Re: Hardware interface constant for Bluetooth

[Marcel Holtmann <marcel@holtmann.org>]

Hi Jaroslav,

> > I like to drive the Bluetooth audio support a little bit and it would be
> > great if you can add a hardware interface constant for us. I don't know
> > what is the correct procedure to do it, but from my first analysis it
> > seems that it is enough to add a patch like the one below to the kernel
> > code and the library code. Everything else can be developed outside the
> > ALSA core.
> > 
> > +       SNDRV_HWDEP_IFACE_BLUETOOTH,    /* Bluetooth audio */
> 
> Do you need really this change? This identifier is only used for hardware 
> independent API (hwdep) - mostly for firmware / DSP code handling and 
> other things which don't fall into abstract APIs.

this is a little bit tricky. Besides the native support for transporting
PCM data over a Bluetooth SCO channel, the headset/handsfree profile
uses a RFCOMM channel (serial port emulation) for additional settings.
On this channel they use AT commands and actually I don't wanna put a AT
parser into the kernel. It is in somekind possible, but I still think
that it does not belong there, because it is profile specific and a
Bluetooth profile is an application and applications should run in user
space. On the other hand you still need to create that SCO channel from
user space with somekind of ioctl, because the kernel can't know when
this channel must be established. It depends on some AT commands that
are send over the RFCOMM channel.

So my plan is to write a kernel module that is reponsible for the SCO
audio traffic. This registers as a soundcard with the ALSA subsystem and
it also registers a Bluetooth HWDEP. There is no initial PCM device
registered with this soundcard. These are only added when a SCO channel
is opened from the peer side or requested through an ioctl on the HWDEP
interface. Besides this the HWDEP would also control the mixer settings,
because these are exchanged over the RFCOMM channel.

Do you think this makes sense?

Regards

Marcel




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