>
>> Right now we are missing a property for a magic-mouse like device. It's
>> valid to have neither direct nor pointer set from kernels 2.6.38 through
>> 3.2 (at least).
>>
>>> Hopefully the above is showing clearly that what was "documented" in
>>> the threads enclosing the protocol patches still holds, and that there
>>> is no use to dwell on it further.
>>>
>>>> The properties weren't documented when they were merged, and they
>>>> obviously aren't clear. However, if either table above is correct, then
>>>> we can't assume that !pointer && !direct means "unknown".
>>>
>>> If all devices fell in the pointer or direct or both categories, we
>>> could. If not all devices do, the problem is rather that some property
>>> bits are missing (or excluded) from the description.
>>
>> Given my last statement above, we have a problem because previously
>> released kernels are reporting the magic mouse correctly, and yet we
>> still can't distinguish it from another device that merely does not have
>> the property bits set. This is the crux of the issue as I see it. We
>> cannot differentiate between "unknown" and a specific type of device
>> given the interfaces from 2.6.38 through 3.2.
>>
>>>> There is a way to fix this in a backwards compatible way: add a new
>>>> property bit called something like "PROPERTIES_AVAILABLE". If any bits
>>>> are set, then it implies that the properties are available (which covers
>>>> older kernels). If no bits are set, then the properties are unknown.
>>>> What do you think?
>>>
>>> It is rather the special properties of the magic mouse that are
>>> missing. All types of devices do not _have_ to use properties; most
>>> types can be figured out by other means.
>>>
>>> Saying "prop == 0" is equivalent to "figure out some other way" makes
>>> sense, but it is also sensible to say "(prop & some_subclass_of_bits)
>>> == 0", since some properties are bound to describe totally different
>>> things. This is what we did with "!direct && !pointer".
>>
>> This may work, but we need to document the classes. The next time any
>> properties are added the documentation must be included :).
>>
>>>>> The same is applicable to other properties as well. If device is telling
>>>>> you that it is a "buttonpad" you can trust it, but if it does not you
>>>>> need to decide for yourself how to treat it.
>>>
>>> Yes, and this will always be true. Old devices or systems that become
>>> used in new ways cannot always adapt to a "if property not present
>>> then dont use that way" policy.
>>>
>>>> No, in kernels previous to 2.6.38 it's clearly unknown. My problem is
>>>> that I believe there was no way to determine unknown properties. If
>>>> unknown properties is equivalent to magic-mouse like devices, then we're
>>>> going to treat a lot of devices wrong. Or, we have to use heuristics to
>>>> determine what a device is, like no properties and MT and REL_{X,Y} ==
>>>> magic-mouse like. Properties was supposed to resolve this once and for
>>>> all, so we didn't need heuristics.
>>>
>>> Properties were added to be able to distinguish usecases that could
>>> not be distinguished at all before. It was never meant to replace
>>> everything else.
>>
>> Why shouldn't we use it for that? The code in evdev for determining the
>> type of device is just a big hack. We'll obviously need it for a while
>> since we don't have all drivers with all necessary properties set, but
>> it seems a waste to have the interface and not fully use it.
>>
>>>>>> Henrik, can you comment on the documentation patches? You wrote the
>>>>>> patch, so you hopefully know what's going on :).
>>>
>>> I wasn't copied in on the conversation, but they seem fairly well
>>> commented on already.
>>
>> It's still not clear to me what the definitions are. It seems it won't
>> be clear until either you or Dmitry give your own definitions in an
>> explicit manner (something that could be copied into the formal
>> documentation). Please help me out :).
>>
> 
> OK, so how about this:
> 
> INPUT_PROP_DIRECT:
> 
> This property idicates that device coordinates can be directly mapped to
> screen coordinates (not taking into account trivial transformations,
> such as scaling, flipping and rotating). Non-direct input devices
> require non-trivial transformation, such as absolute to relative
> transformation for touchpads. Typical direct input devices:
> touchscreens, drawing tablets; non-direct devices: touchpads, mice.
> 
> INPUT_PROP_POINTER:
> 
> This property indicates that the device is not transposed on the screen
> and thus requires use of an on-screen pointer to trace user's movements.
> Typical pointer devices: touchpads, tablets, mice; non-pointer device:
> touchscreen.
> 
> How does this sound?

These definitions sound fine to me. We also need definitions of what it
means when a bit is set versus when it is not set. Does an unset bit
mean "unknown"? As stated before, I don't like that definition because:

* It means we can never get away from device type heuristics in user-space.
* There's no negative version of the properties. For example, there's no
way to say "this device is indirect" because it looks the same as "unknown".

Imagine a tablet driver only has INPUT_PROP_DIRECT set. According to the
"unknown" definition, it's ok to leave INPUT_PROP_POINTER as unset. But
then userspace will end up treating it like a touchscreen instead of a
tablet.

If you are unwilling to backport property bits to stable kernels, then I
don't think we have any other choice than to leave the definition as
"unset bits are unknown", but it clearly puts userspace in a bind.
Because the evdev event codes are clear (BTN_TOUCH means touchscreen,
BTN_TOOL_FINGER means touchpad) and these property bits are not, I doubt
userspace will ever rely on the direct and pointer property bits.

-- Chase