linux-kernel - Re: [PATCH] input: mt: introduce MT event slots

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Date:	Thu, 08 Apr 2010 17:23:53 -0400
From:	Rafi Rubin <rafi@...s.upenn.edu>
To:	Henrik Rydberg <rydberg@...omail.se>
CC:	Michael Poole <mdpoole@...ilus.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] input: mt: introduce MT event slots

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On 04/08/10 16:25, Henrik Rydberg wrote:
> Rafi Rubin wrote:
>> On 04/08/2010 05:43 AM, Henrik Rydberg wrote:
>>> 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.
>>>
>>
>> Is there any particular downside to defaulting to implicit slot ids?
> 
> Yes. The device driver should not have to update every slot between
> synchronizations, or the point would be lost.
> 
>> For drivers/hardware that don't handle tracking, SYN_MT_REPORT could
>> just result in dev->slot++ and a SYN_REPORT resets dev->slot to 0;
> 
> Drivers that do not handle tracking should not use the slots at all. The slot
> concept requires that whatever gets communicated over it is identifiable, or
> else it would not be possible to send changes. Drivers without tracking
> capabilities should stick to the current MT protocol, for which it was designed.

That's unfortunate.

I think tracking upsets are generally quite rare (at least for the n-trig
hardware), and we would see most of the benefit of jitter and bandwidth
reduction even if we use contact ordering for slots.  Tracking upsets would
still flow downstream, a large state change should cause the slot to emit the
new position.

I was also hoping the slotting mechanism might be a good place to inject generic
tracking support later.

Rafi
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