UsermodeHID uncertainty

UsermodeHID uncertainty

[Mike Purvis]

Hi all,

I'm struggling to understand what the UsermodeHID option in BlueZ
does, and I'm not finding a good source of documentation on it. I
understand that using this option has the kernel pass raw HID traffic
to a usermode process for interpretation, rather than doing this
directly in the kernel, and this has advantages as far as
debuggability, pluggability, fault isolation, etc.

However, my questions—

- What is the usermode process? Do I have to start it somehow, or does
it start on its own? Is it an already running daemon?
- What is the communication channel between the kernel and user
process, and how can I listen in on the traffic?
- Is the usermode process directly managing things like devfs nodes
(eg, /dev/input/js0), or does it communicate back to the kernel for
that?
- As a user, what are my options for swapping out the usermode process
if I want to modify its behaviour? For example, handling disconnects
or pairing in a different manner than the default?

Thanks for any pointers or thoughts,

Mike

Re: UsermodeHID uncertainty

[Mike Purvis]

> - What is the usermode process? Do I have to start it somehow, or does
> it start on its own? Is it an already running daemon?
> - What is the communication channel between the kernel and user
> process, and how can I listen in on the traffic?
> - Is the usermode process directly managing things like devfs nodes
> (eg, /dev/input/js0), or does it communicate back to the kernel for
> that?
> - As a user, what are my options for swapping out the usermode process
> if I want to modify its behaviour? For example, handling disconnects
> or pairing in a different manner than the default?

Any thoughts on these questions?

Re: UsermodeHID uncertainty

[Luiz Augusto von Dentz]

Hi Mike,

On Wed, Jul 15, 2015 at 6:10 PM, Mike Purvis
<mpurvis@clearpathrobotics.com> wrote:
>> - What is the usermode process? Do I have to start it somehow, or does
>> it start on its own? Is it an already running daemon?

bluetoothd will be in charge since after all UserspaceHID (not
UsermodeHID) is part of its input.conf.

>> - What is the communication channel between the kernel and user
>> process, and how can I listen in on the traffic?

It communicates using uhid/uhid2, check Linux Documentation for more
details. You can listen the L2CAP traffic using btmon, hcidump or
wireshark, but they probably wont decode the HID protocol

>> - Is the usermode process directly managing things like devfs nodes
>> (eg, /dev/input/js0), or does it communicate back to the kernel for
>> that?

Nope, uhid does that in the kernel, bluetoothd only forward the
descriptors it gets over L2CAP.

>> - As a user, what are my options for swapping out the usermode process
>> if I want to modify its behaviour? For example, handling disconnects
>> or pairing in a different manner than the default?

bluetoothd will take care of paring, etc, of course you can change the
logic by changing BlueZ's source code, but be aware things like paring
is subject of Bluetooth Qualification which is something BlueZ has
passed many times.




-- 
Luiz Augusto von Dentz