7.4 Linux Bluetooth Support

As with many emerging technologies, there are competing implementations of Linux Bluetooth support. The main two implementations are Affix and BlueZ. Affix was originally developed by Nokia and is now hosted as an open source project at SourceForge (http://affix.sourceforge.net). BlueZ is also hosted at SourceForge is (http:// bluez .sourceforge.net) and the official Bluetooth stack of the Linux kernel.

Although Affix is a mature and functional project, BlueZ receives more testing and has more widespread adoption. For this reason, this chapter focuses on the uses of the BlueZ Linux Bluetooth stack and libraries.

This section includes all the information that you need to install and configure Bluetooth support from scratch. It is possible that your Linux distribution already contains preconfigured Bluetooth support, which will save you effort. However, the installation instructions provide useful background information for troubleshooting.

7.4.1 Distributions

As Bluetooth is a relative newcomer to Linux, BlueZ support across commercial distributions varies. Generally speaking, if the kernel shipping with your distribution is older than 2.4.22, it is a good idea to upgrade it. Users of "bleeding-edge" distributions such as Debian Unstable and Gentoo should find that Bluetooth is adequately supported.

7.4.2 Configuring the Kernel

Bluetooth support under Linux requires a recent kernel. If your kernel is Version 2.4.22 or better, or a 2.6 series kernel, then you're all set. Otherwise, you must upgrade your kernel. Alternatively, if you do not wish to upgrade, and have kernel 2.4.18 or better compiled from source, you can apply the patches from the "kernel patches" area of the BlueZ web site (http://bluez.sourceforge.net). Regardless, it's worth checking out the patches, because there are often improvements available that have not yet been merged into the main Linux kernel source.

Patching the Kernel

To patch the kernel, first download the most recent patch for your kernel version from the BlueZ web site (for example, patch-2.4.22-mh1.gz ), and place it somewhere convenient , such as /usr/src/. Change into the directory where your kernel source is unpacked, typically /usr/src/linux , and apply the patch:

  cd   /usr/src/linux     gzip -dc   ..     /patch-2.4.22-mh1.gz   patch -p1  

Next, run this command:

  find . -name '*rej'  

If any of the patches were rejected, you'll find some files ending in .rej . If they were, delete the kernel source, extract it again (be sure that you have the correct patch for your kernel version), and try the patch again.

You can then proceed with configuring your kernel for Bluetooth by running make menuconfig , make xconfig , or make config .

Chapter 2 explains how to configure and compile a kernel. Table 7-3 and Table 7-4 show the options that must be set in your kernel configuration to enable Bluetooth support. You can either configure Bluetooth support to be compiled into the kernel or to be loadable on-demand in the form of modules. Many Linux distributions choose to ship with modules, so we proceed on the assumption that you will use modules. This removes the need to recompile your kernel if you acquire a different type of Bluetooth device.

Figure 7-3 and Figure 7-4 show the Bluetooth configuration options from the 2.4.24 kernel.

Figure 7-3. Configuring Bluetooth support in the Linux kernel

Figure 7-4. Configuring Bluetooth hardware support in the Linux kernel

Table 7-3. BlueZ protocol configuration options

Option	Purpose	Recommended value
Bluetooth subsystem support	Enables the entire BlueZ stack	m
L2CAP protocol support	A basic Bluetooth protocol	m
SCO links support	Bluetooth Audio	m
RFCOMM protocol support	Serial data transfer	m
RFCOMM TTY support	Maps Linux terminal devices (e.g., /dev/rfcomm0 ) to Bluetooth serial ports	y
BNEP protocol support	Personal area networking	m
BNEP Multicast filter support, protocol filter support	Advanced filtering for networking	y

Table 7-4. BlueZ hardware support configuration options

Option	Purpose	Recommended value
HCI USB driver	Support for USB dongles	m
SCO (voice) support	Audio transmission support	y
USB zero packet support	Workaround for buggy USB devices	n
HCI UART driver	Support for serial dongles: either PCMCIA, CF, or RS232 serial port	y
UART (H4) protocol support	Serial protocol used for most PCMCIA and CF cards	y
BCSP protocol support	Serial protocol used for PCMCIA and CF cards based on the CSR BlueCore chipset	y
Transmit CRC with every BCSP packet	Improves reliability of BCSP support, at a slight cost to efficiency	y
HCI BlueFRITZ! USB driver	Support for BlueFRITZ! Bluetooth ISDN	m
HCI VHCI (Virtual HCI device) driver	Support for a virtual Bluetooth device for testing purposes	m

You should not enable the Bluetooth device support in the USB drivers section of your kernel configuration (CONFIG_USB_BLUETOOTH). This is a vestigial driver from very early Bluetooth work and will prevent the BlueZ Bluetooth stack from operating. If you are using a distribution's precompiled kernel, this corresponds to the bluetooth kernel module, and you should prevent it from being loaded. This can be done either by ensuring the BlueZ hci_usb module is loaded instead of the USB Bluetooth module or by disabling the module by adding its name to hotplug's configuration list ( /etc/hotplug/blacklist ).

Once your kernel is compiled and you have rebooted, you must configure Linux so it knows how to load the appropriate Bluetooth protocol modules. Most modern distributions come with /etc/modules.conf already set up for Bluetooth use, but you may be missing the required configuration.

To verify this, ensure that the contents of Example 7-1 are present in /etc/modules.conf . If you need to change this file, run depmod -a to ensure automatic loading of modules by the kernel. You must be the root user to perform this operation.

Example 7-1. Module configuration for BlueZ

 # BlueZ modules alias net-pf-31 bluez alias bt-proto-0 l2cap alias bt-proto-2 sco alias bt-proto-3 rfcomm alias bt-proto-4 bnep alias tty-ldisc-15 hci_uart alias char-major-10-250 hci_vhci 

Linux distributions may vary in the way they manage the /etc/modules.conf file. Debian GNU/Linux, for example, requires you put the contents of Example 7-1 in a separate file in /etc/modutils and run /sbin/update-modules ; however, if you use the pre-packaged Debian BlueZ utilities, this file is already provided for you.

7.4.3 Supporting Subsystems

Depending on your hardware configuration, you must ensure you are running some extra supporting software to initialize your Bluetooth device.

7.4.3.1 PCMCIA

If your Bluetooth adapter is a PC card or a CF card, you must have kernel support for PCMCIA and the PCMCIA card services software installed. This software is responsible for initializing your adapter when it is plugged in and loading the required drivers into the kernel.

BlueZ requires PCMCIA card services to be Version 3.2.2 or higher. If your Linux distribution has an older version, you can update it from http://pcmcia-cs.sourceforge.net. See Chapter 2 for complete instructions on compiling pcmcia-cs from source.

7.4.3.2 Hotplug

The Linux hotplug subsystem enables you to plug in a device and have it immediately ready to use. It is similar in function to the card manager from PCMCIA card services, except it is generalized to include USB, Firewire (IEEE 1394), and network devices. The 2.6 series of Linux kernels adds hotplug capability to even more subsystems, such as SCSI and input devices.

When hotplug detects a new device, it loads the necessary driver modules into the kernel, and it runs any scripts the user has configured. In the case of BlueZ, hotplug is required for certain Bluetooth devices that require firmware downloaded to them, such as the USB dongles based on the Broadcom chipset.

Hotplug ships with most Linux distributions. Version 2002_08_26 or later is required. If you need to install it separately, download it from http://linux-hotplug.sourceforge.net/.