Playing BlueZ on the D-Bus

Playing BlueZ on the D-Bus
Playing BlueZ on the D-Bus
Marcel Holtmann
BlueZ Project
[email protected]
Abstract
The integration of the Bluetooth technology
into the Linux kernel and the major Linux distributions has progressed really fast over the
last two years. The technology is present almost everywhere. All modern notebooks and
mobile phones are shipped with built-in Bluetooth. The use of Bluetooth with a Linux based
system is easy and in most cases it only needs
an one-time setup, but all the tools are still command line based. In general this is not so bad,
but for a greater success it is needed to seamlessly integrate the Bluetooth technology into
the desktop. There have been approaches for
the GNOME and KDE desktops. Both have
been quite successful and made the use of Bluetooth easy. The problem however is that both
implemented their own framework around the
Bluetooth library and its daemons and there
were no possibilities for programs from one
system to talk to the other. With the final version of the D-Bus framework and its adaption
into the Bluetooth subsystem of Linux, it will
be simple to make all applications Bluetooth
aware.
The idea is to establish one central Bluetooth
daemon that takes care of all task that can’t or
shouldn’t be handled inside the Linux kernel.
These jobs include PIN code and link key management for the authentication and encryption,
caching of device names and services and also
central control of the Bluetooth hardware. All
possible tasks and configuration options are accessed via the D-Bus interface. This will allow to abstract the internals of GNOME and
KDE applications from any technical details of
the Bluetooth specification. Even other application will get access to the Bluetooth technology without any hassle.
1
Introduction
The Bluetooth specification [1] defines a clear
abstraction layer for accessing different Bluetooth hardware options. It is called the Host
Controller Interface (HCI) and is the basis of
all Bluetooth protocols stacks (see Figure 1).
This interface consists of commands and events
that provide support for configuring the local
device and creating connections to other Bluetooth devices. The commands are split into six
different groups:
• Link Control Commands
• Link Policy Commands
• Host Controller and Baseband Commands
• Informational Parameters
• Status Parameters
• Testing Commands
422 • Playing BlueZ on the D-Bus
code or to notify of a changed power state.
Applications and Profiles
OBEX
SDP
RFCOMM
L2CAP
HCI
Link Manager
Baseband
Radio
Figure 1: Simple Bluetooth stack
With the Link Control Commands it is possible to search for other Bluetooth devices in
range and to establish connections to other devices. This group also includes commands to
handle authentication and encryption. The Link
Policy Commands are controlling the established connections between two or more Bluetooth devices. They also control the different
power modes. All local settings of a Bluetooth device are modified with commands from
the Host Controller and Baseband Commands
group. This includes for example the friendly
name and the class of device. For detailed information of the local device, the commands
from the Informational Paramters group can be
used. The Status Parameters group provides
commands for detailed information from the remote device. This includes the link quality and
the RSSI value. With the group Testing Commands the device provides commands for Bluetooth qualification testing. All commands are
answered by an event that returns the requested
value or information. Some events can also arrive at any time. For example to request a PIN
Every Bluetooth implementation must implement the Host Controller Interface and for
Linux a specific set of commands has been integrated into the Linux kernel. Another set of
commands are implemented through the Bluetooth library. And some of the commands are
not implemented at all. This is because they are
not needed or because they have been deprecated by the latest Bluetooth specification. The
range of commands implemented in the kernel
are mostly dealing with Bluetooth connection
handling. The commands in the Bluetooth library are for configuration of the local device
and handling of authentication and encryption.
While the Host Controller Interface is a clean
hardware abstraction, it is not a clean or easy
programming interface. The Bluetooth library
provides an interface to HCI and an application programmer has to write a lot of code to
get Bluetooth specific tasks done via HCI. To
make it easy for application programmers and
also end users, a task based interface to Bluetooth has been designed. The definition of this
tasks has been done from an application perspective and they are exported through D-Bus
via methods and signal.
2
D-Bus integration
The hcid daemon is the main daemon when
running Bluetooth on Linux. It handles all device configuration and authentication tasks. All
configuration is done via a simple configuration
file and the PIN code is handled via PIN helper
script. This means that every the configuration
option needed to be changed, it was needed to
edit the configuration file (/etc/bluetooth/
hcid.conf) and to restart hcid. The configuration file still configures the basic and also
default settings of hcid, but with the D-Bus
2006 Linux Symposium, Volume One • 423
integration all other settings are configurable
through the D-Bus API. The current API consists of three interfaces:
• org.bluez.Manager
• org.bluez.Adapter
• org.bluez.Security
The Manager interface provides basic methods for listing all attached adapter and getting
the default adapter. In the D-Bus API terms
an adapter is the local Bluetooth device. In
most cases this might be an USB dongle or
a PCMCIA card. The Adapter interface provides methods for configuration of the local
device, searching for remote device and handling of remote devices. The Security interface
provides methods to register passkey agents.
These agents can provide fixed PIN codes, dialog boxes or wizards for specific remote devices. All Bluetooth applications using the DBus API don’t have to worry about any Bluetooth specific details or details of the Linux specific implementation (see Figure 2).
Besides the provided methods, every interface
contains also signals to broadcast changes or
events from the HCI. This allows passive applications to get the information without actively interacting with any Bluetooth related
task. An example for this would be an applet
that changes its icon depending on if the local
device is idle, connected or searching for other
devices.
Every local device is identified by its path.
