Re: [RFC] LE: Low Latency GATT Write-Sign-Cmd

[Brian Gix <bgix@codeaurora.org>]

Hi Anderson,

On 11/18/2011 12:36 PM, Anderson Lizardo wrote:
> Hi Marcel,
>
> On Fri, Nov 18, 2011 at 3:08 PM, Marcel Holtmann<marcel@holtmann.org>  wrote:
>> I think one of the most important questions that we have to ask
>> ourselves at some point is if we wanna put ATT into the kernel.
>>
>> The potential candidate that forces us to think about this is HID over
>> Low Energy. However I like to see numbers on how the context switches
>> with keeping ATT in userspace will effect our latency.
>
> I still fail to see how ATT handling in kernel would reduce context
> switches. A GATT operation is composed of one or more (possibly many,
> see e.g. discovery procedures) ATT PDUs.
>
> Unless you are proposing GATT on kernel as well?

I don't think having GATT in the kernel is necessarily the logical 
conclusion to an ATT-to-kernel migration.

Remember that the focus of LE is *not* the discovery process.  Discovery 
(of Devices, Services and Characteristics) is important, but what makes 
LE "Low Energy" is what happens *after* pairing and discovery have been 
completed, which ideally would happen One Time Only.

The LE profiles and services are being spec'd so that they can be highly 
efficient over the lifetime of the Low Energy device.  The Linux side of 
the equation will rarely be an actual Low Energy device:  It will just 
know how to talk to an LE device in such a way as to make that 
Button-cell battery driven device last a couple years if possible.

For that reason MOST day-to-day communication between a Linux based 
"Central" device will in fact be single-ATT procedures.  If it takes 
more energy and context switches to do discovery using many compound 
procedures, then we pay that price, because it should only happens Once.

HID devices in particular, will almost certainly NOT use compound GATT 
procedures to communicate after the pairing and discovery point in time 
has past.


-- 
Brian Gix
bgix@codeaurora.org
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum