Re: [PATCH] input: mt: introduce MT event slots

[Henrik Rydberg <rydberg@euromail.se>]

Michael Poole wrote:
[...]
> 
> How would the slot number for a contact be chosen?

The device driver determines how to use the slots. The driver calls
input_mt_slot(dev, slot), sends the data for the slot, picks another slot, and
repeats.

> If the kernel makes
> that assignment, what should a "slot" correspond to from a computer
> user's perspective? "Set[s] of identified sources" is a little vague:
> Does it mean contacts from one hand, contacts in one displayed window
> (assuming the touch surface is a screen), or something else?  (I assume
> it would not duplicate the blob or tracking IDs already defined for MT
> events.)

The slot is only used for data communication. Think of the slot as a combined,
unique identifier. For example, imagine a device driver dealing with contacts
labeled with both a USER_ID and a TRACKING_ID. The driver assigns every active
(USER_ID, TRACKING_ID) contact to a specific slot, and uses it to communicate
all changes to that contact. When the contact is destroyed (for instance by
sending a zero ABS_MT_PRESSURE on that slot), the slot is free to be used for
another contact.

> It seems like those would be important aspects of the protocol
> to document in Documentation/input/multi-touch-protocol.txt --
> otherwise, driver implementers or application developers might get it
> wrong.

Certainly.

Cheers,
Henrik