For the first Bluetooth adapter, this would
be /org/bluez/hci0 and this path will be
used for all methods of the Adapter interface. The best way to get this path is to call
DefaultAdapter() from the Manager interface. This will always return the current default adapter or in error if no Bluetooth adapter
is attached. With ListAdapters() it is possible to get a complete list of paths of the attached adapters.
If the path is known, it is possible to use the
full Adapter interface to configure the local device or handle tasks like pairing or searching
for other devices. An example task would be
the configuration of the device name. With
GetName() the current name can be retrieved
and with SetName() it can be changed.
Changing the name results in storing it on the
filesystem and changing the name with an appropriate HCI command. If the local device already supports the Bluetooth Lisbon specification, then the Extended Inquiry Response will
be also modified.
With the DiscoverDevices() method it
is possible to start the search for other Bluetooth devices in range. This method call actually doesn’t return any remote devices. It only
starts the inquiry procedure of the Bluetooth
chip and every found device is returned via the
RemoteDeviceFound signal. This allows
all applications to handle new devices even if
the discovery procedure has been initiated by a
different application.
3
Current status
The methods and signals for the D-Bus API
for Bluetooth were chosen very carefully. The
goal was to design it with current application
needs in mind. It also aims to fulfill the needs
of current established desktop frameworks like
the GNOME Bluetooth subsystem and the KDE
Bluetooth framework. So it covers the common
tasks and on purpose not everything that might
be possible. The API can be divided into the
following sections:
• Local
424 • Playing BlueZ on the D-Bus
applications
org.bluez.Manager
org.bluez.Adapter
org.bluez.Security
hcid
Passkey Manager
Core Manager
Adapter Manager
Security Manager
Host Controller Interface
kernel
Bluetooth Core
Bluetooth Drivers
Figure 2: D-Bus API overview
– version, revision, manufacturer
– mode, name, class of device
• Remote
–
–
–
–
–
version, revision, manufacturer
name, class of device
aliases
device discovery
pairing, bondings
• Security
– passkey agent
With these methods and signals all standard
tasks are covered. The Manager, Adapter and
Security interfaces are feature complete at the
moment.
4
Example application
The big advantage of the D-Bus framework
is that it has bindings for multiple program-
ming languages. With the integration of DBus into the Bluetooth subsystem, the use of
Bluetooth from various languages becomes reality. The Figure 3 shows an example of changing the name of the local device into My Bluetooth dongle using the Python programming
language.
The example in Python is straight forward and
simple. Using the D-Bus API within a C program is a little bit more complex, but it is still
easier than using the native Bluetooth library
API. Figure 4 shows an example on how to get
the name of the local device.
5
Conclusion
The integration of a D-Bus API into the Bluetooth subsystem makes it easy for applications
to access the Bluetooth technology. The current API is a big step into the right direction,
but it is still limited. The Bluetooth technology
is complex and Bluetooth services needs to be
extended with an easy to use D-Bus API.
2006 Linux Symposium, Volume One • 425
#!/usr/bin/python
#include <stdio.h>
#include <stdlib.h>
import dbus
#include <dbus/dbus.h>
bus = dbus.SystemBus();
obj = bus.get_object(’org.bluez’,
’/org/bluez’)
int main(int argc, char **argv) {
DBusConnection *conn;
DBusMessage *msg, *reply;
const char *name;
conn = dbus_bus_get(DBUS_BUS_SYSTEM, NULL);
msg = dbus_message_new_method_call(
"org.bluez",
"/org/bluez/hci0",
"org.bluez.Adapter", "GetName");
manager = dbus.Interface(obj,
’org.bluez.Manager’)
obj = bus.get_object(’org.bluez’,
manager.DefaultAdapter())
reply =
dbus_connection_send_with_reply_and_block(
conn, msg, -1, NULL);
adapter = dbus.Interface(obj,
’org.bluez.Adapter’)
dbus_message_get_args(reply, NULL,
DBUS_TYPE_STRING, &name,
DBUS_TYPE_INVALID);
adapter.SetName(’My Bluetooth dongle’)
printf("%s\n", name);
dbus_message_unref(msg);
dbus_message_unref(reply);
dbus_connection_close(conn);
Figure 3: Example in Python
return 0;
}
The next steps would be integration of D-Bus
into the Bluetooth mouse and keyboard service. Another goal is the seamless integration
into the Network Manager. This would allow
to connect to Bluetooth access points like any
other WiFi access point.
The current version of the D-Bus API for Bluetooth will be used in the next generation of
the Maemo platform which is that basis for the
Nokia 770 Internet tablet.
References
[1] Special Interest Group Bluetooth:
Bluetooth Core Specification Version 2.0
+ EDR, November 2004.
[2] freedesktop.org: D-BUS Specification
Version 0.11.
Figure 4: Example in C
426 • Playing BlueZ on the D-Bus
Proceedings of the
Linux Symposium
Volume One
July 19th–22nd, 2006
Ottawa, Ontario
Canada
Conference Organizers
Andrew J. Hutton, Steamballoon, Inc.
C. Craig Ross, Linux Symposium
Review Committee
Jeff Garzik, Red Hat Software
Gerrit Huizenga, IBM
Dave Jones, Red Hat Software
Ben LaHaise, Intel Corporation
Matt Mackall, Selenic Consulting
Patrick Mochel, Intel Corporation
C. Craig Ross, Linux Symposium
Andrew Hutton, Steamballoon, Inc.
Proceedings Formatting Team
John W. Lockhart, Red Hat, Inc.
David M. Fellows, Fellows and Carr, Inc.
Kyle McMartin
Authors retain copyright to all submitted papers, but have granted unlimited redistribution rights
to all as a condition of submission.
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