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

From: Ping Cheng
Date: Sun May 23 2010 - 19:47:53 EST

On Sun, May 23, 2010 at 3:27 PM, Henrik Rydberg <rydberg@xxxxxxxxxxx> wrote:
> Randy Dunlap wrote:
> [...]
>>> +Protocol Usage
>>> +--------------
>>> +
>>> +Contact details are sent sequentially as separate packets of ABS_MT
>>> +events. Only the ABS_MT events are recognized as part of a contact
>>> +packet. Since these events are ignored by current single-touch (ST)
>>> +applications, the MT protocol can be implemented on top of the ST protocol
>>> +in an existing driver.
>>> +
>>> +Drivers for type A devices mark the end of a packet by calling the
>> end?

Since Randy brought this question up, I feel the urge to say
something. I know there are X drivers and clients using the type A
format so I am not suggesting that we need to change this format.

What I am thinking is that we only need one SYN_ call for both _MT_
and regular data combined, which is a call to input_sync() at the end
of the whole packet. The SYN_MT_ can be replaced by the following
example, which I think is more "client-friendly". This solution is
based on the fact that the major difference between type A and type B
is whether we need to filter the data or not:


input_set_abs_params(input_dev, ABS_MT_RANDOM, 0, 2, 0, 0);

would tell the clients that they can expect two random touches.

>>> +The end of a multi-touch transfer is marked by calling the usual
>> The end method is done for Types A and B, right?
> How about this line instead?
> All drivers mark the end of a multi-touch transfer by calling the usual

If we use ABS_MT_RANDOM, there would be only one SYN_ event for the
whole packet at the end, regardless of what kind of data is included
before the SYN_ event.

Basically, ABS_MT_RANDOM indicates a non-filtered and non-tracked MT_
event; ABS_MT_SLOT indicates a filtered and tracked MT_ event/slot.

With all this said, I don't have a preference for type A since I do
not need it for my driver. Just to be active in the group if you
wonder if there is any positive rationale :).

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