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

Hi Anderson,

On 11/18/2011 12:58 PM, Anderson Lizardo wrote:

On Fri, Nov 18, 2011 at 4:48 PM, Brian Gix[off-list ref]  wrote:

quoted

On 11/18/2011 12:36 PM, Anderson Lizardo wrote:

quoted

On Fri, Nov 18, 2011 at 3:08 PM, Marcel Holtmann[off-list ref]
  wrote:

quoted

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.

I was actually referring only to the context switch topic. You still
need to send the ATT request parameters from userspace, which is not
different from building the PDU on userspace (vs. building on kernel).

Unless I'm not understanding what Marcel meant with "ATT on kernel"
(which is most probably the case).

quoted

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 discovery procedure was only an example of multiple PDU operation.
Take e.g. "read long". If only ATT is on kernel, it still need
multiple context switches for the multiple PDUs involved for the GATT
operation.

quoted

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.

... and for this reason I don't see much gain in reducing context
switches if *only* ATT is on kernel. (note: I'm not defending GATT on
kernel).

quoted

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

Yes, and also I don't see why LE HID is an example of "many context
switches". For sure, you will need a bunch of them for setting up the
HID device (discovery + pairing), but after that it will "talk" HID
with the kernel input subsystem (If I understood the basics of this
profile).

I don't know about everyone else, but most of the context switching that 
I would like to see avoided is for single ATT-procedures that:
Connect-DoSomething-Disconnect

That is the case for any Write-Signed-Command type of operations.

Perhaps HID is different, although I think the idea for HID is to not 
require the round trip of:

Baseband --> Kernel --> UserSpace --> Kernel --> HIDQueue

Would the Context switching make significant difference?  I suspect that 
on some platforms it would be hardly noticeable, but I would rather 
design something that we *know* requires low latency, to assume it is 
operating in an environment that might be hostile to latency concerns.

I personally think a fully feature rich GATT is too heavy to include in 
the kernel, but that perhaps a specialized HID server (recognized by a 
kernel resident ATT driver as being in the proper ATT Handle range) 
might make some sense.

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