[Bluez-devel] SCO. Some ideas.

[James Courtier-Dutton <James@superbug.demon.co.uk>]

As SCO over bluetooth is only really suitable for audio sound data, I 
see no real problem with alsa being the only interface in linux to send 
data over a bluetooth SCO connection. (only the actually sco data, not 
the bluetooth profile control)
Does anyone see a problem with that?

If people think we should also make it easy to add an OSS driver, and 
also a network socket driver, we could maybe work out some devision 
between the SCO specifics and the ALSA specifics. But for a first 
attempt, we should keep the SCO and ALSA driver tightly matched.
Therefore maybe making the bluetooth sco and alsa-sco module being the 
same module, with an ioctl interface so that a user space application 
can handle the creation of bluetooth profile SCO pairing.
For example, if using a Headset profile, the alsa PCM and alsa MIXER 
device would only appear if the userspace application had already set up 
the RFCOMM connections. The userspace application would also have to 
have set up the bluetooth specific details for the SCO connection, so 
that when alsa opens the PCM, it has enough information to open a SCO 
connection.

I have looked at this for the alsa->sco->bluetooth->hci_usb  drivers.
The alsa driver has to be able to do the following: -
1) open - this should check that all bluetooth connections are up that 
need to be up in order for SCO data to pass. It could do this via 
interaction with the userspace bluetooth profile.

2) close - close everything neatly.

3) hw_params - allocate buffers etc. for the pcm audio as well as the 
hw_params config. Can be called multiple times, so re-alloc of buffers 
should be allowed for. hw_params are things like sample rate, number of 
channels, PCM format. Obviously there are limits on what these values 
can take when using bluetooth. 16bit PCM, 8bit PCM, 8bit A-law, 8bit 
u-Law. Start by also limiting it to 1 PCM channel. I.E. Mono and not stereo.

4) pcm_prepare - actually set the hw_params. e.g. do the equivalent of 
hciconfig hci0 voice XX, where XX depends on the hw_params.
Also selecting usb alt profiles.
The only difference between hw_params and pcm_prepare is that 
pcm_prepare is called for xrun recovery.

5) trigger - actually start/stop usb_urbs (e.g. call 
usb_submit_urb/usb_unlink_urb now.)

6) pointer - get hw_usb_frame_pointer and modify the result to simulate 
an audio ring buffer of the size configured in the hw_params.
When retrieving the pointer, one should also retrieve the valid range 
that the pointer can have, so one can adjust for pointer wrap around.
For usb, this would be the usb_get_current_frame_number() for the 
pointer, and dev->bus->iso_sched_frames for the range of values it can take.

7) period_time_elapsed - hci_usb should call this on each urb_complete 
call. For our use, it seems sensible to make 1 urb == 1 alsa period.

If we can let the alsa bluetooth audio driver have access to the 
hardware pointers (in this case the usb current frame pointers), we will 
get a much more accurate idea of exactly which audio sample is currently 
being played.

Also, letting alsa control period and buffer sizes, and getting that to 
directly determine usb urb sizes, we would give complete control of 
buffer sizes to the application, which is vital for low latency 
applications like Voice over IP.

Summary: -
we need to provide up to the sco level, control of low level usb 
interface isoc parameters. E.g. prepare, trigger, pointer, 
period_time_elapsed, hw_params.

If people are happy with this proposal, I will start writing the alsa 
driver, and also start adding to the bluetooth -> hci_usb driver api to 
allow for it. The api will only change for SCO data handling, as Bulk 
and Int handling don't need such fine controls.

Cheers
James



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