Re: [PATCH 2/2] input: mt: Document the MT event slot protocol (rev4)

[Henrik Rydberg <rydberg@euromail.se>]

Pavel Machek wrote:
> Hi!
> 
>> +Protocol Example A
>> +------------------
>> +
>> +Here is what a minimal event sequence for a two-contact touch would look
>> +like for a type A device:
>> +
>> +   ABS_MT_POSITION_X x[0]
>> +   ABS_MT_POSITION_Y y[0]
>> +   SYN_MT_REPORT
>> +   ABS_MT_POSITION_X x[1]
>> +   ABS_MT_POSITION_Y y[1]
>> +   SYN_MT_REPORT
>> +   SYN_REPORT
> ...
>> -Usage
>> ------
>> +Here is the sequence after lifting the first contact:
>> +
>> +   ABS_MT_POSITION_X x[1]
>> +   ABS_MT_POSITION_Y y[1]
>> +   SYN_MT_REPORT
>> +   SYN_REPORT
> 
> ...
> 
>> +Protocol Example B
>> +------------------
>> +
>> +Here is what a minimal event sequence for a two-contact touch would look
>> +like for a type B device:
>> +
>> +   ABS_MT_SLOT 0
>> +   ABS_MT_TRACKING_ID 45
>> +   ABS_MT_POSITION_X x[0]
>> +   ABS_MT_POSITION_Y y[0]
>> +   ABS_MT_SLOT 1
>> +   ABS_MT_TRACKING_ID 46
>> +   ABS_MT_POSITION_X x[1]
>> +   ABS_MT_POSITION_Y y[1]
>> +   SYN_REPORT
>> +
>> +Here is the sequence after moving contact 45 in the x direction:
>> +
>> +   ABS_MT_SLOT 0
>> +   ABS_MT_POSITION_X x[0]
>> +   SYN_REPORT
> 
> With this proposed system, protocol A is *very* different from
> protocol B. Is there way to make them more similar?

Hello,

unfortunately, the two protocols are different for a reason. Given the point of
the type B protocol, to reduce bandwidth and accommodate filtering in the input
core, the major difference is that type A receivers always expect all data
between synchronizations, whereas type B devices never send all data. This alone
means a receiver wishing to support both type of streams will need to know what
type of stream to expect. This is accomplished by the presence of ABS_MT_SLOT.
Secondly, in order to not break the established semantics of events, ABS_MT_SLOT
is used instead SYN_MT_REPORT. This also makes type B devices in type A
applications stop working in a somewhat graceful fashion.

The usage of SYN_MT_REPORT versus ABS_MT_SLOT is the only syntactic difference
between the protocols, and it stems from the different interpretation of what
comes through the wire. IMHO, it means the way the type B protocol is formulated
is the smallest possible change that accommodates the required effect.

Thanks,
Henrik